Air Conditioning Problem

[TownCarTechi]

My daughter has a 99 GMC Jimmy with the 4.3L engine. Her air conditioning has worked great until today. She said she was driving normally and then heard a "woosh" sound like air escaping from somewhere. She stopped the car and kept the engine running while she checked for any unusual smells, sounds or sights. She didn't see any and proceeded home. The air coming out of the vent was still cold all this time. She parked the car at home and then went out within 10 minutes to check the car again. This time when she ran the engine, the air coming out of the vent was warm. When I came home, she told me about it and I had her start the car while I popped the hood. I noticed that the AC clutch was not engaging at all. I surmised this might be why the air is warm. I've checked all the fuses in the interior fuse block and the engine fuse block that pertain to AC (2 in interior block- 10A & 20A, and 2 in engine block - 10A & 30A) and they were all ok. I also switched the relay in the engine fuse block for the AC with the one for the horn - they had the exact same markings so I assume they can be interchanged - but it didn't help the AC clutch to engage. I also confirmed that the relay that was originally in the AC spot now works the horn - this also tells me that the original AC relay was okay. What else should I look for? My Haynes service manual for GMC Jimmy 1995-2004 shows a picture with three relays but I don't see them anywhere on her engine. I searched the Jimmy forum for air conditioning issues but the last post I found was a few years old.

[old_master]

Check the low side pressure, if it's below approximately 42psi, the system is low on refrigerant and the compressor will not engage. Post your results.

[TownCarTechi]

Hi old master, I finally had a chance to check the low side pressure - under both static (engine off) and dynamic (engine running w air on max cool) conditions, there was no pressure (0 psi). Clearly, the AC suffered a catistrophic leakage - that is probably the "wooshing" sound my daughter heard when she was driving. I don't know what else I can do except take it to my mechanic for repair.

[old_master]

AC repair is not all that difficult however, it does require some special tools to recharge the system. Can you tell where the leak was/is? Follow each hose from the compressor and visually check each component. If the leak was loud enough to hear it when it blew, you'll most likely be able find it. Post your findings.

[TownCarTechi]

Hi old master, my daughter's car also needed some suspension work (ball joints, idler arm, pitman arm) so I just let the mechanic investigate the AC issue. Turns out the high pressure switch blew out from the compressor so we needed another switch and compressor. Car is now completely fixed and my daughter is happy.

[old_master]

Cool, glad you got it repaired, and thanks for posting back with the fix.