2001 Silverado, compressor rapidly cycling, refrigerant pressure quite high. Looking for all potential causes.

BG710 · July 5, 2020

Purchased this truck maybe a month ago, when I bought it the AC worked pretty well. After a few weeks it gradually felt like the air was getting hotter, but still seemed cool enough once I got on the highway and up to speed. It did seem to take quite a while to get there though. Thinking it might be low on refrigerant, I hooked up a pressure gauge and saw that if anything, it was actually a bit overfilled. I decided to keep my eye on it.

Today it started off semi-cool, then became hot within a 25 minute drive. I could also hear the compressor cycling on and off every few seconds. I put the AC on max on the way home, put a pressure gauge on, reading extremely high. When I disconnected the gauge the port hissed for a while afterwards. It's at this point I'm certain I have a problem.

I've dealt with AC troubles on a GMT800 before. I had a 1999 Chevy Silverado where the compressor failed and sharted metal into the whole system, every place I went quoted me a full rebuild of the system for $1000-$1300. Ended up selling the truck before it ever got fixed. Metal contamination sucks.

I'm hopeful this isn't the same situation. I have caught it much earlier than on my previous truck, so I'm optimistic. I have maintenance records for the truck going back to about 130k (currently sitting at about 203k), and while most parts of the truck were very well taken care of, I don't see any records regarding the AC system. I plan to take the truck to an AC specialist for a proper diagnosis, but I like to know what's going on with my own vehicle (do most maintenance and repairs myself. Hopeful somebody on here might have some insight.

Looking around, there seem to be a few things that could be happening.

1) Compressor failure. According to Carfax it's one of the most common mechanical failures on this year and model of truck. May or may not be accompanied by metal contamination of the whole system.

2) Clog somewhere in the system, possibly repairable by a flush and recharge.

3) Fan failure. This doesn't seem to be my issue as the fan and clutch are operating fine, and wouldn't explain why there's hot air now even on the highway.

4) Thermostat failure.

5) Air may be in the system.

6) Blend door failure. I hear no clunking from the dash and air distribution was fine before, so I see this as a bit less likely.

If anybody has other potential causes or has experienced this issue and resolved it, would appreciate hearing what the issue was. Thanks for reading. Like I said, plan to take it to a specialist, but want to be as informed as possible beforehand.

(Worth noting that I have recently changed out the cabin air filter, so doubtful there's an issue there)

Edit: Check replies for updates.

Edited August 7, 2020 by BG710

BG710 · July 12, 2020

Update: Turned out to be a combination of a couple things.

1) System was low on actual refrigerant, meaning the system was likely just compressing air which explained my high pressure readings.

2) Leak from one of the service ports. Both low and high side got replaced according to the shop.

3) Cooling fan clutch had not failed, but was on its way out. The fan would still turn on but which was what threw me off, but with a new clutch it definitely moves WAY more air. Explains why I wasn't getting as effective cooling around town or when stopped. Sounds like a jet at low speeds too. Definitely notice worse throttle response and power now, so I'm thinking about possibly doing an electric fan conversion in the future. That will probably wait a while though.

Anyway, I'm happy to have nice cool AC again.

davester · July 12, 2020

thanks for coming back and posting what the fix was.

BG710 · August 7, 2020

Another update. Following this repair I was still having issues. The AC would work great, but occasionally would stop working entirely or would get quite cold and have trouble maintaining temperature.

Based on all my symptoms, I crafted a new theory. I don't actually think my system was low on refrigerant until I checked it when problems first occurred, I think it was at that point that the service port valve failed and leaked out all my refrigerant. The cooling fan clutch did need replacement, but it had still been working enough to keep the engine and AC cool when I bought it. Thus, based on the symptoms, I began to think the root of my initial issues was the AC cycling switch.

I spoke to an AC specialist about it who told me that it's not so common for the switch itself to fail, but that the original pigtail connectors are prone to failure. Sure enough when I jiggled the connector a bit the compressor turned back on and I had cold air again.

I decided to order both a replacement pigtail connector as well as a new cycling switch. While it is possible I could get away with just the pigtail, based on the one experience of having the compressor repeatedly cycle on and off, and the system having difficulty maintaining temperature even after fiddling with the old pigtail connector, I decided it was probably best to replace both since those two symptoms are not necessarily indicative of just a bad connection. The cycling switch is attached to a schrader valve so it can be removed and replaced without losing much refrigerant or PAG oil.

AC is working great again now. I think I've got all the issues tracked down for now.

sefiroxx · August 8, 2020

When doing this type of diagnostic, and posting, please use a full set of guages and post
- static pressure when system is off
- hi and lo pressures when the compressor is operating
(Include outside temperature).