Air Conditioner troubleshooting

The AC system in my '99 (124,000) Tahoe is not something I have the skills to repair myself. But prior to having a technician look at it I wanted some of the collective insight of this group. The AC has never been very strong on this vehicle, even when new. Yesterday on the highway the cooler was working even worse than before. In the garage the compressor cycles on for about 1 second, then off for about 6 seconds. This seems like much less "on" time than normal and to my small mind would account for the lack of cooling. This system has never been recharged. Can you guys give me any hints as to what the trouble probably is? The outside temp on the highway was about 75 F. Cabin temp was set to most cool, but not to "max" where the air recycles. Air flow seemed normal, just wasn't pushing out very cool air. All else works normally. Thanks.

You need to get gages on this and check the high and low side pressures for starters.

Thanks, maxwedge, but I wouldn't know what to do with them if I had a set. I used the GM manual troubleshooting guide and have concluded that because the input line of the evaporator is cold and the output is warm, that the system is probably low on refrigerant, but that's just a guess based on the troubleshooting chart. My main question for the group is, what usually causes this refrigerant loss, and can it be expensive? And is it common to have to add refrigerant after seven years? I can do a lot to this vehicle, but when it comes to AC it's going to the shop.

Refrigarant only leaves the system if there is a leak. How big of a leak deermines how long it is before you need to recharge the system. The most common offenders are the seals. Check around the hoses where the compressor connects if i is oily then you have a leak. This could be a costly repair depending on where the leak is. You will at least have to replace the drier....

You definately have a low freon situation, but like others have said, it means there is a leak, and you need to have the pressures and freon level checked. The common areas for leakage on the Tahoe's were: 1) the compressor (usually the shell o-rings leak, since it is a 3 piece design the o-rings leak); 2) the manifold hose assembly (hoses that attach to the compressor, look for oily dampness where the hose goes from the compressor over the top of the front of the engine); 3) the cycling switch (mounted on the silver can, pass side of the engine compartment back by the fire wall, has a 2 wire plug); 4) the service ports (part of the manifold hose assembly), or 5) the rear evaporator/expansion valve (least likely case, but seen plenty of them). A/C shops can put in a dye in the system, recharge the system, then when you notice it is blowing warm again, they can inspect the system with an ultraviolet light and tell exactly where it is coming from. Hope this helps.

Thanks, klinn, that is exactly what I'm looking for. The vehicle does have a few of the signs you mentioned, and now if the shop says I need to add dye and run it for a while I'll have some faith in what they say. Tell me this...if the compressor case o-rings are shot is the fix to replace them or to replace the entire compressor? Thanks so much for the helpful reply.

To my knowledge, not even GM makes a seal kit for the shell seals. I would just replace the entire compressor. Make sure you ask about the warranty, and make the additional repairs that are required to fulfill the warranty (if any are needed). Usually the a/c shop will want to replace the receiver/dryer and the orifice tube, do it and that way in the event of another failure, you are covered and it can't come back on you. They commonly leak at the seals and it will be a matter of time until the next one leaks also, but there are so many parts in the compressor, you might want to make sure you are covered in case something else goes wrong, not to create panic or doubt, just make sure you are covered and you don't have to buy another one. Shop around, some shops might offer a lifetime replacement on the compressor, including labor. Hope this helps.

my 96 did that last year. i took it to a local shop and told the guy what it was doing, and without even opening the hood he told me what it was. these compressors are known to leak. he pulled out the leak detector and went all around the system, and the machine started beeping like crazy when he went to the back of the compressor. he told me i had a choice, either replace for about $600, or he could charge it and it'll last till it leaks down again. i recharged for $40, and it's been good for 13 months now. might go have it topped off for summer, but it is still cold now.