2000 Silverado 4.8 AC compressor never stops

[hillytim69]

This just started this spring, when driving with the ac on I will hear the compressor bleed off excessive pressure, sounds like air brakes on a semi truck. i looked under the hood and see the compressor running but never cycles. At idle it seems ok but with increased engine speed the pressure builds up until it blows off with a loud WHOOSH! The refrigerant level is fine just seems the compressor never kicks off. I wonder if this is just a switch problem or what. Anyone see this before on these trucks?

[MD8349]

My 99 started doing that too evently it stoped cooling. Someone told me that the compressor runs all the time I have never notice it before.I always thought it cycles. When it started not cooling as well I have put a can of feon with its own guage and it shown that the feon was full. Then when it stopped cooling I checked it again it was in the red and i stoped useing the airconditioner. When it runs it sounds like a dieal engine the sound increases with speed. I need to have mine serviced but I am afraid of the cost.

[truckdude239]

need a new compressor. from the sound of it

[bwood]

When the AC is on in the truck, the comp will run all the time. It only starts to cycle when the air going over the evaporator (inside coil) is so cold that the suction (or low side) pressure gets low enough to cause the low pressure cycle/cut out switch to trip. It varies, but the usually happens in the 15-23ish psi range. I'm not sure what you mean by a "woosh". There is a pressure relief valve mounted on the back of the compressor that does just what the name implies, it relieves excess system pressure. Excess pressure generally only happens when there is a blockage in the system. This usually happens at the oriface. This is where the noise comes from when you turn the truck off and hear the hissing noise from under the hood. That is the sound of the liquid refrigerant passing throught the oriface and back into a gas. It does this also any time the compressor is running, its just harder to hear when the truck is running. What are you pressures? With the truck off, you should have about 50-60 psi on both sides with the compressor off. And your pressures will vary depending on outside air temps.

[hillytim69]

I only have a cheap refill kit gauge so can only check low side but will do that with engine off. What I can tell you is that it doesn't have a problem at idle and cools normally with pressure around 25-30 but when I increased the rpm, the pressure builds up to 60-70 and eventually releases pressure. It sounds like cracking open the valve on an air compressor. The ac will run like this but makes this noise and I'm sure the high pressures are not good so I don't run it now.

[truckdude239]

it could be over charged i'd pay to have the frezzon pulled out and have system properly charged

[bwood]

It may be overcharged, undercharged ot have some sort of restriction. When you rev it up and the low side pressure builds, that is usually a sign that the compressor shut off and the refrig pressure is equalizing across the system. If the compressor was still running, when you rev it the low side should drop, and the high side will generally go up.

Overcharged will trip the high pressure cutout switch and generally will happen when reving, could also pop the relief valve

Undercharged will trip the low pressure cutout/cycle switch and also happens when you rev it.

Restriction will give same symptoms of overcharge, but will not cool very well or at all.

What does you recharge guage show at idle?

[bwood]

I reread your last post and see 25-30psi at idle. What was the approx outside air temp when you took those readings? Also you say it climbed to 60-70psi, I'm thinking 70 is getting pretty high for an idle system (compressor off). It might be worth it like Truckdude said and pay a shop (use someone that specializes in AC work), and pay them to diagnose it. It would probably be cheaper than buying a set of guages (70-100bucks). Just have them check pressures and recommend a solution, and have them price the solution. Don't take the "you need a whole new system crap". Next time you think about it have someone rev it up and down and watch the compressor and see if the clutch is going on and off.