A/C compressor suddenly stopped running (electrical issue, not mechanical)

[Steveinfl]

Hi Everyone I hope you all are doing well,

I had a weird issue start with my TB today. The air-conditioner would do nothing but give me hot air, not heat but engine bay temp air none the less.

I had my system looked over, the refrigerant pressure was high, they evacuated and properly recharged my system.

Still wouldn't engage the compressor.

Shorted low pressure sensor, No Change

Jiggled and inspected wires of High Pressure Sensor, No Change

Inspected fuses and relays, switched out with known functioning units. No Change

So this brings us to the interesting part.

This refers to relay #44 in the engine bay fuse panel.


With the Key off the only hot wire is pin #30.

With the key on but A/c off pins 86, 30 and 85 ( ground, why would I receive a reading on my voltmeter jumping - terminal to the ground terminal on the relay?) are reading +12v.

Key on A/c on

Same as before.

If i manually jump from the - battery terminal to pin 85 the compressor engages as it should.

Do any of you have some light to shed on this, I am in central florida. It's not terrible yet but the heat is coming.

Thankyou Kindly,

Steve

 

[The_Roadie]

The "ground" pin 85 on the relay is only grounded by the PCM when *it* wants the compressor to come on. It's pulled high to 12V by the relay coil when the compressor is off. So I'm thinking either the fuse block receptable for the relay is bad, the PCM is bad, the wire from the relay to the PCM is bad, or something is making the PCM think it should not be allowing the compressor to engage. Things like overtemp, too high refrigerant pressure, too low pressure that trips the low pressure switch, a flaky control signal from the HVAC control module where it thinks you have the snowflake override button pressed and you really don't. That sort of thing. A high end scan tool or a Tech II could look at all those sensor signals and tell you wby the compressor is being suppressed.

 

[Steveinfl]

I was beginning to think the computer could be the culprit. It sounds like $100 for a dealershop to run their tech 2 at it is money well spent. I'll have to look and jiggle the engine bay portion of the wires a little bit more, but I've almost ruled them out at this point.

Thanks Roadie, I will amend the post once I figure out eactly where the electrical gremlins got loose. I wonder if they migrated from the MG in the garage that needs rewiring to the trailblazer, hopefully not to the same extent at least!

 

[Steveinfl]

I don't have any explanation other than that

A. The electrical fault has not reappeared much to my liking.

B. The system was grossly overcharged initially as they put in refrigerant for a system with rear A/c. I have the rear controls but not a second evaporator assembly.

I'm leaning towards B, as the A/C techs were surprised the system hadn't blown apart due to the higher than advised system pressure. A simple evac and refill along with enough time for something in the computer to reset seemed to do it. A/C vents pushing 45 degree air in 85+ degree central Florida. Working better than before. I'll take it, hopefully it continues as it is now!

Thanks for the responses, I am going to attempt to mark this as solved, I'm not sure if I remember how.