have a 1998 Chevy 3500HD. The air conditioner doesn't work. It is not low on freon; no fuses are blown; it is not the relay. If you jump across the terminals that the AC relay goes in it cools good, but does not cycle. I also tried a different relay that I know is good but it still didn't make any difference. Have any suggestions??
Country: United StatesMake: ChevroletModel: C3500 HDYear: 1998Engine: 454
Hi, sounds like you may have a bad pressure/cycling switch. It is located on the Accumulator and it can be removed and replaced without losing refrigerant. There is a valve underneath that prevents refrigerant loss when you unscrew the switch. Use a new O-ring and tighten to 40ft lbs:
There is also a high pressure safety switch on the compressor itself, which is designed to cut power to the compressor in case the pressure got too high for any reason. That may have failed too. I think the cut-off switch can be removed without refrigerant loss too like the other switch, but I can't confirm that:
Other than replacing these switches, I would recommend taking the vehicle to a shop that is equipped and authorized to service AC systems. Releasing refrigerant into the athmosphere is a felony, an environmental crime, subject to a fine up to $27.500 per day, jail time, and a reward of $10.000 for anyone ratting you out to the EPA if you are convicted :)
Also, how have you determined that the system has enough refrigerant in it? I was just wondering. When it gets low, the low pressure safety switch may prevent the compressor from coming on. There may be enough refrigerant in the system to make somewhat cold air, but not enough pressure.
I have a license to work on AC systems. I have worked on several John Deere tractor systems; but not many newer automotive AC systems. My gauges on the Chevy pickup in question read 27 or so on the low side and around 150 on the high. I had nice cold air at the vents as long as I left my jumper wire on in place of the relay. It did not cycle with the jumper wire on, it just ran steady. Then when I took the jumper wire off and replaced it with the relay it quits. I know the relay is good because I took it off and tried it on the starter and it worked there. They are the same relay, as they have the same part number on them. It can't be the pressure cycling switch because I put a jumper on it and the compressor wouldn't cycle.
Ok, very good info. Those pressure numbers sound low, what is the ambient temperature where you are? The AC season is just starting here in Virginia, we are hitting 88F and most systems run well over 250PSI high side and 35 low side. To be accurate, you would need to suck it all out and refill by weight. There should be a sticker in the engine bay, possibly on the accumulator, stating how much to put in. Pressures are not really an accurate way to measure. I did spend some time online looking for other GM trucks similar to yours, and all of them report 225 or higher high side. I think your system is low. I also found people who had a bad low pressure shut-off switch, the one on the accumulator. Some shops offer free AC checks, they can check the charge level with a machine. Pep Boys for example. It's free, so it doesn't go to great depths in analyzing what is wrong, but it at least should cover the basics, adifferentiate between refrigerant and electrical problems.
20 years of general automotive repair & modification. Chevy owner.