Dell Latitude C640 shuts off all the time

Hi, I have a Dell Latitude C640. It shuts off all the time and can usually only get windows loaded and it’ll shut right off, then it will turn on and turn off right away at boot. I’m very familiar with taking these apart and when this first occurred I suspected a clogged cooling system. So I checked, and it was clean as a whistle… hmmmm…so I’ve been looking for anything out of the ordinary on the motherboard but everything is tight and looks clean. Even took the LCD apart to make sure there weren’t any loose connections. So if you could give me any insight on this it would be a great help! The only thing that “SOUNDS” suspicious to me, is that sometimes I can HEAR the power when I plug it in, a very faint high pitch ringing noise…. so then I figured well if its the plug technically it should still run fine off the battery, but it does the exact same thing. I’m also suspecting that maybe my fans aren’t running fast enough?? Or maybe I need some new thermal grease on the CPU??? HELP!!

Yeah, looks like an overheating problem. By the way, have you checked if the fan spins at all? May be it stuck and causing this high pitch noise?
I would probably remove the hard drive and boot the laptop from a Knoppix CD. Knoppix is a live Linux CD and it will boot your laptop to a Windows like environment. You don’t need the hard drive to boot the laptop with Knoppix, only the DVD/CD drive. If the laptop boots fine and doesn’t shutdown, then something is wrong with the hard drive or with the operating system on it.
Also I would test the memory module. You can run Memtest 86+ and see if the memory fails the test. It’s possible, that the memory works fine for a while when it’s cold, as soon as it warm up it fails. That’s why your laptop works fine for some time, and then just shuts down when you restart it. If the laptop doesn’t run long enough to finish up the memory test, then try to replace the memory if you can find any test module.
Of course, I would try to reseat all connectors inside the laptop. Any loose connection might cause this problem.

167 Responses

  1. DellCA

    Dear Dell Customer,

    I read your post from July 9 regarding your Latitude C640. If you would like to contact me directly regarding any outstanding issues, I will be happy to assist in any way possible. You can reach me at the email below. I look forward to hearing from you. If you would simply like to provide feedback, you are welcome to do so, as well.



  2. I have this same problem. I have tried the following to no avail. Most of the time, I get about 20-40
    minutes before the abrupt shutdown. Any more ideas?

    1. blew out dust with compressed air (not really any)
    2. reformatted hard drive
    3. new hard drive (to see if it was the hard drive)
    4. boot to bios screen (to see if it was windows)
    5. made sure fan was clean and running
    6. made sure all connections are tight

  3. Laptop Freak

    Even though the heatsink is clean and the fan is running, the problem still could be related to overheating. If the laptop is a few years old, it’s possible that thermal grease dried out and has to be reapplied.

  4. That was also tried, the old thermal conductive tape was removed and a new piece was applied, (the really good stuff too, like $400-500 a roll). The surfaces
    were carefully cleaned with alcohol. The laptop is like 4-5 years old, but prior to just a few weeks ago, it ran fine and often was plugged in for days straight without incident.

  5. Laptop Freak

    You mentioned that the fan runs fine. Usually the cooling fan starts as soon as you turn on the laptop, then it turns off and turns on again when the CPU is hot. Does your fan turn on prior to the abrupt shutdown? May be the fan doesn’t work properly or the thermal sensors have failed?
    Just recently I repaired a laptop with a similar problem. Here’s what I experienced: the fan spins only on the laptop startup, then turns off and never come back again. As a result, the laptop shuts down by itself after 5-10 minutes. I had to replace the motherboard.

  6. I think the fan works OK since after startup, I exercise the CPU to see if the fan will kick on, which it does, then I relax it and it eventually gets to a state where the fan goes off. I tried this a few times and it appears to follow what one expects.

    I also tried the I8KfanGUI program on the web and it measured the CPU temp and the peak was like 60 something. This laptop apparently cannot monitor the memory temp so I cant tell if the heat problem resides there. I also am not continuously looking at the fan GUI results as sometimes the machine can stay up for hours. It’s too bad that a running log cannot be kept with fanGUI to help in debug. If so, I could tell what the final CPU temp was before shutting off

  7. Hi,
    I experience similar problem with my Laptop. Its a Presario 700 series. The fan had went out on it. The keys above the cpu would get so hot you could hardley touch but it never shut off. I never let run more than maybe 5 minutes. I replaced the Cpu fan and the other fan. Now, when i try to boot it up it shuts off no shutdown or anything. (I am having to restore harddrive as well) Once or twice I get into the restore program and it will start to do restore then shutoff. Is this an overheating problem or do I need look else where?

  8. I have the same prob with a Toshiba Satellite – maybe increased fan volume and then abruptly shuts down.
    1. Is it possible this cd be caused by spyware or any other kind of virus?
    2. I am working in Microsoft word. Only half the time or so I get autorecovery. Is it possible to ensure the file I am working on has autorecovery every time since half the time I am losing work- v annoying!
    Thank you for your time,

  9. Laptop Freak

    Most likely your laptop shuts down because it overheats.

  10. Hi, I have the same problem with a Dell Latitude C640:
    I can’t keep it turned on: sometimes it turns off during boot, sometimes after loading windows. In any case it doesn’t last more than few minutes. After some tries it does not turn on anymore.

    After I disassembled it TOTALLY (and assembled it again) it started working well, but now the problem is again.

    Probably there’s something wrong with some component, but wich one?


  11. I’ve just made some experiments.
    I disassembled toatally the laptop again:
    now I have only the motherboard and the CPU but when I press the start-up button NOTHING still appens.

    Only few times (in many tries) the power-on led lit.

    I’m afraid it’s something with the MB.

  12. Laptop Freak


    I’ve just made some experiments.
    I disassembled totally the laptop again:
    now I have only the motherboard and the CPU but when I press the start-up button NOTHING still happens.

    That’s a good idea to troubleshoot a laptop this way if nothing else left. For this test you’ll need only three basic components: motherboard, CPU and memory. You didn’t mention memory, so make sure to test it with a memory module. Test the system with an external monitor. If all three components are in a good shape, you should get a video output on the external screen.
    If you disassembled the laptop down to these three main components and still cannot get any video on the external monitor, then most likely one of the components is bad or not seated properly. Try moving the memory module into another slot. Use a test memory stick if you have it. Try reseating the CPU. If still nothing, then most likely that’s a bad board. From my experience laptop CPU failures are rare, motherboards fail much more often.

  13. Unluckely I had already tried everything you mentioned: moving and changing memory module, reseating CPU… I even chaged both fans.
    Laptop (in this case MB + CPU + Memory) seems to turn on only when it decides to do it.

    Probably there’s a faulty circuit or welding on the MB: sometimes, moving and stretching the MB makes it to work, even for a brief time.
    I tried to find a rule (twisting & stretching the MB, applying pressure on several points of it) but I found none.

    I go on trying…

  14. I know what the problem is with the Dell C640’s. I worked for a corp. and had to rebuild about 40 of them before we armwrestled Dell into replacing all of them. The problem as noted above is overheating. Once you start having the problem often it is too late. The processor is done. The processor, heatsink, thermal grease, and fan need to be replaced. I actually have one at home that is now starting to have this issue and will have to find the parts to get back to running shape. Hope this helps!

  15. I work in a callcenter that uses nothing but Dell computers. First, let me tell you, that your when your Latitude cops attitude like that, it’s 9 times out of 10 the motherboard. I know this because we had at one time about 150 C640’s. Out of those 150, 65 of them died in a similar manner. They sort of work their way to not working, starts cutting off at random more and more often until it won’t work at all. What I found that works, at least as a temp fix, unplug the power, remove the battery, push the power button down for 10 seconds. Then replace the battery and turn it on, then replace the a/c power. It will come back on for a bit but eventually turn off. All the techs I spoke with say it’s a power supply issue on the motherboard. The high-pitched noise you hear are bad capacitors, not fans.

    Powers supplies seem to be an issue for Dell. We have 175 GX270 models here in the building. Each one has had it’s power supply replaced. Some have had it done 2 or 3 times in the last year. Cheap parts. However, for those of you who have power supply issues with your dell or other branded desktop, I recommend They carry an aftermarket version for desktops like Dells and are reasonable on the prices.

  16. Jennifer

    Hi, I replaced the heatsink fan in the latitude c640 (was constantly shutting down and I noticed that the fan was not coming on) and now when I hit the power but all I get is the above mentioned high pitched noise. Does this mean the MB has now gone bad? If so can you recommend a good place to purchase an inexpensive replacement? Thanks in advance, J.

  17. Hi I also have this sudden switching off issue with my dell C640. It turns on then at seemingly random time’s 10sec’s to 30 min’s it just turns off (like I have just removed the battery and mains at the same time). so far this eve it’s been working for nearly 3 hours couple of probs earlier on, then it behaved. Something however I have noticed is that the system has started to forget the date and time.
    Does anyone think this may be related to the power off problem? i.e. system will not work unless it has some settings to read etc. Although admittedly if the thing has been working for 30 mins, I would normally have expected the system to carry on regardless of BIOS battery status. I figure it may be worth trying a new bios battery but not much else. Unless I hear otherwise 😉

  18. I am now experiencing this problem with the C640. Last time i used the laptop was two months ago and it worked fine. Now it powers down after 30 minutes after first plugging in and then every few minutes thereafter. Certainly seems to be heat related but more importantly, the power supply light on the laptop goes off as well.

    I suspect that this has nothing to do with the CPU but rather a failed capacitor on the DC input circuit. Can anyone confirm?

  19. Hi
    I have a Toshiba 410 laptop and there are times when I try to shut down but it takes few minutes before it shuts down. It happens about 50% of the times when I try to shut down. Once I choose shut down nothing seems to happen and I am able to go to the Turn Off Computer options again and choose again shut down, but it doesn’t make it any faster. When it’s slow shutting down, it takes about 3 minutes. There aren’t programs that I leave open before shut down.

    Any suggestion what I can try to do?


  20. Laptop Freak

    Probably it’s time to reinstall the operating system.

  21. Hi
    I have an Averatec 6100 laptop with a similar problem. Whenever i run a memory consuming process – for example downloading music, virus scan , windows update, or even play music with windows media player or winamp – or whenever i have simultaneous applications running, the laptop slows down considerably and shuts down unexpectedly with no error. If i leave the laptop on for several hours without running any memory consuming program then it does not shut down.

    From the Laptop Freak: the solution has been published here.

  22. Hi
    Suggestion for all fried (non booting or random shutdowns) lattitude c640’s. It is probably the motherboard- customer service at dell immediately reccommends replacing it when i told them my problem. reccomended fix? – buy a new motherboard on ebay- only about $150, and not too hard to install- use the dell service manual- available at the dell support site.
    good luck!

  23. I too have a C640 that died from heat exhaustion. System is fried but I was able to get my data off by setting the system on blocks of blue ice 🙂

  24. after using my latitude c640 for 4 year I upgraded memory from 256 to 512 and upgraded the bios from A03 to A10, now I too can just manage to log onto windows before it shuts down, have swapped memory, but still having problem, think it must be related to bios update…D’oh!

  25. I recently got a QBook 8589 laptop off ebay (I would rather say it is a desktop, using desktop Intel Celeron D 330 2.66GHZ and desktop ram, but it looks like laptop and a 2.5inch hdd). It shut down itself completely after turning on no more than 1min(the screen display fine). after research on this site, I’ve clean one cpu fan/heatsink and one graphics fan–>and put new thermal grease–>put everything back, two fans running fine(noisy but gives strong air, after a 10seconds, it gives warm air, so i think the fan/heatsink are doing fine), the problem still the same, when it shut itself off, I use my finger to touch the cpu heatsink, it is pretty hot. It is because the laptop have a faulty CPU or a faulty video card or faulty memory? If it is just cpu, i can change one but if it is video card I can’t do anything. I will try to get a desktop memory from my friend’s dekstop, but I don’t think it is the memory problem.
    Do you have any idea why this happens? Thanks in advance.

  26. Laptop Freak

    It’s hard to guess what’s going on. Do you get any image on the screen when you turn on the computer? Can you access the BIOS setup screen at all? If you can, leave the computer with the BIOS setup screen for a while and see if it still shuts down.
    I’m just trying to narrow down the problem.

  27. I’ve had my C640 (2ghz, 1G of Ram, 40 gb)for almost 3,5 years and never got a problem with it. until recently i started to get the same problem, humming noise when i put the power plug in and tries to start, load windows for a couple of seconds and then shuts down. same behavior when on Battery mode as well. i was able to do some circuit test to see where the problem, and i’m sure it’s one of the capacitors responsible for power distribution. You can change the capacitor but it’s hard to find. I changed the Motherboard and it’s back on track running smoothly.

  28. Patricia

    I have the same pre-mature shutdown problem with the Dell Latitude C640 and was told it was my TMobile Wireless Internet card that must have a short in it. But, the card works fine in other laptops. I am going to return this pc.

  29. Patricia

    Sounds like a class action suit is in order

  30. If the fan is working and the cpu is not reaching too high temps (above 80C) it may be that the hard drive is overheating. Using mobile meter my laptop shows moderate cpu temp at 60C but the hard drive slowly rises from under 40C to 60C and shuts down. At first it takes about an hour to heat up but then shuts down quickly if I don’t get it time too cool. I can’t seem to find a solution to keep the drive from overheating, and it picks up temperature even without reading/writing anything. Any ideas would be great.

  31. Hi,

    perhaps, after many experiments (look at my previous posts), I’ve solved my Dell power-off problem, but don’t know how!

    After months of inactivity (I had decided it was gone forever), I reassembled the laptop (I made my experiments using the naked motherboard) ’cause I decided to sell it as “not working”. Anyway, before reassembling it, I checked for the last time: it was really gone.

    My laptop had been always missing of the screws on the back, those used for connecting monitor, serial cable and parallel cable. Perhaps they were lost, or remained attached to the connectors in some office.
    This time I found some screws that could fit and used them.
    I completed the job and, once again, I switched it on, and that’s the MIRACLE!
    Now it works PERFECTLY! I’ve been using it ten hours a day (for job) for a week.

    What can I think? Perhaps screws keep motherboard in a particular position or create a mechanical tension/torsion on it, making it to work.
    This could mean that the problem (in this case) is a faulty welding or something similar.

    Perhaps my laptop will turn-off in five minutes. I hope it will not. I’ll let you know.

    Hope my eperience will help someone.



  32. Jerry

    I have a similar issue with my Dell C640, but slightly different. My laptop works fine on Battery power but it will not charge or run on AC power. I have dis-assembled the laptop, cleaned the board with canned air(attached my electro static discharge cable too). I re-assembled the laptop and plugged it in, it still does not recognize the Power Supply. I have purchased 3 brand new power supplies, and tested them on another Dell laptop( I love Dell). I still cannot get the laptop to run or charge on AC. Can anyone help me out here? Thanks.

    aka Vampnite69

  33. Towser

    Dell has 2 tiers of built in diagnostics that may be helpful to some of you. When the PC first powers up hitting the F2 key will take you into the BIOS setup screen, F12 allows several other booting options (for example, booting from the CD). Also listed under the F12 option is “Diagnostics”. Select this to run either an abbreviated system diagnosis (running from the system board), or, when that gets done, a MUCH more complete diagnostic, that runs from the hidden hard drive partition. In this 2nd diagnostic, you can run tests on individual components (I know the fan is one option).
    An alternative means of getting to the diagnostics is to hold down the blue (purple?) key (lower right of the keyboard) as the system boots up. I hope this can be of some help to those of you experiencing problems out there.

  34. Towser

    Sorry; my previous post left off holding down the blue FN key to boot into the system diagnostics.

  35. I have a Latitude C640 it shut off right at booting. It happend a few days back all of a sudden it just shut off and from then on it does not start up at all. Are there any tests I can do to narrow down the problem any help would be great…….


  36. Shehzad

    I have the same problem with my dell latitude.
    I was working on it that it suddenly off (on battery).
    I tried to start it again it started and on windows startup it was off agian.
    I thought it was coz of a low battery. Again i plugged in the power and started it, it asked that want to start windows normaly, after i press enter, windows started loding and it agian went off. After that it has never come on at all. But sometimes its LEDs lit for a split second as I push the power button. Hi what could be the problem, how to fix it. I hear a very very low sound on pluging the power, as i move very close to the plug socket… What could be the problem.
    is it with the MB


  37. S Chittenden

    My battery is fine, the laptop is cool and the fan spins. After the windows logo and the loading bar pass a blue screen quickly flashes up then dissapears then reboots. I can’t seem to stop this. I am very annoyed as it is a new laptop and i’m not sure what to do. Neither can I hear a problem. Can anyone help?

    S Chittenden

  38. Rennie

    Hi, my problem is even worst, my dell C640 works fine once started but after you shutdown the laptop overnight the next day it may not start. What I notice is the power light will come on when press and that’s it. The power light will just stay on, you have to remove the battery and AC connection for it to go off and even when you reinsert the battery and plug in the AC adaptor only the power light will come on and that’s it.
    The only way the laptop might come on again is if you remove the battery and AC for a long time, very frustrating. Have any one solve this problem ?

  39. Laptop Freak

    First of all, I would check the memory. Reinsert the memory module/modules and test the laptop again. Test it with a known good memory modules. It might help.

  40. I have a Dell Latitude D233SD. I am running XP PRO. CF Card Ram for a harddrive.

    1) If the four lights come on and not HD light. Press down the CPU and the power module on the laptop. It will then boot.

    2) If HD light but runs for 35 seconds place Display adapter in external video adapter and Motherboard will stay running.

    3) CPU heat problem, replace fan.

    Does anyone know how to get the motherboard to run without LCD and external display? Is this an XP Pro and NeoMagic settings problem?

  41. Dell C640, same random power off problems. After sitting for a bit, it will run for about 10-15mins, after that not very long until sitting for a bit.

    May have to unplug the power adapter to get it to start again and / or the battery. Does the same thing under battery or wall power. The power LED sometimes does the following after hitting the power button.

    Amber Green Green Green Amber

    What does this code mean?

    ort11 @

    How many others are seeing this?

  42. My C640 just started crashing with any RAM inserted in the second DIMM slot. I traded several different modules and it always crashes when the second DIMM is populated.

    Has anyone seen a similar situation? Is there a fix other than replacing the motherboard?


  43. Take an EGA or VGA monitor and connect it to the adaptor, the machine should
    stay running. If it does not then your fan is not running on the laptop.

    I have removed the screen and attached wheels. The system runs fine until
    it hits a wall. Also you sometimes need to press on the hard drive daddy.

    I have exchanged the Hard drive for a CF card (50.0) and an CF laptop
    (19.00) adaptor. There are no moving parts except the wheels.


  44. Wow I guess I’m not alone in this dilema! Ok I have a Dell C640 with the A10 BIOS update. Gotta admit that this laptop is a workhorse and has provided more than 2 years of service and I use my laptop @ work and and at home. The only time it is off is when I am sleeping! But alas my poor Latitude C640 is running into problems now. I cam across this forum and thought I’d throw my problem out to the rest of ya and see what you might have to say.

    Ok so about 3 months ago I was working away merrily in Windows and my laptop froze. No bigge.. just tought all I need to do was reboot and I’d be back in business. Fat chance. Every time I tried booting up I would get the CAPS LOCK key flashing 10 times and then I would hear a whipering noise from the laptop and then nothing. Literally nothing comes up on the screen. Nothing boots up. I don’t even see the Dell logo. Nothing.

    First thing I did was to extract my hard drive and move all my essential data to my desktop. All the data was there. Everything is in tack so that rules out any hard drive malfunction. I then reinstalled the hard drive and also decided move the RAM chips. So I opened up the bottom where the RAM is located and I swapped places with the RAM. Switching their slots. Lo and behold everything worked!

    Till today that is. Back to a dead laptop. Now I know that the hard drive is workin. I’ve rebooted a number of times and I was finally able to get it to boot up and go into windows. But it froze up. Then it wouldn’t boot up again after several tries. I reseated the RAM and it still woulnd’t boot up. After trying several times I was able to get it to reboot and am at this very moment running Memtest86. The RAM seems to be ok as the Memtest has not found any errors in the RAM. So does anyone know what I should do next? I should also point out that the laptop has been running this memtest for the past 60+ minutes and the computer still hasn’t froze. Which to me is a surprise because the laptop will sometimes freeze even in the middle of booting up. Heck it sometimes even freezes while loading the BIOS. I was able to log into Windows XP last night and it was wroking fine for about 10 minutes till it froze again. So I’m quite certain that the OS is fine. Perhaps it is a hardware failure? Motherboard fault? BIOS? Any ideas or has anyone else come across this problem too?

  45. Laptop Freak

    In order to narrow down the problem you’ll have to take this Latitude C640 laptop apart piece by piece and test it after each removed part.
    1. Remove the battery and test with the AC adapter. I’ve seen bad batteries freezing the whole system.
    2. Remove the wireless card, test again.
    3. Remove the modem card, test again.
    4. Remove the DVD drive, test.
    5. Unplug the keyboard from the motherboard, test.
    6. Remove the hard drive and see if the laptop freezes in the setup menu.
    7. Unplug the display assembly and test with an external monitor.
    In short, find out witch component is freezing the system. If the laptop stops freezing after you remove the wireless card, most likely this is the problem.
    Finally, after you remove all components, you’ll have only three major parts: motherboard, processor and memory (I assume a good one). If the laptop is still freezes when you test it with an external monitor, I would assume that you have a problem with the motherboard. CPU failures are not very common and we assume the RAM module is good, so there must be something wrong with the motherboard.

  46. Hi,

    on may 20th I wrote
    “…perhaps, after many experiments (look at my previous posts), I’ve solved my Dell power-off problem…”

    but I was not right, ’cause after one month the problem was again.

    Finally I changed the MotherBoard (keeping all the rest) and this – as I expected – solved the problem for real.

    Again, I think that most of these power-off problems are a MB issue.


  47. i’ve the same problem with my dell C640. the soluce is really simple : the ram disconnect from the JDIM(1or2)
    each time you move your laptop. To solve this problem use one of your biz’card, cut it in 8 pieces, and stuck them over the ram. no joke.

    bye, had

  48. hey there had.

    I suspect there is something wrong with the RAM disconnecting and connecting too. I’ve taken the laptop apart and tested each component and there is nothing wrong with any of the peripherals etc. I still suspect something wrong with the RAM connectors because when I swap the RAM from one slot over to the other the laptop works fine. For a while till it freezes up again. Is there an overheating issue in the RAM area at the bottom of the computer perhaps? And could you explain a bit more or do you have a photo of the biz card cut into 8 pieces solution which u have used? cheers m8

  49. hi,

    i upload two low-res pics (sorry) :

    that’s still not the best solution : i haven’t time to do it for good, and this freaky tip could (maybe) create an overheating (?). I think about build something more clean. but that’s work fine ! Don’t forget to hardly close the back-door.

    bye, had

  50. this is also explain in this post :

  51. Laptop Freak

    Find a known good RAM module. Test the laptop with this module in both slots. First, install the module into the slot 1 and run it for a while, then do the same with the slot 2. Find out if the laptop freezes with your known good memory module, find out witch slot is bad. If the memory slot is bad, most likely you’ll have to replace the whole motherboard or use the laptop with one working slot.
    I have the same problem with one of my IBM ThinkPad notebooks. One of the slots is bad and the laptop cannot see RAM module installed into this slot. I was able to fix the problem by placing a small piece of pen eraser on the RAM module installed into the bad slot. When you close the RAM hatch, it presses on the eraser and the eraser applies pressure on the RAM module installed into the defective slot. It was enough to make a good connection between the memory and motherboard. It’s just a temporary fix but it worked for me.

  52. Success!!!!!!! I cut up an eraser lengthwise and placed it on top of the RAM sticks and then screwed the cover back on. So far so good! No problems and lets hope it stays this way! Thank you for the all the help guys! You’re all the very best!!

  53. I took some photos of how i fixed my laptop. hope this will help anyone else who might run into this problem.

  54. I bought a used Dell C640 not working condition with no hard drive When I turn it on (using the AC adapter)it goes on for seconds and then shuts off I notice the chassis fan doesn’t spin but then when would it since the unit doesn’t stay powered up long enough to make a judgement re the fan So where should I start?

  55. Jonathan Evans

    I’ve read all the problems people have been having, and many of them look like the problem I’ve had with my Dell Latitude D600. Just like “Laptop Freak” and “Steve,” what I found was if I roll up some cotton gauze about the length of the memory stick and put it on top of the stick, then screw down the hatch, the pressure anchors the ram stick in place and prevents the weird random shutdowns. It was clearly a loose connection between the memory slot and the mobo: when it was working, if I simply tilted the laptop over onto one side or the other, it froze immediately. Now I can move it all around upside down, sideways, etc., and no freezeup. I imagine a good laptop or mobo repair technician could identify where the loose connection is–probably a solder joint has come apart–and resolder it solid again.


  56. I have a DELL C640 the same problems just keep shuts off
    but I’ve just found a simple experiment i use an external monitor and plug it to my C640 laptop and turn the laptop on the computer boots and runs great with the external monitor. This could be a good and quick check for the CPU, Fan and the memory but i am still working on WHY? the laptop works only when there is an external monitor plugged in to it and why not with it’s built LCD screen. Please if someone out there knows how to deal with this issue please POST


  57. I’ve got a C640 1.7GHz/512RAM. I don’t know its history since I got it used for free, but the thing works great and I leave it on all day. It does like to run its fan a lot more than my Compaq Evo N620c does, but it never dies unexpectedly. The one weird thing it does is that the touchpad will randomly freeze up on occasion (everything else is still fine).

    And to the person who asked about the amber/green flashing light, that generally indicates a battery failure.

  58. Hello
    Can you help me out please
    I want to remove the dust from the heatsink of a Dell Latitude C640 but I don’t know where it is located.
    Do you have to remove the keyboard and everything or is there a little door just for the heatsink 🙂
    I know I gotta ground myself and remove the battery before to operate but that’s it 🙂 lol
    I’ll try this because I have #M1004 Error Code.
    Thanks a lot for your answer

  59. Laptop Freak

    Here’s the service manual for your laptop. It looks like you can access the fan if you remove the keyboard, it’s not very clear. If not, you’ll have to remove the top cover.
    This service manual has step-by-step disassembly instructions.

  60. romanian

    I had the same sudden shut down symptom at my C640 laptop. I tried the “cheap” solution indicated in this forum, namely to increase the mechanical pressure over the memory modules by adding some thin rubber over the modules. The computer, which I was going to throw away, works fine since then. Thank you!

  61. I have the same sudden shut down symptom at my C640 laptop, i found it doenst matter what bay the ram is in, it does nt like both sticks. mine works perfect when only one stick is in, they are 2 matching 512mb that worked fine before now.

  62. I also have a c640. I noticed that during the boot up process it stayed on 4 a few seconds before the screen goes blank. After waiting about 5 seconds of the blank screen I hold (fn+f5)and the screen blinks. I would then go through a series of (fn+f5) then (fn+esc)for the screen to remain on.

  63. Laptop Freak


    I noticed that during the boot up process it stayed on 4 a few seconds before the screen goes blank.

    Take a closer look at the screen when it’s blank. Is it really blank or just very dark and you still can see a dim image? It’s possible that you have a problem with the inverter board.

  64. It’s possible that you have a problem with the inverter board.

    You are correct.. The screen has a faint image, hence the screen is very dark.Is the inverter board integrated or it can be purchased? I pulled the system apart and reassembled it…The problem is still occurring.

  65. It’s possible that you have a problem with the inverter board.

    You are correct.. The screen has a faint image, hence the screen is very dark.Is the inverter board integrated or it can be purchased and replace? I pulled the system apart and reassembled it…The problem is still occurring.

  66. I realize that if I adjust the brightness of the screen and set it to the dullest and turn the system on it stays on.. however if I was to turn up the brightness when its been operating it wound shut off..What could this mean?

  67. i have a c640….the small fan in the rear never comes on and the laptop runs very hot. Ive never experience the random shutdowns discussed here but can seem to make that small fan work. i did disassemble and check and there is not binding in the fan. is there a “switch” to enable that fan. i installed the fan controller and it only has setting for the large fan on the right rear.

    Thank you,

  68. stephenz

    i have a dell c640 thats does not charge the battery but runs fine from both the bios and the windows…please can any one tell me the solution to this..thanx

  69. Cody D.

    I have a few problems with my C640 too but none of your problems.

    1)Battery won’t charge at all and they are known good batteries. Whenever I insert a battery in the laptop it just shuts off. When a battery is in it and try to start it, it won’t start. Charge light stays on for about 5 seconds. The CD-ROM works fine in either port though so it just shuts off when a battery is inserted. Used 3 different good power blocks.

    2)Power port has loose connection as whenever I slightly touch the cord it just shuts off. Is it possible to resolder or should a laptop repair guy so it? Are the refurbished c640 motherboards on ebay as close to new I can get without spending $400 for one from dell?

  70. Hi!

    I have a problem with Dell Latitude C640 as well. After reading this whole forum I believe that the thing happening to the laptop is the pre-phase of overheating problem.

    The problem looks as follows:
    When I load the processor heavily for a longer period of time (around 10 minutes) the fan starts to work at maximum speed, emits a lot of noise and that is normal. The problem is that when the processor is idling after doing some effort, the fan still rotates at the max speed until I shut down the system. The Fn-z combination slows it down only for 20 seconds.

    The HDD temperature is between 40-56C and the motherboard sensor shows no more than 52C.

    The worst thing is that once the fan starts to spin at max – the computer’s performance goes down dramatically. After rebooting everything is fine… until the fan spins up…

    I would be greatful for any advice.

  71. Conket

    My c640 is having problem with shutdown and restart, after that that the time it get up is shorter and stop. I narrowed it to the fan, when you power it up the fan is not working at all, or after that. I took everything off bare, right before I take the fan out I thought let try to power it up one last time, the fan is working. I said to myself that is weird, so I put the heat sink back. Power it up, the fan is not working. Removed the heat sink, power it up, the fan works. Could someone tell me what kind the problem I have? Is it circuit on the mb bad?
    Thank in advance!

  72. tusja

    Hi,my C640 has a problem with some keynotes: b,n and the spacebar. It’s so uneasy to write..Could someone please help me…Or tell me whether it can be solved or not?

  73. Laptop Freak

    If only some of the keys stopped working then most likely you have a keyboard related problem. You’ll have to replace the keyboard OR use the laptop with an external USB keyboard.

    This also could be related to the keyboard controller failure, witch means you’ll have to replace the whole motherboard, but it’s not a common failure. You cannot tell witch one the keyboard or the motherboard is causing the problem until you test the laptop with a known good keyboard.

  74. Richard

    This is such an awsome thread. I just got my C640 last week and everything runs perfect except for a few things. It came with a 512mb stick installed in jdim2 which seemed odd. I added a 256 stick to jdim1 and got memory read errors. Ok so I assumed the ram I had laying around was bad. A few days later I moved my 512 stick to jdim1 and id either get caps blinking or memory errors. So now I knew it wasn’t the ram that was bad and said “ohh well, I’ll just live with 2ghz, 160 HDD and 512mb ram” untill I read this thread. I tried the pressure on ram method and NOW JDIM1 WORKS!!!! I’m typing this message on the c640 by the way

    The second weird thing is, not a problem but just a weird thing. Someone else mentioned this earlier in this post. When I go to turn on the laptop at any time, 95% if the time it won’t turn on the first time I press the power button. Instead the battery light blinks “Amber Green Green Green Amber” but as soon as I hit power again, it powers right up. The battery I was told by the seller is brand spanking new and I can squeeze over 4 hours out of this battery if not doing anything intensive. The battery charges just fine and it discharges just fine. I just don’t understand the blinky thingy and why it does it.

    Other than that, the laptop is perfect and in new condition. I just ordered a Intel Wireless b/g mini-pci card for dells since the stock 1150 series I have sucks in range, speed, and encryption support. It can only do WEP..eww.. Anyway, any thoughts? And I am another one to confirm that the “pressure on ram” method works!!!

  75. Scott

    I had intermittent shutdown and RAM problems for my Dell C640 similar to those reported in this thread. Adding pressure to the RAM using some thin slices of a foam block has resolved the problem. Thanks!

  76. Daljits

    Guy’s I’ve solved the problem on the DEll C640. After hours and hours of research on the net I concluded to open up the box of electronic and do some investigating.

    I found the problem There is an electronic component called u25 (It’s number is M5236 -3.3BM) it’s a voltage regulator IC manufactured by mitsubishi electric. It is used in tandem with another adjacent to it (on the under-side of the motherboard).

    The chip gets hot and burns out causing a short circuit. I have managed to find a replacement chip on the net (ebay of all places) and I’m waiting to fit it.

    Now be careful, use a solder braid. don’t use a wacky desoldering tool!!! because you’ll burn out the pcb and the copper tracks will lift off.

    The braid sucks up all the solder leaving a very thin layer to tack the component. Next, get a tooth pick ot fine jewellers screw driver and prise-up each leg 1 by one.

    When it comes of, replace it with the new chip. In all honesty, It might cook again until I find the fet or transister that is causing this overload.

    I’ll let you guy’s know how I get on (today is 21/6/08) thanks..any questions email me on

  77. Rupert

    I have same problem with voltage regulator chip and am trying to salvage one from old boards that a dealer gave me. Chip sits under VGA socket. Wish I could find new

  78. Rupert

    The real number is MIC5236-3.3BM manufactured by Micrel.
    I am going to remove chip and place it in test circuit.
    Datasheet is available.I suspect a short circuit downstream. Anyone got schamatic for C640

  79. Rupert

    Problem was not MIC5236 but transistor further down the line. Look For U45 short circuit 70mm due north.

  80. I use my C640 Dell laptop to try cloths – while i am using it. it gets very hot at the bottom
    laptop’s bottom 😉 !

    and every time i power on – it asks me to go to setup with a message that setup is wrong.
    Whats wrong with this fellow?
    Does a laptop uses its big battery to provide power to CMOS settings? (like the small penny size batteries do for the motherboard of a pc?)

    another thing i have noticed that, if I switch off the power – sometimes the battery power won’t work, it just switches off the laptop.

    any help appreciated.

  81. Laptop Freak


    and every time i power on – it asks me to go to setup with a message that setup is wrong. Whats wrong with this fellow?

    Apparently, the CMOS battery is bad and has to be replaced.

    it gets very hot at the bottom laptop’s bottom

    Clean the heatsink with compressed air, I think it’s clogged with dust.

  82. I have a c640 with identical problem as Conket described on 5/1/08. With heat sink attached fan does not come on (even at startup). Remove it and fan works. Does not come on even if heat sink is in place but not screwed down. Must be a short, but don’t see anything obvious. I could test likely components but it overheats too quickly. Perhaps supply external air to keep pc going, and then poke around for for shorts. Any suggestions would be most appreciated.

  83. I have the same problem as Jimbo. With the heat sink on the fan will not come on, but if you take it off or even unscrew it it will come on. You can even unscrew it with it running and watch it come on and then screw it down and watch it turn off. IF anyone has a fix let me know please. It is driving me crazy.

  84. I actually tested my laptop and noticed that it has the same issue that Ben (July 14th, 2008 at 8:14 am) is having. There’s something with putting that screw in that doesn’t make any sense to me.

  85. If 4 of us on this site alone have the same symptoms, it was probably a significant defect with the c640’s and perhaps someone out there found a fix or workaround. One possibility that might cut the fan off when tightening down the heat sink is that the added pressure might be causing an short or open circuit in one or more of the socket to board connections. None of my module pins looked bent, and I don’t have a spare, but I did reseat it several times as a test. I suppose we could prevent overheating by using another 5V power source for the fan, but then it would run at full power all of the time and kill the battery in short order- but I guess that would at least retire the laptop to a desktop instead of leaving it for dead.

  86. I have an compaq 2500 laptop, everytime i try to install windows updates or do any high activity my monitor will go black with a faint image and the power light is still on and so is the battery light but the hard drive light shuts off and the computer is locked up. If i hook it up to a external monitor it works fine.

  87. My ABS Mayhem G3 has been doing something similar. I fixed one problem where my CMOS battery wasn’t making contact with another piece, so that fixed the weird freezing up and flickering, but now my laptop won’t run on AC power. It works on batter power well enough, just a little slower and not nearly as long. It charges normally, too. It runs on AC power in bios and in safe mode, and I’ve reinstalled Windows XP pro 3 times so far, so I think it might be my hard drive. I would just like to know if there is anything I can do with Windows to get it to like the big plug in the wall. If they could make up and just be friends then we could all get along just fine. (whoa, those hippies are starting to get me) I tried looking at msconfig, but I don’t really understand most of it. I would try using a different hard drive if I had one; I might have to sooner or later. If you have any ideas, please let me know. Thanks!

  88. GerryP

    I have the same problem as Jerry.
    My D640 had been running fine but the last time I used it the battery was powering the laptop, even though the AC cord was plugged into it. I noticed the green light on the power transformer was not lit and suspected a bad unit. I replaced it and same thing…as soon as I pluged the AC cord into the laptop the green light on the AC transformer went out. The d640 runs fine on battery… Does anybody know how to correct this AC power issue?

    Jerry Says:
    May 28th, 2007 at 7:20 am
    I have a similar issue with my Dell C640, but slightly different. My laptop works fine on Battery power but it will not charge or run on AC power. I have dis-assembled the laptop, cleaned the board with canned air(attached my electro static discharge cable too). I re-assembled the laptop and plugged it in, it still does not recognize the Power Supply. I have purchased 3 brand new power supplies, and tested them on another Dell laptop( I love Dell). I still cannot get the laptop to run or charge on AC. Can anyone help me out here? Thanks.

    aka Vampnite69

  89. Richard

    Those of you that mention the fan “problem.” From what you mention it doesnt sound like a problem at all. My laptop works perfectly, and when I power up my laptop my fan doesn’t spin. It shouldn’t have to. My fan only starts spinning when the cpu starts to get warm. The same EXACT thing would happen if you were to take off the radiator fan on a car. You guys say that the fan starts spinning when you take off the fan. Of course, because since you have the fan off, the cpu is getting hotter quickly, therefore the fan kicks on.

    Power up the laptop with the fan in place, go into Windows, and try doing something cpu intensive and see if it spins. Install a cpu temp monitor program (such as I8kfanGUI)and monitor the cpu temps. I always keep it open on my taskbar next to the clock. If it starts hitting 60 degrees celcius and the fan is still not spinning slowly, then yes you do have a problem.

    I understand that some laptops upon first turning it on, does a FULL fan speed test, but of the 3 C640’s I have, none do that. Only my Toshiba and Gateway do it. I also own a Dell C610 and that one also doesn’t do the full speed fan test upon powering up the laptop.

  90. MikeonTV

    I fixed it guys. I stored this computer on the shelf for a year and now thought I would try to fix it. I did it…

    ….All I did was remove the back door that is there for adding more RAM. On the rubber strips that are on the door I folded up a piece of paper and taped it on the strips (making it thicker and thus press on the motherboard harder) and now the machine has been running an application for over three hours.

    see this and this for a (bad) image of what I did

  91. Rupert…..Thanks for saving me some time on troubleshooting the C640 with shutdown problem. The U45 was shorted to 60 ohms and freeze spray would make the unit work. The only number on the part is S60. Do you have a part number or know what it is? This is an IC and not a transistor and I suspect it is a regulator but not sure. Any help appreciated and thanks again for the time saving tip. Please leave comment here or email me ……..Thanks again

  92. Lau See Ping

    I had a c840 that would freeze after about 10 minutes even with the fans on manaul control set to fast. After that it would not show the dell start up screen and would shut down. It would need to cool down for an hour or so to get it going. I dismantled it and put it back together with cleaned out fans and heatsink, more thermal grease etc. It worked fine for about an hour and then the same thing. I tried the taking the battery out, removing the power and holding the button for 10 seconds which would allow it to post but it would lock up soon after (2 minutes or less). So I tried the ram. Already I had only one slot working as the other does not recognise any ram that I put in it. I ran the memory testing software from a linux distro which crashed. Since swapping the ram (from 512mb to 256mb) I have had no crashes and the mem tester ran for an hour ok with no problems. I will now go and purchase an other 512mb ram stick to replace my 256mb as it is a bit slow and makes games laggy. I hope this helps others.

  93. OK – a number of us appear to have a slightly differnt problem than most. The c640 runs perfectly off of a charged battery, but the laptop doesn’t get any power from a known good ac adapter. Would it be worth the time and money to have a pro put a new power inlet in or is it more likely a problem on the motherboard?

  94. Youngblood

    VERY interesting & informative thread!

    My 2GHz C640 started flaking on me about a month ago … the Time/Date problem along with the Sudden Cutoffs followed by No Starts.

    It has lived most of its life in a C/Dock getting turned on, primarily, for file synchro, OS updates and backups.

    My month-long testing has indicated to me that the issue is directly related to the AC-to-DC circuitry on the mobo.

    My C640 runs fine on battery and the mobo circuitry charges my batteries while the unit is off and plugged into a PA-6 alone or thru a C/Port or C/Dock (PA-9).

    My C640 does not operate reliably for any length of time on AC … and the way(s) that it fails after the initial abrupt cutoff, strongly suggest that heat is a factor.

    When I started this troubleshooting journey, I installed SpeedFan and was SHOCKED at the temperatures inside the laptop and the fact that the laptop’s fans were not kicked into HIGH because of it. I quickly searched the web, discovered I8KFANGUI and have been using it ever since … with the fans forced to highspeed.

    Also, during that period (first thing, actually) I replaced the CMOS Battery Pack and, while I was rooting around in there, cleaned up the primary fan + path and the heatsink … and upgraded the CPU to 2.4GHz.

    The two 512MB SO-DIMMs are also new.

    I will try the addtnl-pressure-on-the-SO-DIMMS trick, but I seriously doubt that this will solve the issue with my C640. In my case, everything seems to point to one or more mobo AC power supply components having become heat-sensitive and flaky.

    I am hoping that someone will be able to naildown the culprit and post about it so that I(we) can changeout the one or two board components and cure our laptops’ ilness.

    Another reason that I am concerned about this issue is that I recently bought a partially stripped C640 and have been rebuilding it into a fully functioning laptop for my sister & her family (as a surprise … I have accumulated a lot of C-series accessories/components over the years).

  95. Youngblood

    “I will try the addtnl-pressure-on-the-SO-DIMMS trick …”

    Goodness! In my case THAT turned out to be a Bad Idea.

    I cut one of my old business card in thirds, lengthwise, and, after trimming them to length, installed them under the access panel over the RAM sockets. I replaced the panel, tightened the captive screws and installed the center keyboard screw.

    The system cutoff in mid-BOOT and would not start. I removed the center keyboard screw (which proved itself to be an Item Of Interest awhile ago in my troubleshooting) and the system almost got into XP before cutting off. It was behaving like it does on AC power even though it was on battery power.

    I removed the pieces of cardstock, replaced the panel but not the KB screw and the C640 still presented with the No Start syndrome (even after removing & replacing the battery) …

    … so I pulled the PCMCIA covers and directed a fan-blown steam of air into that area to cool off the components in the AC power supply area.

    That was about an hour ago and now my laptop is back to where it was, running fine on battery power.

    Apparently, my mobo does not like to be flexed like that … and I will have to look closer at that center KB screw.

    BTW, one thing that I failed to mention in my initial post is that my BIOS is version A10 and has been since 2004(?).

  96. Read my post above about U45. This is a small IC that looks like a 3 legged transistor. All of the posts above about pushing on board near memory are all related to the U45. Resistance would change when either pushing or freeze spraying the part. When AC is used the regulator is always on supplying 3.3V to U45. When battery is used voltage is on U45 only when on. This is why the regulator has a chance to cool down when AC is removed with the battery still installed. If the battery always works and the AC doesn’t then the AC power input posts probably have a cold solder joint. I have been unable to find out what that part U45 is from anybody. I got mine to work by removing U45 and putting a diode across the in and out and it has worked flawlessley for hours at a time with no shutdown. If anyone knows what that part actually is please let me know. Left side is ground, top 3.3V and right 2.9V. Taking it all apart is a pain. Hope this helps others.

  97. Youngblood

    Yes, before posting I read all of the posts, especially Daljits’, Rupert’s and yours, with great interest.

    I, like you (and probably many others), am hoping that someone will be able give us an ID on the U45 part so that we can source a replacement and then change it out.

    That failing, if your Diode Workaround continues to work, hopefully you will be able to provide more details to those of us that would like to repair our C640s.

    Currently, this is not a major issue for me since I have several other systems that I may use … but I would like to get it properly repaired, eventually, and thereby possess the knowledge & experience JIC my sister’s “new” laptop starts exhibiting like behavior.

    To you, Daljits and Rupert, THANK YOU for taking the time to share your discoveries with us!

  98. Another victim of the c640 Caps Lock flashes 10 times. I inhered the problem from someone how did not want it any more. I tried everything suggested and finally the answer. The memory does not seat right in the notebook. Cleaning the contacts did not work. I finally resolve it by adding constant pressure to the ram chips by using the spongy foam and double-sided sticky tape to the bottom access lid.
    Some pics that might help.

  99. longstand

    for all with the C640 SHUT OFF PROBLEM try this….

    when it starts to boot hold the function key (fn) at the bottom left of the keys beside the lett alt key and press the down arrow a couple times. (fn+down arrow)

    For the more technical persons press f2 after turning on the system while it boots. follow the instructions to say press (alt+P) to change the pages till you reach the brightness settings when power is plugged in and when on battery. adjust and see what happens!!!!

  100. Youngblood

    Thanks for the tip longstand, but I have tried that several times, now, and my C640 still still treats me to Sudden Cutoffs.

    Hopefully, it will work for some others, though.

  101. Youngblood

    FWIW, after giving up completely on my laptop ever managing to boot-up with either the PA-6 or PA-9 power supply, I decided to try a lower wattage PA-2 unit (I figured that it might be easier on whatever PS component is flaking) … and I have had quite a bit of success, although once the C640 cops an attitude(about every other day), neither the battery nor the PA-2 succeeds.

  102. Hello John,
    I found on an working motherboard the U45 replaced with a different marking code FCAA which means Maxim MAX6326 CPU reset IC with 2.93V. I hope this will help.


  103. Thanks MadMax…I was almost 100% sure that was a reset for one of the microprcessor in one of the chips. With the IC out of the circuit the unit would not come on but momentary touching with the diode across the in and out would allow the unit to run. Once it was cut off this had to be done again so the temp fix I did was to leave the diode in the circuit. Reset circuits leave the voltage their so I figured it did no harm to leave the diode. I have operated a tv repair service for over 26 years so I have dealt a lot with having to redesign and make circuits work. This is great knowing what the IC actually is so I can replace it properly. Looking this up it is a very low current device @1ua. I think I will try to put something bigger in their so it will last. I searched and called numerous dell motherboard repair centers and none of them went that intense into board repair. This forum solved the problem and I appreciate the time saving advice. Thanks again to all.

  104. Youngblood


    Once you find a source that will sell QTY: 1 or 2 or 3 of the part upon which you decide, would you please share the details with us?


  105. The part I actually ordered is a Fairchild FM809SS3X…If you look at the datasheet this has a reset threshold of 2.93V. The part is $.42 each so I ordered 7 of these as spares and the total was less than $9. The URL is….
    The part is in South Carolina. What I did was run 2 wires from the solder tabs to locate the part between the memory so I would not have to dismantle if this goes out again. The ground I used a spot on the chassis near the shield for the left leg. The Newark part # is 58K1509…I put this in today and works fine. Hope this helps others.

  106. Youngblood

    Many thanks for the info, John!

    For those of you wondering about the actual location of U45 on the mobo, here is a pic:

    It is the little 3-contact IC between the “large” PCIbus & SMSC ICs.

  107. John thank you for the info!

    Can you put a picture of the setup you made.


  108. Thanks to Madmax, rupert and John for all the work on this; it can rather hard (to say the least) to figure out an intermittent failure like this. Maybe I can fix my brothers Latitude now. Methinks I’m gonna need a darn small soldering iron tip.

    John, I guess you ordered the Fairchild because it was a) available as a compatible part (active low reset at 2.93V threshold) and b) a lot cheaper even though it used twice the supply current (2uA – don’t think that’ll be an issue).

    I don’t think I’ll be able to source the Maxim chip here in Ireland anyway (assuming they have the 2.93V version out of the 21 versions of that chip) so I’ll go look for that fairchild now!
    Thanks guys…

  109. Hi all,

    Great thread.

    I’ve been given a C640 to repair and upgrade. After upgrading the RAM, to my horror, I started having random reboot problems.

    After reading this thread, I can confirm that the putting several bits of business card on top of the RAM to apply pressure works 🙂



  110. Hi All,

    I don’t think that it is demanded to build in the Maxim IC. There are a lot of similar parts on the market. It is simply a 3V3 voltage supervisory IC that has a typical theshold of 2.93V. For Europe the easiest way would be RS Components having TI ICs on stock packed to 10 for about 3 Euros. For US I think digikey would be the Source of choice.


  111. Hi,
    yesterday I picked up a not working DELL C640. No power up with a full charged battery and if I plug in the power adaptor the led in the power adaptor goes out. Maybe a short circuit??
    In the night I dismantle the complete notebook and found U45 on the motherboard.
    Today I try to replace it with a diode. Report follows 🙂
    Touch wood!
    Thomas (Germany)

  112. Hi, I am from Argentina and I have the same problem. I found the datasheet for the U45 and it is a simple transistor PNP with an 10k resistor in the base. I don’t have a replacement for that transistor so I put a 2N2907 with a 10k resistor in the base. Now the PC works great and don’t shutdown any more so thanks for the help and I hope this help. Sorry my english. Saludos.

  113. I,also, would like to thank Madmax, Rupert and John for all the work on this.

    I had the shutdown problem. I saw this thread, ordered 5ea of the Fairchild IC’s (under $2.50 delivered).
    I disassembled my motherboard, took one look at the component. No way I have that kind of skill but I have a telephone. Made 2 calls and found a computer repair person who said he’d do it. He charged $25.00!!! What a deal. I’m writing this on my C640 which has been on, continuously for 36 hours now. All for less than 28 bucks!!! I cannot thank you enough!!!

  114. Hi, I’ve been having a similar problem with my mom’s c640 but it will freeze in the bios and when I boot it up the Dell logo is about 1/4 it’s normal size.

    My mom has gotten a replacement after we decided that it wasn’t worth the time fixing but about a week ago I took another look. I asked one of my Linux buddy’s and he told me to unplug the bios battery but to no avail. I don’t know what’s wrong with it but I put new thermal grease on it and that did nothing. After I read this thread (btw very interesting) I think it has to be something with the mobo.

    I have been able to boot into windows but it wont boot to livecd.

  115. Follow up of my post on July 15th, 2008 at 11:01 am:

    I went ahead and ordered the part from Newark as stated above. I just changed the thing and put it back together and VOILA!!! This thing has been up for about a half hour (WHICH IS WAY LONGER THAN EVER BEFORE) and still going strong.

    For me, the actual replacing of the part wasn’t too difficult, it was actually taking the thing apart. You pretty much have to take the laptop apart COMPLETELY! Even with the service manual it was kind of a pain. Anyway, this fix works well. THANK YOU AGAIN!!!

  116. Emiliano

    The U45 transistor is FJX4010R from Fairchild. It’s very similar to 2N2907 with a little diference, the 10k resistor. I put this on my DELL and it works fine. Probe this if you don’t have the same transistor. Don’t forget the resistor.

  117. I would like to thank all those who have been involved in the diagnosis of this problem which I know would have been many hours of work, I have replace U45 transistor and the laptop is working great.


    Phil K

  118. Steve C

    Hi. Big respect to the chaps who sorted out the u45 problem.
    Can I run this little “feature” of my pair of c640’s past you all and ask for comments? I used to get surplus latitudes from a well known software company, by way of a clearance, and kitted out my G/F with a pair of c610’s some time ago, runnning at just over a Ghz.
    When I got some c640’s running 2.2Ghz processors I expected a huge speed increase.. Not only didn’t I get that, but I started getting complaints until I eventually reinstated her old laptops. I’ve been using this c640 for a while now, and most times it’s fine, but every so often it just slows to a crawl for no discernable reason. I got a mate (who should know, he used to fix laptops all day long) says that p4’s are just plain rubbish compared to p3’s. Anyone have a similar experience to me? Cheers, Steve C.

  119. I have a Dell C640 that will not run for long on ac only, generally not complete boot up, with the battery in place and no ac it appears to work ok, with battery and ac it will run but the LCD screen intermittantly darkens or goes blank the back to normal brightness. When I have the ‘power meter’ displayed it shows ac much of the time but switching to battery and back to ac. I suspect a dry joint on the ac input plug, wiggling the adapter cable has no effect, and have followed the Dell instructions as far as removing the system board, however the actual mother board has metal plates (covers) top and bottom and as far as I can see both need to be removed to get at the plug’s solder terminals.
    There appear to be many screws and nuts to remove and some plates seem spot welded.
    Is there any guide as to which to remove and what to leave alone? From the many entries above working at component level several people have dismantled that far.
    Any advice gratefully received.
    regards Mike

  120. Kamran Brohi


    I am going to apply the grease on the heat sink but I do not know exactly where to apply it. Should I apply it on the bottom of the whole heat sink or just over the rectanglular area in the middle which is placed on the CPU. One more thing I don’t understand while cleaning the heatsink is that do I have to completely remove the black layer below the heatsink or just wipe the grease in the rectangular area in the middle? If anyone has an answer with the picture, it would be much helpful for me.

    Thanks in advance.

  121. Emiliano

    Mike, this can be the power ac wire is cut somewhere. I had a similar problem with my ac adapters and it was cut somewhere. I was able to repair this. Test the wire and look very well because it is hard to see. If you can’t understand what I’m saying is because I don’t speak english very well. Regards.

  122. Kamran Brohi

    Dear John,

    Can you please tell me which soldering device you used for dismentaling the u45 IC?

    Thanks in advance.

  123. Mick Bush

    Hi all you have done a good job finding the route cause of this problem but I am finding it very hard finding a component supplyer in the uk who stocks them can anyone help me.

    It would be cool getting the c640 up and running again.

  124. Emiliano..
    Many thanks for your interest and suggestion, I have checked the cables on the ac adapter and they appear good. I still have’nt taken off the tinware from the motherboard to investigate further.
    Regards Mike

  125. There seem to be a couple of different issues in this thread, so I’m having trouble making sure I’m following it…

    I have a C640 that won’t start up: no apparent response to pushing the power button. Should I be checking the U45 device? Can I check it in place on a powered down board with a standard multi-meter, and if so what am I looking for in a good and bad device?

    I’m happy stripping the board down (I’ve previously soldered a new USB port onto another C640). Assuming I determine the device is bad is this an equivalent part (from RS in the UK):

    or perhaps this:

    Many thanks for any help on this.

  126. To Mike K…

    Nothing is actually spot welded to the board: a couple of the tin bits are tacked to each other but everything comes away in the end.

    I’ve stripped these boards a couple of times, and everything has gone back together neatly. The fiddliest bit is a few miniture cable connectors for things like the loudspeakers. Remember to reattach these before putting al the screws in…

    I don’t have a formal stripdown guide, but the tools I use are the largest cross-head from a set of jewellers’ screwdrivers, plus a ‘fishing tackle’ type box that has a number of shallow indentations in the top. Work methodically and put each set of screws in a separate tray and it helps you keep track of putting it back together (and if you seme to have a screw spare at some point).

    IIRC, the main order of work is:

    1) Remove cover to top rear over switches and graphics board.
    2) Remove HDD caddy housing and PCMCIA slots.
    3) Loosen vertical trim around front/battery connectors.
    4) Remove remaining screws and detach wiring connectors.


  127. lau See Ping

    To Mike Bush:
    I live in Australia and I ordered the part from off the web and they delivered very cheeply and quickly. I would recommend purchasing from them. Now I have to find someone to solder it on for me with the resistor. Again if someone has a pic of how they fitted the resistor it would fantastic and greatly appreciated.

  128. To Ian C
    Many thanks for your info.
    I eventually chickened out and put it all together again.
    I had reached the point of having only one large tin plate to remove (this has riveted to it a small section that surrounds the power plug) but there appeared to be many more screws to remove that had their heads under other small circuit boards, so I felt that I was adventuring too far into the unknown, and with lots of little heaps of screws and sections carefully set out on the furniture I gave up.
    However the frequent flashing of the screen means that I will have to have another go at it, this time as you suggest with a nice little compartmented box.
    Thanks again Mike K

  129. Hi,

    Thank you all for sharing your knowledge here.
    My C640 also had that shutdown syndrome.
    I`ve replaced the U45 (S60) with a U67 (FCAA) from a Latitude C600 as shown here before.
    Now the notbook is running again since hours without any shutdowns so the problem seems to be fixed!

    Regards Cab

  130. Here are my notes for complete disassembly of a C640 to bare motherboard. They’re written in a pretty terse style, but will hopefully be an aid to disassembly, and more importantly reassembly.

    The first section gets as far as Dell’s own instructions for replacing the mobo that can be found in the online C640 service manual. Uisng that guide with its pictures may be helpful.

    Case legend
    [K] means ‘Screws labelled K on plastic/stickers’, etc.

    Screw legend
    (K) : M2.5 x 15
    (P) : M2 x 3
    (D) : M2.5 x 5
    (Q) : M3 x 5
    (R) : M2 x 16
    (S) : M3 x 3

    Remove motherboard assembly
    Keyboard screws [K]: 5x(K) on base (4 if memory cover not in place)
    Keyboard, unplug ribbon cable
    Filler/button strip
    Screen ribbon cable screws: 4x(P), screen cable plug
    Display assembly screws [D]: 5x(D) on rear
    Palm rest screws [P]: base 3x(K), drive bay 2x(P), top rear edge 2x(P), riser near heatsink (P)
    Palm-rest ribbon cable & assemmbly
    Base screws [B]: internal (D), fan surround 3x(D), base 6x(D)
    Speaker assemblies: push-fit
    Mic/headphone sockt fitting: ease case around to avoid damage
    Motherboard assembly: friction / moulding fit

    Remove PCB from assembly
    CMOS battery cable
    Button/GPU shield screws: 5x(P), NB: two holes empty for securing display cable
    GPU ribbon cable: slide retention tabs
    GPU assembly: push-fit to motherboard
    Front guard screws: 3x(P)
    Speaker/mic r/h assembly: 2 x m/b plug
    Speaker l/h assembly: 1 x m/b plug
    Dive cage: 3x(Q), PCMCIA cage (R)
    Rear trim: 3x(P), PCMCIA cage 2x(R)
    PCMCIA cage: (R), detach cage
    CPU heatsink
    Fan cable: m/b plug at 2 locations
    Hex stand-off: silver @ JTV1, bronze @ C56
    Rear port connector retention hex bosses
    Lower metal assembly
    Modem cable: 2 connectors @ base

    Test PCB
    Attach video/button assy
    Attach LED assy
    Use stand-offs to attach heatsink
    Put battery/power in place

    Drive caddy
    Caddy retention: (Q)
    Disk attachment: 2x(S)
    NB: If (S) is unavailable then (Q) is compatiable with many drives

  131. Hi Guys;

    I’m trying to fix a Dell C640 (that I bought off eBay) that turns on, then off almost immediately.

    The longest it stays on is about 15 seconds or so.

    When it stays on for 10-15 seconds I get the msg that the time is not set, try to enter the BIOS, then before I can save any changes the laptop shuts off.

    The battery has a charge and I’ve tried two other batteries — but it won’t run for 6-10 seconds on battery. Once it shuts down, I’ve got to let it set for several minutes to get it to start again.

    When I plug the AC adaptor in (I’ve tried two that I know are good) the AC light on the laptop sometimes comes on and then goes off — but most of the time the AC light doesn’t even come on.

    I read on Google about the C540 having this problem when the CMOS battery dies — not sure if this is true for a C640 — but I replaced the CMOS battery with one from another C640 that does work — no joy.

    I also read about the power jack. I took the laptop apart and re-soldered the pins on the MB — no joy.

    I am thinking I need to replace u45 and am wondering if someone can point it out in this photo (my eyesight ain’t so good these days):

    Oh… and if anyone has any other advice, I would appreciate it.

    If I get this thing fixed, I definitely let everyone know what the problem was and what it took to fix it.


  132. Hi Charles

    In that picture you need to be looking in the lower quarter, and about central left to right.

    Near the bottom of the image there are two large ICs. To the left is an SMC device, and to the right a TI device. Dead right of the side-on ‘M’ from SMC is a small device in a white rectangle. This is the U45 device. If you look really closely you can just make out the FCAA printing on the top.

    There’s another picture about halfway down this Polish forum thread showing a 10k resistor wired across the damaged deveice, which apparently acts as a sort of fix:


  133. Hi IanC;

    Thank you for the reply and info — I see it now.

    From my description of the problem and what I’ve already done, do you think this is the component that needs to be replaced?

    BTW — does anyone have a schematic and/or troubleshooting manul for a Dell C640?


  134. Youngblood

    “For those of you wondering about the actual location of U45 on the mobo, here is a pic:

    It is the little 3-contact IC between the “large” PCIbus & SMSC ICs.”

  135. Lau See Ping

    For C840 owners,
    I have been following this thread because I have a Lattitude C840 which has similar problems to everyone elses C640s. I tore it down to find the U45 and found an empty location near the battery/floppy connection with U45 printed near it. So it looks like for C840’s there is a different IC that might need replacing.

  136. For Lau See Ping…

    On both the C600 and C640 the device that fails is just above the ‘M’ of a square package with SMsC written on it that is about 15mm square, as seen in the picture that is linked to above.

    If you can see an SMSC package on the C840 then take a look in the same place. On the C640 this is U45, on the C600 it is U67.


  137. To clarify my last post…

    The SMSC and U67/U45 device are in the same position relative to each other on the C600 and C640, but they appear in somewhat different locations on the motherboards for the two models. Finding the large package on a ‘new’ type of mobo hopefully makes it easier to locate the smaller one.

    BTW, just actually fixed a C640 by taking the U67 from a scrap C600 mobo to replace a presumably duff U45. Fiddliest bit of soldering I’ve ever done, but a pleasing result. Thanks to everyone who provided the info.


  138. Hi Everyone;

    First… let me thank everyone for their help and replies. But… I could use some additional help!!

    I 10 of the FM809SS3X devices from Newark and have replaced the device. (Man that’s one little sucker — hard to see, hard to get a soldering iron on each little pin!!)

    Anyway… now the laptop does nothing — no green led when powered on from battery, nothing when I plug the AC adapter in.

    I am wondering if I overheated the FM809SS3X when soldering the new one in place (I have spares to try again if need be).

    Are there some voltages I can check to make sure I did the job right?

    Does anyone have a schematic or service manual to assist in repairing this thing?


  139. Lau See Ping

    I am getting good at tearing the C840 down to the mobo. I stripped back the black sticky to get to the SMSC chip and had a look around for a three pronged IC Chip. There is one directly diagonal NE (towards the back of the mobo) with U28 with 2 prongs on the north side and one in the middle on the south.
    Here are some pics

    Ok now I know the part I am going to take it to get soldered. One thing I am unsure of is if it needs a resistor. I found a Polish forum with a pic that I have stolen for everyone. Sorry for stealing your pic.
    Cheers all.

  140. AIUI that resistor is an alternate way of getting the machine to work, but it creates some sort of slow startup/switch off issue.

    If you replace the U28 device (and it was faulty) you should be OK without the resistor.


  141. Hi,

    Can anybody help? My Dell C640 seems to be short circuiting. Please see photo below.

    When I plug the AC adapter in this component gets *very* hot to the touch and emits a small ticking sound. The LED on the power brick also flashes on and off. The laptop will also not run from a fully charged battery.

    Here’s what I think is written on top of the chip,

    F P238L

    The 8 could be a B however.

    I’m not sure if it’s related to the U45 component fault or not.

    Please advise.


  142. I guess that’d be one of these:

    Sounds like a different problem to the U45 (both symptoms and component), but perhaps worth trying to replace it.


  143. Thank you everyone for you help and thanks to you I’ve fixed my lappy !!!!!!!!!!!! Dell C640 -U45 shut down after 2 mins problem.

    I tracked down some of the FM809SS3X devices, they are hard to see & hard to get a soldering iron on each little pin.

    I’ve never soldered before so I looked up a few you tube clips on the technique, practised on some old bit of wire, stripped the lappy ( struggled a bit but it was not working so what the hell )

    Anyway now been running it for two spells of 5 hours each and its great. I think I was a bit lucky as I the solder didn’t look text book. ( I used a magnifying glass to see thins more clearly if it helps anyone)

    I’ve some FM809SS3X devices for sale but I don’t think you can give details on this site.

    If anyone is interested I could pop them on ebay. ( Mod is this ok )

    Scopio ( UK )

  144. Hello,
    Pressing memory method was working for me too.
    Thank you every body.

  145. Scopio i could do with a couple of those chips if the mods allow you can contact me by email @ bilko2001(at) change the (at) to @ obviously:)

  146. Does anyone know if this will do the job? or this as i can not get the fairchild ones in the uk.

  147. Lau See Ping

    I have had my C840 laptop soldered to replace the U28 chip which is NE of the SMC chip on the left side of the board, under the CD-ROM slot (a bit different to the C600 and C640). I am little bit happy to say that it seems slightly better. I reinstalled XP, but it still freezes, especially if bumped or knocked. DIMM A slot still doesnt work, but I have been using the RAM pressure trick to angle the stick so it sits in the slot differently. I have also tried cleaning the RAM dimms contacts with a pencil eraser (but not the mobo pins). This makes it seem a bit better (will run for longer without freezing up). I did find that running it with a 60W power supply instead of the 90W (hence a slower CPU clock speed) seemed to help, and battery seemed to be even better. I had it in a C-Docking station all night and day and it was still running, but when moved out of the dock it froze up. I think that the 6 year old, abused laptop is terminal and should be retired.

  148. chanclon

    So the C640 was freezing when using JDIM2 . I would get the blinking caps light and some HD noise but then would turn off. The very few times I actually got it to boot, it would simply freeze within a minute.

    I used a piece of formed plastic about 3/4 the size of ram chip. The formed bubble on the plastic put extra pressure down on the chip. It worked immediately. Hats off to HAD!

  149. I have a dell c640 laptop and was wondering does any one know the normal running temp that these computer should run at before it will crash or burn up components? Also i have had the same problems of having the computer crash all the time and i replaced the processor and now seems to be running good but still heats up but it wont crash.

  150. Hi there BDOG

    It is difficult to say what the normal runing temp is , because it all to do with CPU/dvd/gaphics usage , useually the fan cuts in at around the 50 to 60c , when you say heats up i am not sure what you mean , heats up to what temp !! .
    have you got fangui loaded !! , ( this is a programe to control the temp ) better than dells bios , if you have not e.mail me and i will sen you the programe .

    stanley.binns AT



  151. frankie

    i want to put a resistor on the (bad)u45..
    like the polish website..

    they say this is a “kind of fix”

    my question is:

    is this a permanent fix or do i have to take the c640 apart
    again after a while?

    i also order the 2N2907 for this board..

    is this 2N2907 a better solution?

    gr frankie

  152. Youngblood

    VERY tardy update …

    Changing out the U45 SMC successfully repaired my C640.


    Previous posts in this thread: 94,95,97,100,101,104,106 … I think that’s all. 😉

  153. frankie

    thanx for the reply..

    i just replaced the u45 with a c600 ic..

    i shall see what happens..i ll put my lappy together with no
    screws ect , just to see how long he will be in the air..

    does anybody know where i can buy the u45 FM809SS3X or equivalent in europe or the benelux? without the horiible expensive shipping costs?
    (newark asks 25 dollars for five ic’s)


  154. I have the same problem with C640s. Can you tell me how the C640 senses the lcd/lid is closed? Is it possible that the sensing mechanism can fail causing these symptoms?

  155. apache admiral

    l found out that usually when it switches off and on the problem will be the 3 power ic’s that work with the hissing noise from the inductors so it’s best you check those ic’s and replace them l’ll give you a picture later

  156. Ahmad Nabeel

    My Dell C640 is not battery charging i was allrady change Battery and charger but problem is not resolved. Please provide salutation.

  157. The problem is the DC Jack, it is not working properly. Dell used faulty DC Jacks on the C640, and they have been known to eventually fail to receive the current needed to charge the battery. I’ve recently purchased a new one on ebay, I will now have to Desolder the old one and install and solder in a new Dc jack. I’ve purchased new cords and batteries to no avail, after much research and finding out that DC Jack Replacement is a service offered all over the net, I knew what the problem was.

    Do a search on Yahoo: Dell latitude c640 dc jack repair (or copy and paste the links below..

  158. sarah kinley

    i have a similar problem, i have a dell microsoft windows xp. i don’t know anything about laptops so.. when i press the on button the windows loading thing comes up and once its done it goes black, i think the screen might be broken but i’m not sure,i was wondering is there anything i can do before i go to a shop?

  159. frankie68


    me again,

    i fixed several c640 with replacing th u45 chip..
    but now i don’t have a u45 chip anymore the fix another c640
    but i have here 3 motherboards from toshiba , (sp6100, tecra9100)
    is the u45 chip on these boards also?
    if yes, where are they located?
    and are there more maxim chips for 2,9 volts on a c640/c600/c610 board? or is it just one?

    thanx a lot for any reply!!


  160. frankie68

    i fixed another c640 with a maxim chip from a dell d600!!
    it’s located at place U20

    Under the black wide tape where the pcima socket is..
    it’s the same chip as the c640

  161. ali alazzawi

    in the first my languge is not very good
    so i will try to explane my problem as i can
    i have laptop dell latitude c600 and when i plugg the AC adaptar it will turn off and i will plugg the ac adaptar in another laptop it will work fine
    then i know the problem in my moatherboard so iwant to learn how i fine the short in my board

  162. frankie68

    hi ali,

    i had the same with a c640 board..
    as soon as i plugged the adapter the adapter goes off.
    so i soldered some things from another c640 mother board as a experiment and put it on the c640 ..
    at least he turned on ! but the short came back after two minutes..
    i replaced the 8-leg ic right behind the jack (on top)
    and one of the 8 8-leg ic’s on the very right side (close to
    the video connection)
    so the short should be somewhere there (i think)

  163. Flusensieb


    @112 Emiliano: Thank you for this information.

    I fixed a C640 in the same way Emiliano did.
    I replaced the damaged IC on position U45 on the board with a very small selfmade circuit board.
    I put a BC 807-16 SMD (similar to 2N2907) transistor with a 15K Ohm (SMD) resistor in the base on this selfmade circuit board.
    I used some wires to connect U45 and this small board.
    No it works great.
    Thanks for your help!

  164. frankie


    i scored a dell c810 quitte cheap ..
    he turns on and off after a few seconds..
    i opened the lap and i saw there a small powerboard at the front onder the touchpad
    might the small board be the problem or is it a fet to the cpu-power?


  165. Hi
    Other things can cause a laptop to turn on and off. Like the ram is going bad in it. I had one of those a few weeks ago. Tested the computer with pc check v6 disk and the ram failed.

    My question is that I am not sure on…Is if you have a latitude c640 and it isnt upgradeable to run windows 7 but you are running it witn only 768 mb ram and 100 gb hd. with its original motherboard and cpu…Would it slowly fry the motherboard because its running something that needs more ram then what the system can provide?

  166. so when the fan isnt moving, that means it’s an overheating problem? because when i blow in it , it goes, then stops….

  167. FIXED IT!!!!… YES…YES….YES!!!!!! …..and within 5minutes!
    It started like this: My Latitude C640 was working fine. I seldom used it except on vacations and it was kept in its case. However, I took it out and updated everything since it hadn’t been on for about 6 months when I was in Florida. I turned it on two days later and it wouldn’t boot. Sometimes for maybe 2-3 seconds up to a minute, maximum.
    I tried about everything and was ready to kiss it goodbye until I read a posting. The solution just seemed to simple but lo behold, it worked. To boot it, I pressed and held down the power button for 20 seconds and it booted for about a minute. I tried it again and continually held it until it completely booted up.
    It is working great now.