Automotive Air Conditioning Information Forum

Just bought this used truck and trying to fix the AC.

Compressor looks as if it's been replaced at some point (still has the warning sticker about oil on it).

It runs, but none of the lines get cool.

75F, apx 70% RH.
45psi low side, 120psi high side.

Blipping the throttle a bit causes the low side to drop a couple psi, but the high side really doesn't change.

No abnormal noises. At first I was thinking they replaced the compressor but not the o-tube and left junk in the system. But now I'm wondering compressor maybe..

You will know nothing until you determine the amount of refrigerant in the system. The only way to do this is to extract the refrigerant and weigh it and compare to the amount listed on the under hood sticker. You have a 65% chance of being low on refrigerant due to a leak according to my 3200 GM car database.

An S-truck (as is yours) used an HT-6 (on 6 cyl engine- a V-7 on 4 cylinder engine- you don't specify) in yr 2000- and they are known for belly leaks around the body- since it has been replaced you say, then I's study hard all the other lines and fittings- looking for oily/greasy residue from a leak. If not cycling and 4 cyl engine- most likely due to low refrigerant (V-7s do not cycle, by design)

Sorry, it is a 4 cylinder so the V7.

I automatically assumed it wasn't too low since it wasn't cycling. That's what happens when we assume.

I don't have anyway to recover what's in the system. So it's either add some and watch pressures, or totally refill.
I'm fine with emptying it, flushing it, replacing the otube and accumulator but don't want to find out the hard way it's a compressor.

You don't need to flush etc. Have someone recover and recharge by weight. If the low side still doesn't come down and start cooling, the control valve in the compressor is likely the problem. Diagnosis of a variable system has to start with knowing the charge is correct by weight.

once charged by weight, how would you determine its the control valve?

By reporting the pressures high and low........as the compressor is running.

I really have not had much time to play with the truck. I'm wishing I grabbed some static readings before I turned the AC on.

If there is little to no change in low side readings from compressor off, is charge weight still suspect, or does that more point to the compressor/control valve.

GM Tech mentions a variable system, does this mean this truck uses some sort of thermal expansion valve instead of an oriface tube? (Sorry for the stupid questions).

I called a couple local shops about recovering, they all want a nice chunk of cash just to recover the system. I have all the tools for the job, flush gun, vacuum pump, etc. Just not my own recovery machine.

Compressor is variable, orifice tube is fixed....not a TXV system.

Okay, so I had a local shop recover and recharge. (Which kills me, a recovery machine is the only thing I don't have.)

They said it was very slightly over charged, and recharged to spec.

Drove it home with no cooling, put my gauge set on it and readings look nearly identical. High side hovers around 130 with no change from RPM, and low right around 45. The low side will slowly bounce by 5 psi or so at 1500 RPM.

Whats my next step looking like, control valve/compressor?

So not knowing what next, I pulled the control valve from the compressor to inspect.

Mine was not color coded. There was also no small screen on the end like I see in some photos of them.

There was a good bit of black, gunky grit on the tip of it, and packed in around the outermost o-ring.

I'm leaning towards a new compressor, o-tube, accumulator, and a very thorough flush.