Truck AC

[John Pratt]

I am a little embarrassed to admit this, but I really know nothing about cars. I can change the oil, plugs, wires, and a couple other things, but that is as far as my knowledge goes (I know, I know, how do you call yourself a man without knowing this stuff). The AC in my truck just won’t work properly. I checked the pressure of the R134 and I am good but I only get about a 5-6 minute blast of cold air (unusually cold air) and then nothing but hot air after that. Sometime if I let the truck sit for 30 minutes, I get the really cold air again for about 5 minutes then nothing again. This is Oklahoma and after retiring from the Army I’ve gotten soft, so I need my AC. I would kind of like to know what to expect from the mechanic if I have to take it in. Most around here are part replacers not mechanics so by the time they’re done it could be a whole new system. Any thoughts?
John

[Paul Ryan]

It is tough to diagnosis with out looking at the truck. You need to find out if the compressor clutch unlocks causing loss of cold air. That is pretty easy to check. Find the compressor start the truck with the AC on. The cluch should be turning with the compressor. Watch and wait until the cold air would normally quit blowing. If the clutch unlocks you will see the clutch just sitting idle while the pully on the compressor behind it continues to turn. You will also hear a click and then the compressor clutch will unlock and lock. It is normal for the clutch to unlock and lock back up again a few moments later. But if it unlocks for an extended period of time and you loose cold air then more than likely you have an electrical problem. Something is telling the clutch to unlock. Too high pressure, is also possible that would indicate an obstruction in the AC system, possibly the drier or orfice tube depending on model.

If the clutch never unlocks and keeps running the compressor, I would check out the condenser (radiator looking thing in front of the radiator). That may be full of bugs, dirt, or other crap. This would be restricting air flow to cool off the 134 when being sent through the condenser. Check behind the condenser between the cond. and radiator the front of radiator gets fully of crap sometimes with out effecting the condensor. Dont ask me how this happens but it does. Usually on diesels though.

[Joe Angrisani]

As Paul said, it's hard to diagnose without having the vehicle in front of us. My crystal ball is broken, but if you could hold your computer up to the truck.....

If you get truly cold air before it "goes away", you most likely have a high pressure-related problem, and the high pressure switch is shutting down the system. If you never get truly cold air before it dies, it's a low pressure shut-down situation. No offense intended, but how did a guy that "knows nothing about cars" determine the system has the proper pressures on the high and low sides?

Did the problem start all at once, or did the A/C performance fade away over time? If it started all at once, I doubt it's bugs and crap obstructing the condensor.

What year and model, John?

[Matt Meiser]

I had a clutch problem that did exactly what you are describing on a car I had several years ago. The clutch is electromagnetic, and when the coil inside would get hot from ambient heat, not from the current flowing through it, it would open and unlock. The hotter the day, the quicker it would cut out--gee thanks! Changing the clutch fixed it. The only gotcha was that the puller required to pull the clutch off the compressor cost more than the clutch!

I also had a somewhat similar problem (though I don't recall the exact details) with another vehicle. The cheap recharge gauge I picked up showed the pressure was fine. I took it to the shop around the corner and they found that the high pressure side, which I couldn't check, was low (or maybe it was high--can't remember for sure.) $45 later I had great AC.

[John Pratt]

No offense intended, but how did a guy that "knows nothing about cars" determine the system has the proper pressures on the high and low sides?

What year and model, John?

I know just enough to be dangerous I guess. I picked up one of the easy R134 refill cartridges and hose kits. The truck is a 2001 GMC 2500HD. I use it a lot for work and hauling stuff around, so AC is a much needed resource in OK.

I hadn't even thought of too much pressure being a problem. Are you saying it could have too much coolant in the system? I will probably end up taking it in, but I really don't want to let the mechanic "see me coming" when I drop off the vehicle for repairs. The only thing worse around here is to let your wife drop off the vehicle. Then it would be that you need a new engine. There used to be one honest guy in town, but he decided to pack up shop after 45 years in business. I appreciate the responses, it will give me a starting point.

[Paul Ryan]

John,

You could have too much refrigerant in the system, but unless you added some I wouldn't worry about that. If you did add some with that kit you are talking about then you could have too much in and after running a few minutes the pressure will get too high and it will trigger the high pressure cut out. 2 options then drain the system and add what is supposed to be in the system, and then recheck. Or take it in. But I imagine you put on the refill cartridge after you had the problems so draining and recharging will only get you back to the beginning. Better off taking it in. AC gets expensive though. Good Luck