AC problem - yet another electrical gremlin?

[Maverickxeo]

I'm back with another (possible) electrical gremlin on my 04 4.2L Envoy.

I noticed that my AC compressor was not working yesterday. I turned on and off the HVAC, turned on and off the AC, and nothing. I did notice that the clutch would engage when the engine first starts, but would stop after a few seconds.

In doing some testing, I swapped the relay with a known good relay. No difference. I took the low pressure switch and jumped the connector. No difference. I took the belt off and spun both the clutch and the pulley - both seem good.

I checked the low side pressure (no way to check high side - yet) and it was sitting around 100PSI (of course, the compressor was not running).

For some reason, I decided to disconnect the battery. I did so, and then reconnected it 30 seconds later. I tried the AC, and it worked. Low side pressure measured around 25-35PSI. I was able to switch the AC off and on, HVAC off and on, and engine on and off and it worked good.

Today, I noticed the AC stopped working again. The first thing I did was disconnect the battery and reconnect it. Again, the AC worked as it should. I check both with the engine hot and cold.

As a side note, on my scan tool, I could see that the 'AC control relay' was showing 'off' even when I had the AC switched on via the HVAC unit (when the AC was not working). With the AC working, the 'AC control relay' would show 'off' when not running, and 'on' when running.

What could this be? I am leaning toward a wiring problem between the HVAC unit and the relay.

 

[Mooseman]

What about the high pressure switch? Maybe it's flaky or is actually seeing too high pressure.

Another thing that I know of that would stop the PCM from commanding the compressor on is ambient temperature below freezing, actually below 3 or 4c. If you can, check to see what it's reporting.

 

[Maverickxeo]

What about the high pressure switch? Maybe it's flaky or is actually seeing too high pressure.

Another thing that I know of that would stop the PCM from commanding the compressor on is ambient temperature below freezing, actually below 3 or 4c. If you can, check to see what it's reporting.

You know, the ambient temp on the ATC was showing -1... I'll keep an eye on that.

 

[Maverickxeo]

You know, the ambient temp on the ATC was showing -1... I'll keep an eye on that.

So today I have an update!

I noticed the ambient temp reading +4... AC was working. Get back in after a short drive, ambient temp was reading -20... AC not working. I jumped the AC relay using a short wire, and the AC clutch engaged and worked flawlessly.

So I am certain now it is the ambient temp sensor.

Just curious, are there any other systems that could be affected by that sensor not working?

 

[Mooseman]

Just that and the exterior temp display AFAIK. Engine has its own IAT sensor.

 

[Maverickxeo]

Just that and the exterior temp display AFAIK. Engine has its own IAT sensor.

Okay, kind of figured that was the case. It seems odd that the AC system does not work at lower temps - I always assumed that the AC works at lower temps to defrost the windows and stuff faster in the winter...

 

[Mooseman]

In winter, any condensate would freeze and the outside temp would be colder than the A/C anyway. Instances where having A/C running is helpful are very rare, like when it's humid and there's a cold rain outside. Might have happened maybe a couple of times in my lifetime in vehicles with no A/C. I usually pull the A/C relay once fall arrives as it's a waste of fuel and unneeded extra wear on the system when using defrost.

 

[Maverickxeo]

In winter, any condensate would freeze and the outside temp would be colder than the A/C anyway. Instances where having A/C running is helpful are very rare, like when it's humid and there's a cold rain outside. Might have happened maybe a couple of times in my lifetime in vehicles with no A/C. I usually pull the A/C relay once fall arrives as it's a waste of fuel and unneeded extra wear on the system when using defrost.

Ah, good to know.