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Thread: Need help please GMC 1988 4x4

  1. #1
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    Need help please GMC 1988 4x4

    I have a friend that has a 1988 GMC 4x4 1500 auto. His 4x4 stopped working a while back and he replaced the thermal actuator with the upgrade and the "new" wiring harness. Not sure what made him start there but he did. I read yesterday the thread about the actuator on this forum and it was very helpful about making sure there is 12V at the plug that goes into the top of the tranfercase and the plug on the actuator and how to jump the plug if the ball switch is bad on the transfercase. Well, we are not getting power at the plug at the TC.

    I have a 1990 Chevy 4x4 1500 auto and everything on the truck is just about the same. We compared the fuse pannel and there are 3 differences, 1 being I have a fuse for 4WD at the pannel and he doesn't, 2 I have two places marked "power" and only 1 fuse pluged in and he has two fuses, 3 the he has a fuse marked "spedo" and mine says something else, can't remember this morning.

    Where is his fuse for 4WD????? We looked for another pannel under the hood but do not see one on either truck. After reading yesterday on here the problem may be power related because we are not getting power to the TC to send power to the actuator to engage it. He has 2 wires at the connector on the TC, a brown and a lt blue. The connector on the actuator has 3 wires, a brown a lt blue and a black. He spliced the brown from the new harness to the brown going to the TC switch.

    I'm hoping that one of you can assist us in telling us where the wires go from the TC back to the fuse and where the 4WD fuse is.


    If you need more info please ask and I will do my best to answer.


    Thanks a bunch!!!
    Last edited by Not Sure; 05-06-2010 at 05:51 AM.

  2. #2
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    Welcome aboard. Try this


  3. #3
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    Thanks, can someone tell me where the fuse for the 4wd is? it is not in the panel on the dash like mine is. If we are not getting power to the transfercase it leads me to beleive the problem is upstream back toward the 4wd fuse. Am I correct with my thinking??

  4. #4
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    Its either under the hood or in dash. Its under the instrument panel to left of the steering column.
    Last edited by davbell22602; 05-06-2010 at 07:34 AM. Reason: added info

  5. #5
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    in my 1990 it is in the panel left of the steering column, in his 1988 it is not. Do you know exaclty where under the hood??? We have looked all over under the hood and do not see one or any fuses at all.

    Thanks

  6. #6
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    It would be in black box near one of the fenders or it could it be on the firewall under the dash on the drivers side.

    Is this a carb or fuel injected engine?

  7. #7
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    No carb, fuel injected, throttle body or whatever it was used after the carbs before true fuel injection. Same as my 1990. Bare with me here, I'm assuming that in this black box there will be fuses correct? We did see a black box looking thing under the dash, drivers side that has wires going to it but the only writing on it was colors, blu, wht, brn etc where wires connected, no fuses.

  8. #8
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    Welcome to the forum Not Sure.

    Don't believe you will find any fuses under the hood
    except perhaps the fuel pump fuse, not panels or boxes of them.
    Earlier trucks did not have a labeled/dedicated 4wd fuse,
    I'm surprised yours does. Started around 93 I thought.

    The reason your friend or anyone else replaces the thermal actuator
    with the upgrade and the "new" wiring harness
    is the new one ("Fast Axle Acuator") is a motor not thermal dependant.
    Was also a warrantee upgrade in past if slower than 30secs to engage.

    I'd recommend rechecking the new wiring because that's also typically
    the source of power for Tcase switch. Make sure you are not getting
    fooled by a bad ground connection for your meter, that happens lots.
    Double check all fuses since on that truck I think it's shared something else.
    Do that using a meter looking for voltage on both sides. be aware of key on.

    Not following the "black box" under driverside dash thing...pictures rule.
    Here's some of nearby year fuse panels, as a talking point. Color is a 94.
    .
    Attached Images Attached Images   

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    Thanks, the color fuse pic looks just about like my 1990. I should have power/12v (with key on and in 4H) at the switch that plugs into the ball switch on the top of the transfercase, correct? And to check it I use a volt meter using the + and - leads to side of the connector, correct?

  10. #10
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    For the purposes of this thread you should be making sure that
    the voltmeter is getting connected to a good ground
    with it's "-" or "neg" lead. Unless you give the meter an extension,
    this will require a good ground point be found inside and another under.
    Good testing proceedure means you have to know that it's good,
    so with the neg connected to your choice of grounds,
    touch the +/pos lead to something you know is hot
    and see if the meter agrees. If not, ground point is not good,
    and your testing of fuse or 4wd will be useless based on that.

    When I said check both sides of fuses, that means leave
    the ground/- lead in one place and move only the +/pos.

    When checking under the truck at t-case, set -lead to ground first.
    Then after confirming ground is good, remove t-case connector and
    look for the hot coming to it. If not there, nothing will work.
    With connector off, key in Run, one will be hot and other two nothing.
    Does not matter if in 4W or not, this is the wire that makes
    power available to the whole process. 4W selection only effects
    where the basic hot goes after picking 4LO or 4HI.

    Not sure if that truck has points in ign (?) if so, don't leave in Run long.
    On the 88 there is no 4wd fuse but there is a hot coming from somewhere.
    It's a matter of following the wire back unless someone can tell you where.
    That's why I said check all the fuses first, saves grief.

    Was the 4wd working when he put the new axle part on ?
    Has it ever worked for him ? If he did the upgrade for personal reasons,
    most likely spot of trouble is where he touched it.
    If he did it to "repair" the truck, he should have some history to tell you.
    May want to follow the new hot from the upgrade to see if good,
    may be related to problem.

    The drawing below is for 94 not 88 so of course some differences,
    but the basic idea is same, just different on original hot to t-case.
    Like most drawing this one has some errors. In the lower left corner
    is a box showing how the t-case switch works. It's wrong.
    The connections are right but the internals aren't, PCM only gets a hot
    signal from it when in 4LO not in 4HI. Wire to axle is hot when 4LO or 4HI
    In the lower right corner is shown the thermal actuator which
    your friend swapped out of his truck for the new style.
    New style is 3 wire motor that requires a splice to a hot.
    Hopefully he saved his instructions for reference.

    In the lower middle is optional connections to RWAL (antilock brakes)
    that he may not have on that truck.
    .
    Attached Images Attached Images  

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