2000 Silverado - AC intermittent - what else to check

[JIHarris]

I am trying to figure out a problem with the air conditioning on this truck.   2000 Silverado, LT, 5.3 with about 220,000 miles.  The AC will, when operating show about 40-48 degrees on a thermometer in the vent depending on outdoor temperature and humidity (in Tennessee both get high). 

Normally the AC works.  However, once the truck is at operating temperature the AC will shut off particularly at red lights, at drive throughs, even just running at RPM's around 1500.  When it does this, turning the motor off and back on does not reset it.  Turn the AC off on the dash does not reset it.  But it will normally reset if I a) turn it off and let the engine cool off, b) turn the AC off and leave it off 10-15 minutes while driving, or c) rapidly accelerate to bring RPMs up over about 3500 (might have to do this a few times). 

I have borrowed a set of R134a gauges and when tested this week at about 75 degrees outdoor temps, the low side was at 40psi and the high side was around 150psi with the compressor engaged.   I have also replaced the fan clutch this spring for other reasons but it is a new GM fan clutch.

This week when I got home from work (about 45 minute drive) the AC was running normal.  I was on a phone call so I pulled in the driveway and AC was running fine.  I left the engine running while on the call. About 10 minutes of idling I noted the windows fogging and no AC.  I left it idling, finished the call and popped the hood.  The AC clutch was not spinning.   I tested the wires on the low pressure switch (replaced it last summer) and nothing changed.  I unplugged low pressure and plugged back in - nothing changed.   I unplugged low pressure and jump across the plugs - nothing.   Without driving, I revved the engine to over 2500 about 3 times and the AC engaged.  

 

I am wondering perhaps what else it could be?   Perhaps AC clutch itself?  Perhaps the AC relay?  Perhaps the harness?  Perhaps even some other electrical component in the circuit?  Not sure how to go forward from here b/c when the AC is running its both cold and has a normal sound so it seems mechanically ok. 

John

Nashville, TN

[Jsdirt]

Sounds like your high pressure cutoff switch on the compressor is either failing, or there really is super high pressure there.

 

Many causes for too high of pressure. Inadequate heat exchange with condenser due to debris built up between rad and condenser, bad fan clutch or electric fan motor or control inputs, or excessive air mixed with refrigerant (charging without pulling a vacuum will cause that, among other things).

 

Fan failures are the #1 cause of high pressure spikes at idle. Lack of airflow makes for big, and rapidly climbing high side pressure.

[txab]

Check your clutch air gap for being excessive. One test. Be very careful! While engine running, AC on, but clutch not engaged, tap on the face of the clutch. If it engages. Weak clutch or excessive gap

[JIHarris]

Thanks for the suggestions.  I have not checked the high side switch.   I did last night wonder if the RPM increase tending to resume engagement might indicate a bad or weak relay.   I pulled the relay and swapped with it the horn relay which was the same part number.  Then, at least here, our temperatures went from 70 degrees yesterday to snow last night and 20's this morning so it will be a few days to see if this helped.

 

 

[Jsdirt]

My trick for checking electromagnets was to take a feeler gauge with a .004" flat blade and hold it near the unit, then activate it. If it sucks the feeler gauge blade into the metal, then the assembly is good. If you have to get super close, or it will only suck in a .002" gauge, then it's weak and/or failing.

 

Of course that would be a great time to measure your air gap first - if the gap is too wide, then no point in trying my test.