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  1. #1

    Default 99 Gmc Suburban K2500 7.4 , Electric cooling fan issues

    First off thanks in advance for any help I may receive, My problem is this I have a 1999 GMC suburban K2500 7.4L with front and rear A/C. The electric cooling fan is not working. I am not getting any power to the unit, I have verified that all fuses and relays ( by swapping with starter relay ) are working. I have supplied 12v directly to the fan and it works, I have also replaced the temperature sending unit by the thermostat housing. I feel I am a pretty good do it yourselfer, but my "brain is burning on this".

  2. #2


    little more info on what relay setup your using?
    and what Electric fans are they?
    2000 ECSB Silverado 2WD
    LQ4 with Isky 215 Turbo Cam
    4L80E KB Racing S475 Turbo Kit

  3. #3


    Jonathan, thanks for getting so quick. I am referring to the Factory relays in the engine compartment fuse and relay box and the fan is the cooling fan on the condenser in front of the radiator

  4. #4


    ohhh i didnt know those come with an aux fan.

    Try and jump out pin 87 to pin 30 in the fuse box. that will send power straight to the fan. If that doesnt work then find the fuse that controls it.

    If you have a voltmeter you should see 12V at pin 30.

    - - - Updated - - -

    ohhh i didnt know those come with an aux fan.

    Try and jump out pin 87 to pin 30 on that relay socket in the fuse box. that will send power straight to the fan. If that doesnt work then find the fuse that controls it.

    If you have a voltmeter you should see 12V at pin 30. That will let you know the fuse is good and sending power. If jumping pins 87 to 30 works then you have a issue with the trigger commanding the relay to energize.

  5. #5


    All right Jonathan, I just test 87-30 and nothing happened , I tested for 12v at pin 30 and nothing. I put 12v straight to pin 30 and the a/c clutch engages and nothing else. I took my 12v tester and checked " EVERY " fuse in the engine compartment box and the fuse panel in the cab, all are good. ??????

  6. #6


    hum... what happens if you put power to pin 87?
    Power should come from a fuse to pin 30.

    unless the fan is suppose to come on when the AC clutch is engaged... lemme find a wiring diagram.

  7. #7


    I have power at 85 and 86, with he relay out the a/c quits, jumping power from 85 or 86 to pin 30 nothing happens

    - - - Updated - - -

    Jonathan, I think I have found the issue, There was another relay attached to the engine fuse block but NOT inside it, so it was not on the schematic. anyway I jumped 87-30 and wa-la the fan came on. So I'm off to Oriellys and get this relay and go from there.

    - - - Updated - - -

    ok I'm back the new relay is in and still nothing

    - - - Updated - - -

    is there another item besides the temp sending unit that may control when the fan comes on ????

  8. #8


    i think im confused now.

    but try to find your AC comp clutch relay... jump it out pin 87 to pin 30..
    then jump your Aux fan relay pin 87 to 30.. does your fan come on now?

    im thinking something has to be on first before it gets power.

    no point in the fan being on if the clutch isnt pulled in and the comp pumping.

    - - - Updated - - -

    Dont try pins 85 or 86... those are your trigger.. can burn the pcm or something up doin that.

    power comes in at pin 30.. power out at pin 87 or 87A if its a 5 pin relay.

  9. #9


    Hey Jonathan, I have done what you said in the last post, 1) took out comp. relay and jumped pin 87 to pin 30 in the fuse block. 2) took aux fan relay out of outside harness and jumped pin 87 to pin 30 in the harness. YES the fan comes on ???????????

  10. #10


    Problem solved, after a boat load of web surfing and replacing every relay in the fuse block it turns out that the fan will only come on when the high pressure switch on the back of the compressor say its to high. I blocked off the air flow for the condenser by inserting a piece of card board between the condenser and radiator and turn the a/c on full tilt. The truck had not even made it to operating temp when the air started to blow warm from the high pressure, fan kicked on and the cold air returned. So I guess the fan does NOT run full time when the air is on like it does on other vehicles I have had. Thanks for the Help anyway Jonathan.

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