97 SLS AC compressor

[dvc500]

I have a 1997 SLS with 47k miles. A few months ago the AC started changing output (from vent to defrost and then back) all by itself without any changes being inputed by myself. Sometimes it would clear itself and go back to vent and other times I would turn the Auto control off and back on and it would correct itself. I'm thinking great, the controller module is starting to fail.Then this week, the air stops working all together and the Driver information center reads "AC Compressor shut off... very low refridgerent". I take it to my local shop and they test the system and say it is full problem could be the controller and may need either a re progam or perhaps a new controller +$1,600!. It s at the Caddy dealer today for their diagnostic. My questions; 1) does this sound like a bad controller module? If so, is it worth putting a new/rebuilt one back in or what if I found one at a local salvage yard? I realize I will still need the GM dealer to re program either scenario.

[BodybyFisher]

First thing to do is to check for codes, you do not need a code reader, you have an on board diagnostics system, if you do not know how let us know

Once we see what your codes are, that might shed some light on the problem

You must check all vacuum lines, the vacuum canister, check valve, hoses and reset all vacuum lines on the ACM. Resetting the colored hoses at the ACM on my 96 made a big difference

[Ranger]

A few months ago the AC started changing output (from vent to defrost and then back) all by itself without any changes being inputed by myself.

It is vacuum controlled and defrost is the default mode. Sounds like a vacuum leak. The parking brake release is vacuum operated and is on the same vacuum circuit. The diaphragm is a common leak point. Does it release when you put the car in gear?

[KHE]

Usually when the system says it is low on refrigerant, it is low on refrigerant. How did the shop determine the system was "full"? Did he recover and weigh the amount of refrigerant that was recovered? There is nothing to "reprogram" in the system. You have two problems - a vacuum leak which is causing the air output to change locations, and a low refrigerant issue which is shutting down the compressor so that it does not self distruct from a lack of lubricant. The low refrigerant code will need to be cleared before adding refrigerant.

Look at all components of the system - condenser, lines, connections, compressor for evidence of leaks - they will appear as greasy/dirty spots. If you cannot see any evidence of a leak, it is possible the system has such a slight leak that it has taken all these years to leak enough to trip the low refrigerant message.

[dvc500]

Thanks for all the posts. The parking brake does release O.K. The Caddy dealer said it was a little low in refrigerant (almost negligeable) but they added a tiny amount and reset the system. So far it is running again and the changing from vent to defrost has also stopped.

I'm not convinced that was the problem (or only problem) as the car still doesnt feel right at idle and when backing up. It feels like it is running rough yet no codes (except the low refrigerant one) are being tripped per the dealer. it does appear to run smoother when I turn off the HVAC system manually. Also when I turn the car off, after about 3 seconds, there is a very defined whine lasting 1-2 seconds coming from the engine. I know thats hard to pinpoint but thought I would mention it.

First thing to do is to check for codes, you do not need a code reader, you have an on board diagnostics system, if you do not know how let us know

Once we see what your codes are, that might shed some light on the problem

You must check all vacuum lines, the vacuum canister, check valve, hoses and reset all vacuum lines on the ACM. Resetting the colored hoses at the ACM on my 96 made a big difference