A/C Clutch Always Engaged

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

  1. azaviator

    azaviator New Member

    The A/C clutch on my 1999 Suburban engages with the key. If I remove the A/C relay in the underhood fuse block, it goes out, but when the relay is in, the clutch stays engaged regardless of any of the settings on the dash controller. I also disconnected the pressure switch connector (at the accumulator) but it still stays on(so it doesn't seem to be the pressure switch that's bad). I followed the wire from the relay to the ECU and found that it becomes live with the key on position. Should I get power through it with the key on, or is it supposed to be controlled through the dash switch? If not, will replacing the ECU fix the problem, or could it be just a wire?

    I went to a dealer and was told that I could replace the in-dash controller, but if it didn't fix the problem then I'm out ~$250. I could also replace the ECU to see if that's the problem, but if not then I'm out ~$300(plus the cost of programming). Since the parts are electrical there's no chance of returning either of them once they are used. I'm pulling my hair out trying to figure this mess out!!! Any ideas on how I can troubleshoot further to be certain of what needs to be replaced?

    Thanks for any replies!!
  2. RallySTX

    RallySTX Member 100 Posts

    Funny the shop didn't offer to diagnose the issue before throwing parts at it. Seems like you need to find a shop that has a competent tech that can diagnose the issue and tell you whet it needs without the head games. Does the wire you followed go through the indash controller before it goes to the ECM? If so, and changing the positions of the controller make no difference, I would bet the controller is bad.
    Brian L.
  3. azaviator

    azaviator New Member

    I followed it into the wire bundle and back out ~8" later where it goes directly into the ECU. It is possible that it breaks off somewhere else in that 8" (since the bundle is actually a "Y"), but I couldn't get to the wires on the lower end, so I hope not. It was wound pretty tight.
  4. RallySTX

    RallySTX Member 100 Posts

    Well mabye the truck is wired so your commands to the control panel are sent to the ECM which then controls the clutch directly. Now all I can offer is yank the controls out and test the switches. I'm surprised to see an ECM failure that only effects one aspect of the truck. Usually they fail as a result of an inproper jumping procedure, and all heck breaks loose. Again, I would seek another service option, one that can do better than just offer to waste your money by blindly throwing parts at it. There has to be a way they can flowchart it intelegently.
    Brian L.
  5. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I would have to guess that the controller is bad also. I was having the same issue when I bought my 03 yukon xl. it turns out that my climate control is odd. The ac (snow flake button) actually turns my ac off. So, unless I press the button to illuminate a snowflake with a line thru it on the display then my ac runs all the time. It took about 4 months of asking around to find someone that knew what was going on with it.
  6. azaviator

    azaviator New Member

    One more piece of info(sorry I didn't think to include it in the original post): With the dash control unit removed ENTIRELY, the clutch is still engaged and the a/c works, albeit without blowing air. This leads me to believe that it is NOT the controller, but the ECU.

    Any other ideas? I was quoted $70/hr for troubleshooting and, because it is an electrical issue, "it would probably take atleast a few hours to track the problem down"... I'd like to save some of that money to go towards parts.
  7. RallySTX

    RallySTX Member 100 Posts

    I had to deal with a shop like that once, never again, they raped me for over $700 because my mech fuel pump had failed on me. I was stranded on the big road, with a woman I barely knew aboard, they was the only truck repair shop open, that would look at it the same day, and even though I knew it was just a fuel pump, they would only say that unless I paid for diagnostics by the hour, that they couldn't gaurantee the truck would get us home. I ended up paying for a new timing chain, water pump, fuel pump, and three hours of diagnostic time. That was the last time I ever left town without my tools. Now I just get burglarized every seven years. If the clutch is engaged with the controls removed then either the circuit is wired to arm unless grounded out, or the ECM is at fault. You can test the circuit theory by cutting the black wire somewhere you feel comfortable with repairing, and grounding it out with the AC turned on. By your statement it appears that the AC is directly controlled by the ECM. I would be very embarassed to admit I had built a vehicle that defaults to a constant on condition, to a power robbing device like the AC compressor. What you need now is a schematic diagram to show you the signal wire on the harness going to the ECM, so you could determin whether the ECM actually has control, of the circuit, or has failed to a constant on condition. So, what I would do is slap it back together, and find another shop, and see if they can offer you a better deal than just to soak up your diagnostic wallet. Very sorry I can't offer you anything better that that. Best of luck, and please feel free to tell us more. I will look for a diagram.
    Brian L.
  8. azaviator

    azaviator New Member

    I didn't think my Haynes manual would have a schematic but, lo and behold, there it was...

    I'll try to explain what I see as accurately as I can, since I can't take a picture of it at the moment. The line that controls the relay is at fault, rather than the one that powers the A/C clutch. I know this because when the relay is removed the clutch stops (also because I can hear the click of power in the relay, and I have swapped it with another that is known to work, so I know it's good).

    So, since it is the control for the relay that is at fault, I'll cover that circuit backwards (from the ground to the powered side). The ultimate ground wire for the circuit is the black wire at the dash control. The A/C switch on the dash panel connects or disconnects the ground. If connected, it completes the circuit via a light green wire from the low pressure cutoff switch at the accumulator (under the hood). The ground is fed to the L/P switch via a dark green/white wire from the Vehicle Control Module (what I have been calling the ECU/ECM). The VCM is fed via another dark green/white wire from the A/C relay under the dash. Power is supplied to the relay along a pink wire that comes from the fuse block behind the drivers door, which is hot when the key is in "run" or "start". Hopefully this makes sense, so far.

    I know the problem shouldn't be in front (on the powered side) of the relay, because that would just blow the fuse. So, the short has to be behind the relay. I have disconnected the L/P switch and the clutch stays engaged, so this points to the fault being before the L/P switch (though there could be more faults further down the line...). This leaves me with:

    1) The dark green/white wire that goes from the relay to the VCM is shorted. Can I pull the connector on the VCM to test for a short on this wire, or am I likely to cause more problems by doing that?

    2) The VCM itself is shorted on the inside. I hope this is not the case, $$$.

    3) The dark green/white wire from the VCM to the L/P switch is shorted. Again, my mind tells me I must disconnect the VCM connector while things are on to verify, but I'm afraid I might short something...

    What is the best way to tackle testing these wires? should I start with the one from the relay to the VCM (makes the most sense to me), or the other? Should (could) I test for continuity to ground with the electronics off (sounds feasable, but I'm not sure)?

    Thanks for being patient with me on this issue, I've never been good at dealing with automotive electrical problems!!
  9. RallySTX

    RallySTX Member 100 Posts

    First rule of workin on sa trucks electrical system, Always disconnect the battery ground wire before beginning any work. Thank you for the correction, I will use VCM from now on. The VCM is usually built into the dash under the glove bos, or the kick panel on the passenger side, right beside where your right foot would rest against on the passenger side footwell. When you get to it, you will see at least two big harness plugs on one side if not more. These may require the use of a screwdriver to remove, and each one is bound to have over fifty contacts in a row, some powered, some not due to switch positions and such, as well as ground wires, and signal wires. The work area will be crampt and dimly lit, so care must be taken, so as not to make things worse. With the battery unplugged you can remove the plug you're after and check the wire for a short. Just ground one lead, and touch the proper pin on the connector. Since power is off, and the switch is off, there should be none, unless it's a ground wire. I would try to check the diagram for a pinout for the controls first, and then use it to test for resistance in the circuit. Does the control center light up properly with the vehicle running? or is it dark? If you can try to trace where the effected wires go through the firewall and look for missing gromets, or chaffed wires. Be very careful with these big connectors, the pins are soft, and easily bent or broken. Correct orientation and insertion are key to preventing further issues. That's why I don't mess with these vehicles because I only have one arm. You don't need to feel bad, looks to me like you're dealing with it just fine, it just isn't your strong side, fact is, it isn't mine either, but I have had some training, and lots of experience working on vehicles like it. They was getting scrapped! On a merely component oriented side, it looks like you have eliminated nearly everything but the switch panel and the VCM. If you can't find a bad wire issue, then it looks like you should take the easier of two roads and get another switch assembly. The VCM is the most expensive part that could be bad, and it's so deep within the vehicle that it's unlikely it could be easilly damaged without your knowledge. Like I said before, the only way to really damage a VCM is by giving a sloppy jump or getting one. And it would most likely effect more than one subsystem in the vehicle. Usually it's the relay that fails, but after that, you just work your way up the stream, testing what you can, and replace the culprit. Today's vehicles are so complicated, that you need to be very careful just to keep from making things worse. The tradeoff, they are nicer to be in, safer to drive, and they look fantastic. Reliability, that's another story. I'm sticking with ya, because I don't walk away when I see a drowning man, [not you, you're making progress toward shore, and learning to swim at the same time,] and because I'm hoping another member with better knowledge will see the activity, and chime in with a really big life preserver! Cheers mate, let us know what ya get with the DMM. I gotta brush up on my theory.
    Brian L.
  10. azaviator

    azaviator New Member

    You're close, but not quite right. Chevy, in their infinite wisdom, decided to put the VCM in the engine compartment on the drivers side on these trucks. It's up high, next to the hood, but still exposed to vibration, water, road grime, dust and (worst of all) engine heat. A quick internet search shows that it is not uncommon to have to replace these. If water got inside or it got heat damaged, it is quite possible that the VCM has a short inside it. The one good thing about where it is, is the ability to work with it while standing rather than upsidedown and on my back.

    It's good to know that I can test it without power.

    I'm not sure what you mean when you say "control center". If you are talking about the A/C switch panel in the dash(aka "Heater and A/C Control Module" as it's called in the diagram), then I believe I have removed it as the main problem (but I won't say that it is not ALSO faulty!!) since it is downstream of the L/P switch. When the L/P switch is disconnected, the A/C clutch stays on. Unless I'm mistaken(I very well could be!), the short must be before the L/P switch if the system still has ground when the switch is unplugged.

    Very smart plan!! I'll do this for verification as well, since the previous owner installed a couple of aftermarket mods. It won't hurt to check that he did things right down there.

    I love this analogy! Thanks for toughing it out. As long as I see someone on the beach, I'll keep on swimming!!

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