Stuck on Defrost

Discussion in 'Chevy C/K Truck Forum' started by jimshoe52, Jan 8, 2012.

  1. jimshoe52

    jimshoe52 New Member

    I have a 1990 Silverado (Ext Cab), V8, auto. It seems that I have heat, a/c & fan but all air is currently going through the defrost. When I push any one of the buttons (there are no knobs) to change from defrost to dash vents or floor nothing happens. I'm not highly skilled but can probably figure this out with a little help. I've seen a number of posts on similar problems but the responses were mostly "I have the same problem" and no concrete answers. So, is there anyone here that can offer some suggestions or fixes? Plus maybe someone could offer a web link to the locations of the components that may be at fault?
  2. Scott_Anderson

    Scott_Anderson Rockstar 100 Posts

    Well, you could have a bad pushbutton assy, you could have a fuse/power problem to the control module, the actuator doors could be stuck, or the actuator drive motor could be bad.
    I'd look for any blown or missing fuses, but I'm betting that if you can control the fan/heat/ac on-off, you are probably ok with power to the unit.
    That leaves bad buttons, bad control unit, or jammed linkage in the air housing, or bad motor that connects to the linkage, or a bad wire connection anywhere in-between.

    I suppose that if you can get to the actuator motor, check the wire connections to it. It's located on the left side(drivers side by the gas pedal) of the air duct assy above the floor outlet. This controls the air flow for the defrost/dash vents/floor vent.

    Here's a generic duct layout.

    This should be a more detailed layout of the duct assy itself.
    You will be wanting to look at part #24 where it connects to #28 and #29.

    If these diagrams aren't quite right, go the site home page and put in your specific vin and then look from there.
  3. outalne94z71

    outalne94z71 Member

    with the push button systems like the 88-94 trucks have you will get a flashing display when a actuator has a fault, stuck on defrost i would bet to be the linkage above the petal, common for the plastic slider pieces to fail or just fall out leaving the steel parts to just flop around
  4. 613werley

    613werley New Member

    i had same problem. i went to junk yard and got another pushbottun assy. and it fixed it.
  5. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    It is probably a vacuum leak. IIRC, the door in the ducts operates on engine vacuum. When there is a leak or if the line is plugged, it will always default to defrosters. Look for a rigid vacuum line on the passenger's side around the hood hinge. Oftentimes, the hinge will rub against the line and wear a hole into the line. I have had this happen on several trucks and the vacuum line was ALWAYS the culprit!
  6. outalne94z71

    outalne94z71 Member

    no gm truck uses vacuum since the end of the 73-87 body style, the gmt400(88-98 body) is all electric with the exception of the non a/c trucks, those are cable operated blend doors but still no vacuum.
    Last edited: Mar 4, 2012
  7. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    That's interesting. While I never had that issue on a Silverado, my '97 Express van DID have that problem and it was vacuum related. Since they were so similar, I just assumed that the trucks used the same system.
    Last edited: Mar 4, 2012
  8. wayned

    wayned Rockstar 100 Posts

    you can try a google youtube search

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