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GM Computers

Discussion in 'Electronic Repair' started by BOB URZ, Oct 5, 2003.

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  1. BOB URZ

    BOB URZ Guest

    My 96 S10 seems to have some issues when its cold and i am suspecting
    some potential problems with the computer under the hood.
    Normally, i would have no problem with tearing one apart and checking
    solder joints or such. But, i don't not have a spare and do not want
    this to
    be a one way trip. Anyone tore down a mid 90's Gm vehicle computer
    soldered it up and inspected it, and put it back on line successfully?
    Any pictures internally if you did? Any tips? This is a OBDII unit.

  2. David

    David Guest

    You have to have the equipment to remove the potting compound off the
    circuit board before you can do anything with it.

    The connectors getting corrosion is a very common problem. Also a weak
    battery or slightly low voltage will cause all kinds of erratic symptoms
    (especially when cold). Battery MUST be over 12v for proper computer

    There is also a good chance the grounds (any of about 6 or 7) could be bad.

    The GM computer is a very low failure item, look to it last for your

  3. BOB URZ

    BOB URZ Guest

    Is the bottom of the board hard potting, or just the semi flexible
    coating? Do you know, or are you guessing what's on it?

    The problem i am having on the 96 is when it very cold, i get a
    error code that the IAT air temp sensor is full scale. The thermistor looks
    ok and the connector on the thermistor looks fine. I connected and
    put cramolin on the computer connectors. So it seems to be a thermal
    condition. Other sensors use the same ground as the IAT, and i am
    not getting a code off of them (so i assume the grounding is not an issue)
    Though not impossible, i doubt the thermister in the air intake is
    thermally intermittent. I sure would like to see what the inside of
    the computer looks like before i attempt to take mine off and disassemble it.
    I smell a bad solder connection somewhere.

  4. Franc Zabkar

    Franc Zabkar Guest

    I would think that the computer should be spec'd for operation right
    down to cranking voltage (9V or less?).

    - Franc Zabkar
  5. Jeff

    Jeff Guest

    Most of the GM ones I have looked at were a pain
    to dissassemble without damaging something that
    only a reman center would have and then after that
    I found nothing noticable wrong inside.
  6. David

    David Guest

    Nope, they are not. During cranking the computer is not operating in any
    kind of closed loop mode. The voltage references for all the run sensors
    require a full battery voltage. Only a couple of digital sensors are used
    during start up, oil pressure switch, crank and/or cam sensor. The computer
    does not need any of the analog circuits to be operating during start up.

    Back the to question, the Air temp sensors on GM cars are very problem
    prone. It is highly possible it could be intermittent when cold.

    No I am not guessing on the GM computers. All of them I bothered to take
    apart had both a soft rubbery coating on a large part of it and a hard
    potting on part of the computer board.

    At least the battery problem should be easy to figure out, a good voltmeter
    right before and after a cold start up. The cigarette lighter is an easy
    place to double check the voltage.

    The sensor in question also should be verified per the troublshooting guide
    in the vehicle service manual. If you do not have it, I think alldatadiy
    gives the same troublshooting guides online, first purchase is $25 for one
    year use.

  7. Asimov

    Asimov Guest

    "Franc Zabkar" bravely wrote to "All" (06 Oct 03 06:54:01)
    --- on the heady topic of "Re: GM Computers"

    FZ> From: Franc Zabkar <>
    FZ> Subject: Re: GM Computers
    FZ> Organization: Bachelor Life Ltd
    FZ> Reply-To:
    FZ> Xref: aeinews

    FZ> I would think that the computer should be spec'd for operation right
    FZ> down to cranking voltage (9V or less?).

    I saw a commercial gadget today which plugs into the cigarette lighter
    to preserve the computer data when the battery is removed. Basically it
    consists of a small 9 volt battery with what I presume is a series diode
    and with an accessory plug on the end of the cable. So obviously the
    data can be preserved even for a voltage slightly less than 9 volts.


    .... Over a hundred billion electrons were used in crafting this tagline.
  8. BOB URZ

    BOB URZ Guest

    That's the kind of info i am trying to find out. Without a spare handy, i
    don't want to disable my truck. I sure would like a donor to play with
    before i did the real thing.

    And yes, i do have the factory shop manuals. They are good only up to
    a point where your brain has to take over to fill in the blanks.

    This problem with he IAT (Intake air sensor) only happens when its
    very cold. ANd it only sets one code when it happens. And other sensors
    share the same ground with this one and no other codes are set. So i know
    the grounds are ok. Connectors seems to be ok and i shot D5 into them.
    This thermistor shows a full scale code error, so the hot wire to it
    is either open or shorted when very cold, or it has a bad solder connection
    or component in the CPU that's driving it. It has not got that cold yet,
    but the clock is ticking.....
    And i refuse to pay hundreds of dollars for a remanufactured one
    not knowing if that will solve the problem.

    I need to get me a autotap kit to go with my laptop.

  9. k_teppo

    k_teppo Guest

    Sorry in advance for the slight off-topic
    Bob, for your consideration:

    I have heard of numerous problems with GM's computers/proms. Most are
    late-80's early 90's, but the same could apply for mid-90's. I had a
    1989 Oldsmobile Cutlass Ciera 3.3L that ran very poorly. A mechanic
    scanned for codes and it was found that the coolant temp. sensor was
    reading full hot ALL the time. This mechanic changed the sensor (and a
    few other sensors) and it didn't solve the problem. I decided I would
    finish the repair. I went to the scrapyard and picked up a replacement
    computer (with prom). I swapped the proms and put in the new computer.
    SAME PROBLEM. Swapped proms, and it ran like it should. Corrupted
    proms do weird things.
    Another nearly identical issue was with my co-worker's 1991 Olds
    Cutlass Supreme. The car ran poorly and smelled of gasoline. If he
    didn't keep it revving, it would flood out and die. He swapped out
    sensors from a wreck to no avail, and once again, swapping PROMs fixed
    the issue. Seems the corrupted prom was keeping the fuel injectors
    open either constantly, or just too long.

    BTW: the computer on most of these vehicles is located behind the
    glovebox. I'm sure you know this, but when removing the prom, keep in
    mind that they are **particularly** sensitive the ESD.

    One last note: I opened both computers and found no potting compound
    and all solder joints looked great.

    Take care and good luck with your repair!
  10. In reference to the IAT error when cold...
    I know it seems practically impossible, but I've had 2 bad ones in the last
    couple of years, one on a '93 Geo Metro, the other on a '89 Grand Prix. I
    made a quicky test rig consisting of a 9v battery, load resistor, and the
    IAT and my o-scope. I pulled the IAT out, and sure enough, when I'd cool it
    down and smack it around a bit, the one from the Metro would go open, the
    one from the Grand Prix would go shorted. I can only guess that the
    thermistor internally broke down somehow, like a really big cold solder
    joint in the sensor itself.

    My .2 cents worth...
  11. BOB URZ

    BOB URZ Guest

    Thank for the insight. I find that very interesting. I suppose i will try a new
    if the code pops again and see what happens. I have never seen one
    fail in that manor before.

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