Why does my A/C compressor only engage on defrost - 99 Suburban

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by i_build, Jul 15, 2009.

  1. i_build

    i_build Rockstar

    Started today. The only way the compressor kicks in is in defrost mode. Do the controls need to be replaced?
  2. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    If the AC works and pumps cold air out the defroster vent with the temp set to cold you probably have a bad switch. The easiest way to check the switch is to put a scanner on it. The scan tool data stream will show the state of the switch.
  3. bill190

    bill190 Rockstar 100 Posts

    What kind of A/C system do you have?

    Front and rear? Rear working?

    What are your button settings when this is happening?
  4. i_build

    i_build Rockstar

    Auto, front and rear work and the a/c button is pushed in. The a/c button only illuminates and the compressor kicks on only when the direction knob is set to defrost as soon as i turn the direction knob from defrost the a/c light on the a/c button turns off even though it is pressed in.
  5. bill190

    bill190 Rockstar 100 Posts

    Test the electrical connections on the A/C on switch. The contacts on this switch may be dirty and not making good contact.

    I don't know if this switch would be easy to get to or not?
  6. i_build

    i_build Rockstar

    Jimmie, can autozone do that scan or is this a different scanner that can tell the state of the switches?
  7. Jimmiee

    Jimmiee Epic Member 5+ Years 1000 Posts

    Their scanner won't give that data. You need a GM scanner or a Snap ON style scanner that reads manufacturer data. Autozone just uses a code reader that gives generic OBDII data only. Most repair shops use the type of scanner that will read manufacturer specific data.
  8. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner 1000 Posts

    a little off point but be glad your not in California. They had made it illegal for autozone to check your codes, so they switched to having the scanner part of their loaner tools. Well at the end of last year the a-holes in Sacramento made it illegal for them to even do that, so that you have to go have a mechanic check the code:grrrrrr:. It made me finally break down and buy my own. Ok rant done (sorry not much help for your actual question).
  9. i_build

    i_build Rockstar

    Does the scanner loaner tool track the switch status or is that the simple obd ii tool?
  10. bill190

    bill190 Rockstar 100 Posts

    Scanners you buy at auto parts stores usually only scan the engine computer. Some newer models also scan the ABS and air bag computers. Some may scan additional things on the vehicle.

    Newer vehicles can have 25 different computers and 2 or 3 computer local area networks. Upscale vehicles can have near 80 different computers.

    A Tech II scanner with the right adapter (for GMLAN) can scan all of these. These cost $3000.

    For your A/C problem, this is just a simple switch which is wired in parallel with the defrost switch which also turns on the A/C.

    Try pressing firmly on the A/C switch and turn it on/off several times and you might get it to work.

    If you are going to invest in any tools, I would suggest buying a GM factory Service Manual set for your year/model vehicle, then you could look at the wiring diagrams yourself. Also get a multimeter.

    As to fixing this problem, perhaps you could go to a wrecking yard, find a vehicle with the same A/C control panel, take it apart and see how it works inside. See if there is a way to clean the A/C switch contacts.

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