A/C Clutch Always Engaged

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by azaviator, Jun 25, 2012.

  1. azaviator

    azaviator New Member

    Thanks for all the help Brian!!
  2. bazar01

    bazar01 New Member

    That does not look right if we follow the diagram. It appears terminal 85 is the one that goes to the VCM and not teminal 86.
    With 0.13V at terminal 85 only means the VCM is providing a ground path for the relay coil. It confirms what you were saying that the relay is always pulled in even with the A/C switch OFF. The VCM is always providing a ground path for the relay at terminal 85. Something is shorted inside the VCM. We need to find out if the climate control is integral to the VCM.

    Was the cabin blower running or not when the compressor clutch is engaged?
  3. bazar01

    bazar01 New Member

    I think I got the terminals backward because the diagram is a little blurred.

    Your voltage measurements are correct. It looks like terminal 85 is the one that is connected to the VCM. With that reading, it only proves that the VCM is constantly providing a ground path for the A/C compressor clutch relay to get pulled in with the ignition ON. the problem is internal to the VCM. There could be a shorted component inside that is providing ground to the relay coil, thus, your A/C compressor clutch is always engaged. We need to find out if the climate controller is integral to the VCM. You have a bad climate controller.
  4. azaviator

    azaviator New Member

    All switches and dials were off on the dash control panel. From what I can tell, the dash control and VCM are seperate. They are connected only by a wire through the L/P switch. My thinking is that the wire from the relay to the VCM is bad. Is there a way to bypass it momentarily to determine if the system works correctly outside of that wire? That would show me if the problem lies with the wire or the VCM.
  5. bazar01

    bazar01 New Member

    Pull the relay out of the socket and do a resistance measurement on relay socket terminal 85 to ground. Test your meter by probing one ground post (bare unpainted chassis bolt) to another ground post (another bare, unpainted bolt) in the engine compartment. Compare that reading to your term 85 to ground resitance reading.

    With the relay pulled and If you can unplug the connector #3 from the VCM and do another resistance measurement from term 85 of relay socket to ground, that will be the best test for dead short on the said wire.

    This will tell you if that wire has a dead short to ground from terminal 85.
  6. azaviator

    azaviator New Member

    Ground to ground gave me 003, 85 to ground gave me nothing, 86 to ground gave 004, 86 to connector on VCM gave 022.
  7. azaviator

    azaviator New Member

    Also, forgot: 85 to VCM connector gives nothing.
  8. bazar01

    bazar01 New Member

    Are these readings with the ignition switch OFF, just the relay pulled out and C3 on VCM still connected?
  9. azaviator

    azaviator New Member

    yes to all except C3 on VCM was disconnected. What difference should it make with it connected?
  10. bazar01

    bazar01 New Member

    With C3 on VCM disconnected is better.

    With 85 to ground not giving any reading means the wire is not grounded or shorted with another ground wire in the harness. The problem is internal to the VCM.
  11. azaviator

    azaviator New Member

    Sorry to run off for a few days. I needed some time to reflect... Actually, I needed to step back from the problem for a bit, since I found out that I had been checking the wrong connector!!!

    Today, I got out and restarted the troubleshooting: checked the relay pin 85 for continuity against the REAL C3 pin1. Perfect continuity, so no short in that wire as I had previously determined. To verify, I hooked the C3 connector back up after pulling the pin1 wire out of it: no power to the relay. Then I shorted the pin1 wire to ground, and it triggered the relay. The problem was definitely with either the VCM or the wire to the L/P switch. Then, I tested the wire from the VCM to L/P switch for continuity(perfect) and for a short to ground(none existed). Finally, I had the answer you've been saying all along, bazar01: the VCM is the problem. Called a local junkyard and a couple of hours later had a new(used) VCM. Ran the VCM auto learn procedure and a half hour later my A/C compressor clutch relay problems were solved!

    Now I have identified a couple of additional problems.

    The related ones are:
    1) The light on the A/C switch on the dash control is intermittent, but does not correlate to the actual A/C system(A/C works fine when the light is on or off).
    2) The blower gives nice cold air but after a while, a door near the blower(not sure which one) moves and blocks the air flow. It is still cool out the vents, so probably not the blend door. The recirc door seems to work normally when the button is used(we can hear it working but haven't gotten under it to verify). It doesn't seem to come out of the floor vents, so it's probably not that door, but the flow is reduced though the dash vents by ~90%. I'm stumped at this one. My immediate idea was that something is inside the blower assembly which is getting sucked into the airflow after a time. Once it's there, it won't let go unless the air is turned off for a while, which makes for an uncomfortable ride.

    I'm hoping that replacing the climate control panel will fix both of these issues, but would like some verification before I spend the money.


    The unrelated issue is: when I installed the new(used) VCM and ran through the auto learn, there was a noise that I hadn't heard before... ever. I traced it to the A.I.R. pump running in the spare battery tray on the driver's side. Never even noticed it before, but definitely never heard it run prior to today. I assume the old VCM had a short in the control for that as well. I did some research on it and found that it is supposed to run until the system goes into closed loop(warms up to normal temperature). The problem is, it doesn't seem to shut off. I checked with my UltraGuage to see when it went to closed loop, pulled over and it was still running. I don't have a problem with it working normally but if it is bad, I want to fix the problem. Am I testing it correctly, or is there a more effective way to make sure it is working right?

    At least I sorted out the original problem. I wanted to update this thread with the fix, in case someone else has the same issue, since there isn't much to be found about this problem on the WWW. Thanks to all who chimed in!!
  12. bazar01

    bazar01 New Member

    Glad you got it figured out.

    You did very well on your electrical troubleshooting.

    I was about to suggest that you pull out wire to pin 1 of C3 connector and basically short it to ground and see if the relay pulls in, but I hesitated because I did not know if you have the tool to push out the pin and may end up damaging the connector and pin. But you managed to pull it out. Hats off to you.

    As far as the air dampers, there could be servos or vacuum control valves operating the air dampers inside the blower/evaporator assembly that are not functioning properly.

    Good luck.

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