I have a 2001.5 CTD with 70k miles with an air conditioner problem. Here are the symptoms. The system has the correct pressure of r134 as checked by a cheap walmart gauge. The high pressure side metal hose doesn't get hot, and the low pressure side hose doesn't get cold or drip. The A/C fuse gets power, the A/C relay is working.

It sounds as if the compressor is never turning on, i.e. the clutch doesn't lock up. I checked power to the compressor clutch w/ the voltmeter, and nothing. The ground side of the plug still goes to ground, but I don't get any voltage between the two prongs, or between the hot side of the plug and the frame.

What controls power to the compressor? I read about some kind of fin probe in the evaporator. Is this common? Is there a mechanical switch somewhere, like a pressure switch, or is compressor cycling done by the computer?

There is a pressure switch in the system and if it isn't closed your compressor will not engage. I always disconnect the pressure switch on my truck in the winter time to save a little extra fuel since the air conditioning would otherwise always be running when the heater is in the defrost position.

I would check that and make sure the switch is closed. If it isn't, then either the switch is bad or you don't have enough freon in the system.

There is a pressure switch in the system and if it isn't closed your compressor will not engage. I always disconnect the pressure switch on my truck in the winter time to save a little extra fuel since the air conditioning would otherwise always be running when the heater is in the defrost position.

I would check that and make sure the switch is closed. If it isn't, then either the switch is bad or you don't have enough freon in the system.

The a/c turns on in defrost mode for a reason. It drys out the air to keep the windows from fogging up.

And there should be a cyciling switch. On chevys it is threaded into the evaporator. Its easy to replace.

The a/c turns on in defrost mode for a reason. It drys out the air to keep the windows from fogging up.

And there should be a cyciling switch. On chevys it is threaded into the evaporator. Its easy to replace.


I know why it turns on. But I'm sure you know as well as I do that you don't need the A/C to be on every time you have the defrost on. I can remember only one time this last winter when I actually needed to have the A/C on to dry out the air so my windows would defog...

Besides, what do people with vehicles that don't have A/C do? :flipoff2:

Well, the truck must "know" when the refrigerant is low, and keep the compressor off to prevent damage. I jumped the two sides of the pressure switch plug to make the compressor cycle manually (thinking it was the pressure switch). This must have reset something, because it started cycling every five seconds even after I replugged in the pressure switch. So I filled it back up w/ r134 and it's as cold as ever.

Of course, I'm an idiot because I misread the gauge in the first place. The needle was in the Green. I thought, "Hey, green is for good stuff like go, and money, and healthy grass. So, it must be good to go." Well, it needed to be in the blue. I'll read the directions first next year at this time.

Well, the truck must "know" when the refrigerant is low, and keep the compressor off to prevent damage. I jumped the two sides of the pressure switch plug to make the compressor cycle manually (thinking it was the pressure switch). This must have reset something, because it started cycling every five seconds even after I replugged in the pressure switch. So I filled it back up w/ r134 and it's as cold as ever.

Of course, I'm an idiot because I misread the gauge in the first place. The needle was in the Green. I thought, "Hey, green is for good stuff like go, and money, and healthy grass. So, it must be good to go." Well, it needed to be in the blue. I'll read the directions first next year at this time.




It was cycling every 5 seconds or so because the pressure in the system was flucuating enough to make the pressure switch open and close...

Now you need to figure out where the old freon went. Otherwise you will be doing this once a year or more often...