A/C not working, compressor & clutch are OK

[Gunn]

Several days ago, and of course with record temperatures, the Camaro A/C just quit working. Since then, I've been studying up on the A/C system and have been diagnosing the issue.

To start, the clutch was not engaging. From these forums, I found where the fuse was and checked it. It's good. Then I found the clutch relay next to it. The relay itself bench tested just fine. When I apply +12V to pin 85 and ground to pin 86, the relay clicks. Without power, I have no continuity across pins 30 and 87. With power flowing through 85/86, I do have continuity.

If I have the key in the run position (but engine off) and I jump pins 30 and 87 with a wire, the clutch clicks and moves inward on the pulley. I then started the engine and did this and the clutch engages, the compressor runs and I have nice cold air blowing in the cabin. I can only check the low pressure side currently, and when the compressor was running, it was indicating ~35psi...so there is some refrigerant in there.

So then I checked the slots of the relay in the fuse box. Here is what I came away with. This is with the engine running and the A/C engaged (snowflake light on, recirc light on, and fan on highest setting):

Pin 86: +14.7V
Pin 85: +3.52V

Pin 30: +14.7V
Pin 87: GND

I'm assuming in this configuration, Pin 30 must be connected to battery power and Pin 87 goes to the compressor.

My confusion is...why is Pin 85 registering at +3.52V? I'm guessing this is the pin that, if the A/C is commanded to be on, that it should go to Ground? And if so, then there is the problem, I guess? If this were ground, then the relay should click and the compressor would start.

So this is where I'm at. Where do I go from here?

Thanks,

Tom

[SS Blinder]

Check your low pressure switch on the AC system, GM likes to use this as the ground for the compressor when you have good low side pressure for compressor protection

[hunter1660]

There’s a 5 volt connection with the a.c. That is used as a tap for some air and fuel mixture gauges. I think the 3.2 volt your seeing may be tied to that…just a guess

[Gunn]

Originally Posted by SS Blinder

Check your low pressure switch on the AC system, GM likes to use this as the ground for the compressor when you have good low side pressure for compressor protection

Do you know where I might locate this low pressure switch? And how would I check it, just jump across it?

Thanks,

[Gunn]

I found a high pressure sensor by the radiator hose and windshield washer fluid container near the front driver side. It has three wires going to it (orange, beige and green). Anyone know how to test it?

Haven't found the low pressure switch yet...

[gtstorey]

see post #6

https://www.camaro5.com/forums/showt...1#post11052551

[Gunn]

Originally Posted by gtstorey

see post #6

https://www.camaro5.com/forums/showt...1#post11052551

Ok.... I had found that in my searches before ever making this post. So right, I know not to jump it. I'm guessing those wires are ground, signal and reference. Still trying to figure out how to test it...

Beyond that, is this switch the next most likely cause of the compressor clutch to not engage?

Thanks,

[SS Blinder]

Originally Posted by Gunn

Ok.... I had found that. So right, I know not to jump it. I'm guessing those wires are ground, signal and reference. Still trying to figure out how to test it...

Beyond that, is this switch the next most likely cause of the compressor clutch to not engage?

Thanks,

Have you tried to verify actual AC system pressure? If it is good using an HVAC gauge set, then you can jump that switch/sensor and see if it kicks on the compressor

[G8One2]

Its a sensor. Not a switch. Jumping any of the wires at the AC pressure sensor is not going to do anything but possible damage something. It surely wont command the compressor to cycle. AC systems are no longer like they used to be where the pressure sensor acts as a switch. If you want to command the compressor to cycle, do it at the relay, or use a GM diagnostic tool . You can get a cheap china version fairly cheap that works just fine for sort of stuff. Hell I think HP tuners can command the compressor on for that matter.

[G8One2]

Could be as simple as your evap temp sensor faulty or unplugged, in which case it would read -40f and disable the compressor.

[SS Blinder]

Do you have any DTCs set?

[G8One2]

Most DTC's wont show under the ECM as it is not what controls the AC system, and will only show faults for the compressor and relay circuit its controlling. The HVAC module, which is part of your infotainment system, is what controls it, and will have the stored codes for it, if there are any. The HVAC module monitors ambient temp, cabin temp, evaporator temp, pressure sensor and probably a few other things as well. The ECM only receives a command to turn it on through the CAN buss, and then sends a ground to the clutch relay to energize the clutch coil, when requested by the HVAC module.

[Mouse330]

So u have power on 86 and 30?
87 should be ground, 85 is ground provided by the ECM which is the clutch relay control circuit when the HVAC control module sends an A/C request message. Do u have ground on 85 when requested?

[Gunn]

Originally Posted by Mouse330

So u have power on 86 and 30?
87 should be ground, 85 is ground provided by the ECM which is the clutch relay control circuit when the HVAC control module sends an A/C request message. Do u have ground on 85 when requested?

Thanks for the great info. I went out and performed another set of checks:

Key in the off position:
30: Battery voltage (12.6V)
86: 0V (GND)
87: GND
85: 0V

Key in the run position, engine running:
30: 12.6V
86: Battery Voltage (14.91V) (whether A/C switch on or off)
87: GND
85: 3.49V (A/C switch on or off)

So 85 does not seem to be ground, but instead this odd 3.49 volts...?

I also did a bit of testing with the high pressure switch plug, as best as I could with just a multimeter:
Gray: +5V
Beige: GND
Red/Black: signal wire, couldn't test

I hooked up my A/C gauges and jumped pins 30 and 87 on the relay socket with a wire to get the compressor spinning. With a temperature outside of 84F, I had these high and low pressure readings:

Low Pressure: 35psi
High Pressure: 205psi

My instructions indicate that I should have 45-55psi low and 225-250psi high at 84F. So my readings look a little low, but doesn't seem too low to kick on the compressor, right?

Hope some of this is helpful, I'm not sure where to go from here...other than a shop (ughhhh).

Thanks,

Tom