2001 Silverado 1500, 6-cyl engine.

On a 3 hour trip the other day (Texas 95 deg. F, 65% humidity- Heat index of 110+ deg. F.) - I noticed air dwindling from my vents. Figuring the coil probably froze in the dash - I tapped the AC button, to turn off the compressor to let it thaw out. The AC light went off, yet the compressor stayed on. (other control was on recirculate).

If I switch the air source to outside air intake (vent) - the compressor will shut off. (and that's what I had to do to get the coil thawed out to allow air flow again).

When the AC is running - it also no longer cycles the compressor on/off. The compressor just runs constantly,

Thinking maybe the AC relay was stuck, I went ahead and replaced it today. New relay = same symptoms. Would there be some signal being sent through the air recirculate switch (or the other dash controls) that would be keeping the compressor engaged 100% of the time? Like I mentioned - it will turn off if I switch to outside air for my source (and have the AC button turned off).

Thanks,
Rex S.

 

I'm gonna bump this post again - since there were no replies to the original post - and after having a new AC system put in (Compressor, Condenser, Orifice tube, and lines - evaporator coil is original - and was flushed 2x) - I noticed again - that the compressor stays on after I hit the AC button to turn it off... because the coils were freezing over. (Yes - I changed the cabin filter, also). Normally, the compressor should cycle on/off - but it's not. It just stays on constantly. Would this be a freon charge issue? Or should I look at the relay? Or somewhere else? (i.e. does the signal get grounded on the 2001 Silverado 1500's through a VCM / BCM / ECU, etc. ?)
Thanks,
Rex S,

 

Further updates....
At the suggestion of my local shop - who is helpful in suggesting things I can do myself to save $$ - I have swapped out the Low Pressure cutout switch = No change in the situation
Also swapped out the compressor relay, as well as the hi-pressure circulation switch (the one that's in the line from the condenser - not the one built into the compressor) = also no change in the situation. If the A/C is on - the compressor is on 100% of the time and does not cycle on/off. I believe that it eventually overheats the compressor, and it eventually shuts down on it's own thermal overload. I'll test that by driving with it on for 30-40 minutes, and when it stops blowing cold - will pull over and pop the hood and see if the compressor is on at all.

My local shop owner sent me over the attached A/C diagram... From this diagram - am I seeing it correct - that the signal to cycle the compressor clutch is sent from the PCM ??? *WHY the heck do they have an AC system going through the PCM? Seems like it should be it's own external module)

On a whim - with the a/c running - I disconnected the wiring harness from the low-pressure switch (on the accumulator) - the compressor cut off as expected. plugged it back in - and it came on. I did the same for the hi-pressure switch - and the compressor stayed on. That tells me that (possibly) somethings is elsewhere - holding that line's signal as a closed circuit - and not allowing the compressor to cycle... Bad clutch inductor? Bad PCM? Frayed/chaffed signal line that's shorted/grounded? Do I need to back-probe the signal lines while it's running?

I'm all about doing what diagnostics I can - just needing some direction....



2001 Silverado A/C Circuit

 

I needed some other work done - so had the shop re-look at why the compressor doesn't cycle properly.

They essentially threw a parts-cannon at it - and it still hasn't fixed the problem... but an interesting thing surfaced... Here's what we know so far:

The compressor runs 100% of the time when the A/C is on. It does not cycle on/off properly.

The shop:
Replaced the compressor. Clutch stays engaged.
Replaced the high-pressure switch (on the back of the compressor) - Clutch stays engaged.
Replaced A/C Controls in the dash. Clutch stays engaged.
Disconnected the blower motor resistor - and the compressor started cycling properly.. Of course, there's no air blowing, because the resistor circuit for the blower motor was removed.
They replaced the resistor and it's pigtail connector. And the clutch, again, stays engaged.

I don't know if there's anything else to check - but the PCM.... WHY do these circuits need to go through the PCM to cycle that clutch? That's asinine... SMH...

The only good news so far - is that we've eliminated several culprits - AND the shop was able to return the items that didn't make any difference. (I kept the resistor and pigtail intact, and the new HP switch). I found replacement PCM's on RockAuto for $160 (includes core charge). And it says needs to be programmed.... Is this something an independent shop can program with a high $ scanner? Or is it a "dealer-only" type of programming repair?

 

I know this is an older thread. I wanted to update - that I still have not figured out the problem of the AC compressor not cycling at all. I do, however, suspect it's my ECM.

I don't know the innards of the ECM - if the relay / switching controls are actual relays - or if they're IC based signals - but I'm suspecting a relay within the PCM output signal has carboned contacts - and is not releasing UNLESS it's cold outside (thermal based movement of contacts?)

I had replaced the low and high pressure switches, the AC relay, the AC fuse, and the shop replaced the compressor 2x.

Does anyone else have experience with the ECM/PCM being the culprit?