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Thread: p0641 fried PCM, please take a look at my tune so I don't do it again

  1. #1
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    p0641 fried PCM, please take a look at my tune so I don't do it again

    So I have been trying to figure out why the car is so hard to start, keep running, etc. This is a swap vehicle so I don't have a BCM to tell me i'm in reduced power mode, but after some research it appears the p0641 will cause the REP to kick in. Not sure if I fried the PCM or if I got hosed by the seller (bought it on a forum). I was getting 8v at the map gray wire, I tried switching the wires to two different 5v signal pin (a/c and evap) and they were putting out 6v. So I decided to try a new PCM.
    I'm looking for one of the resident experts to give me an early xmas gift and check out my tune to make sure i'm not going to fry the next pcm. I'm thinking I may have transferred data from a different operating system which may have fried it (again just reading on other posts). I have pulled data from all different tunes and places to get started.

    This is a 2004 silverado PCM and engine, I am running the DBW pedal with TAC from an 04 silverado as well.
    Not sure if it is necessary but here are a few details:
    Running 3 bar SD, no MAF, no narrowband o2 just ngk afx wideband
    Stock bottom, turbo cam/springs LQ4 with TBSS intake, ls7 silver tb with x link, IAT in intake behind throttle body
    s480, meth injection, 80# siemens, aeromotive afpr at 58 psi, eboost2, RX monster catch can
    Truck accessories, 145 amp with TONS of grounds

    Also I may have overlooked, but I always "write entire" and was wondering if I should only be using "write calibration"
    Thanks for your time, the support on here is amazing, I hope I can contribute one day
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  2. #2
    Advanced Tuner blownbluez06's Avatar
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    PM sent. PCM can be un-bricked. Set your MAF fail DTC's to something other than 3 or your vehicle will never run right.
    Hsquared racing engines RHS 427, Procharger F2, Moran Billet Atomizer injectors, Alky Control,Mast LS7 heads, Nitrous outlet kit,Tilton quad disc clutch, DSS shaft, RKT56 ZR1 trans, RPM Quaife diff. Built and tuned by yours truly.

  3. #3
    Senior Tuner Russ K's Avatar
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    P0641 is for a 5 volt reference circuit open, shorted to ground or voltage, not a fried PCM. There are other steps to check before replacing the PCM, first check the 5 volt reference circuit at the oil pressure sending unit with the key on. If over 5.2V, unplug the Map sensor. If still over 5.2V, then check either 5 volt reference circuits for a short to voltage (as they are bussed together within the PCM) If that checks out ok, then replace the PCM.

    And as mentioned, set your Maf codes to other than 3, as the PCM needs to see a Maf DTC to fall into SD mode. So with no Maf, the engine won't run.

    Russ Kemp

  4. #4
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    Thank russ I have unplugged sensors and the outputs from the pcm exceed 5v, 8.2 on the map signal, 6.2v for oil pressure, a/c and evap. This lead me to believing the computer was toast as the internal outputs were over 5v, I performed the readings at the pcm to confirm this is not a wiring issue, thanks again

  5. #5
    Advanced Tuner blownbluez06's Avatar
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    I didn't look up the code. That's good advice Russ. OP If I'm reading this correctly, you removed the pins on all your 5v supplies at the PCM? That's the way to properly isolate the circuits and prove them for sure.
    Hsquared racing engines RHS 427, Procharger F2, Moran Billet Atomizer injectors, Alky Control,Mast LS7 heads, Nitrous outlet kit,Tilton quad disc clutch, DSS shaft, RKT56 ZR1 trans, RPM Quaife diff. Built and tuned by yours truly.

  6. #6
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    Yes I ruled out sensors and wiring and verified the 5 volt signals on the blue connector were all putting out more than 6v

  7. #7
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    I am afraid I wrote something to the pcm which caused this and want to make sure before I load this tune on another pcm

  8. #8
    Advanced Tuner VodeAn's Avatar
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    I would go step-by-step and copy these values into a new PCM, checking them as you go (ask yourself, why did I make this change?), rather then doing a write entire.
    You will need to do one write entire to load the custom OS but then only write the calibration as you make changes, it is much quicker and safer that way.

  9. #9
    Advanced Tuner VodeAn's Avatar
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    Did you make changes to the ETC control portion of the PCM? http://www.hptuners.com/forum/showth...rick-PCM-Tech2
    I always change the opening rate, usually under torque management, but don't mess with the other parameters.

  10. #10
    Senior Tuner LSxpwrdZ's Avatar
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    Just do a write entire with the stock file back into the PCM to verify... however there isn't anything in the calibration that would alter the voltage of the outputs to my knowledge.
    James Short - [email protected]
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  11. #11
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    Okay, thanks everyone. I have other issues. Plugged in the new pcm and was getting 6.2v with the key on for blue pins 7 (oil pressure) and 48 (map). Plugged in my original computer and same thing. I get 5.0v with key off and 6.2v with key on. I have a trunk mounted battery with direct frame and body grounds on battery. Motor has multiple block to chassis as well as a firewall to head ground. I am thinking about tapping into all the factory pcm ground wires right off the pcm connector and adding another in line ground. Not really sure what else could be causing this, will do a few forum searches..

  12. #12
    Advanced Tuner c.u's Avatar
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    A short to voltage on the fuel tank pressure sensor will backfeed through the sensor into the 5 volt renference circuit. May be worth a look.

  13. #13
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    2nd gear pull 1 5 13.hpl2nd hit 1 5 13.hpllq4 s480 base 3 bar-1.hptupdate, my issue was not ground related which is obvious now that I think about it (I had excess voltage not inadequate...)
    Anyway I messed up the wiring when trying to use my factory gauges and tapped it into the oil pressure 5v signal
    Had the car running today, very happy first time hitting boost, had my rev limiter set low (6200 rpm) so I hit it on the one hit of 60% throttle, cam is spec'd for 6500 shift point, so I raised that now after getting no knock or other issues as 6200 rpm
    This is my first ls swap, my first legit turbo build and I'm so happy after 13 months to have a running car. I only have one code now p1518 which I have chased a bit and is something with my x link for running the ls2 silver throttle body on the older truck motors. Car runs great above 3000 rpm, need to work on my rookie tune for low partial throttle.
    I appreciate all the help everyone, happy new year!
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  14. #14
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    Having problems with staying running

    I just did a lq4 4l80 swap into my 86 squarebody. I have hptuners and I removed the vats off my pcm and then it started right up then dies after about 3 seconds. I used the stock harness for the swap and I?m stumped. I?ve got good fuel pressure at the rail, my battery is charged and the only dtc is is p0641. I have a video of me when I got it to run for about 15 seconds. Any help would be appreciated.

  15. #15
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    With that kind of ignorance to not post a new thread nor do any diagnostics you may never get it solved if left to your own devices

    common sense goes a long way
    decipha @ EFIDynoTuning
    http://www.efidynotuning.com/

  16. #16
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    Quote Originally Posted by Cade merrill View Post
    I just did a lq4 4l80 swap into my 86 squarebody. I have hptuners and I removed the vats off my pcm and then it started right up then dies after about 3 seconds. I used the stock harness for the swap and I?m stumped. I?ve got good fuel pressure at the rail, my battery is charged and the only dtc is is p0641. I have a video of me when I got it to run for about 15 seconds. Any help would be appreciated.
    In order to troubleshoot this, I'd use the same procedure that Russ mentioned in this thread...
    You might also try to disconnect the sensors one by one until the code goes away. You might also look at the MAP sensor output, as well as the TPS output...are they reading correctly?