Service AC System / AC Compressor Off
 

Well Guys, still having an AC problem. I get this message. When I push the Auto/Econ button, the AC comes on and the compressor kicks on. But sure enough whether immediately or after 5-10 seconds, it goes back off. The book says that the message means that a sensor is not functioning. I have changed the low and high pressure switches and the relay. Fans kick on and all. I took it back to Express Oil Change and they checked the freon level. It still reads full at 2.0 lbs (according to the Caddy's Manual). And they say the pressure levels are good for a good cool. I'm stumped, what other sensors/switches could it be? It's starting to cool off in Alabama, so AC is not an emergency at this point. But it does effect how the Defrost and Defog options work. Very humid here.

RE: Service AC System / AC Compressor Off
 

How can they check pressures when the compressor is not producing any pressure being off? Is there a specific method of recharging listed on the A/C decal underhood? On my 91 youcannot haveA/C related codes current in the system in order to have the compressor stay engaged while recharging. I would say the sensors are working because they provide the reading to the ECM to open that circuit, which they seem to be doing. And yes, the compressor dehumidifies the warm air directed to the winshield to speed up defrosting.

RE: Service AC System / AC Compressor Off
 

What codes are showing in the computer? Why are you taking it to a place that drains and refills motor oil?

RE: Service AC System / AC Compressor Off
 

when you push the button, the compressor does engage, it just doesnt stay on long. There aren't any codes being showed other than the Service AC from the tread.Express also does AC service, cheaper than taking it to Cadillac. $80 to diagnose.

RE: Service AC System / AC Compressor Off
 

"There aren't any codes being showed other than the Service AC from the tread"

I'm sure you know what that sentence means but it is a mystery to me. The oil-changers charged you $80 to tell you everything is fine? It isn't though, is it?Good-bye $80.I'd like to help, but you give very little to go by. You replaced sensors for some reason and the compressor cycles Off after a short run. That is a classic 'low refrigerant' symptom. There should be AC codes shown in the diagnostic readout.......

RE: Service AC System / AC Compressor Off
 

The codes statement was in reply to "what codes are showing" the only code I'm getting is Service AC System / AC Compressor Off which is the topic of this thread. Sorry for the confusion. I was saying that Cadillac charges $80 just to diagnose. I agree with the low refrigerant, in the past with other vehicles this was a classic and simple symptom and fix. Not with the Caddy. As stated, when hooked up to the machine and the compressor cycles on, it reads full at 2.0 lbs. I did the Off / Warm button diagnosis and no codes were displayed. In every catagory, it says No Codes.

RE: Service AC System / AC Compressor Off
 

Something is telling the system to shut-down the compressor or the compressor is shutting down by itself. No codes is pecuilar. How old is that compressor? Have you checked the voltage 'to' the clutch/compressor past the switch? Wiring/terminals clean/tight?

RE: Service AC System / AC Compressor Off
 

As far as I know Dave, its the original compressor. Not that advanced to check voltage and all, but if it comes on it should be getting power, shouldn't it? The connections are tight as the switches have just been changed. I will check the wiring harness to the compressor, but I don't think thats the culprit. May have to just break down and take it to Cadillac. There is a shop here who claims they know all things Cadillac. Their diagnostic is $78, but they will waive it if you get it fixed right then.

RE: Service AC System / AC Compressor Off
 

If there is voltage to get the clutch engaged, but then the clutch dis-engages: the clutch may be just losing power (the reason to check voltage),orif there is voltage to the clutch, but dis-engages,it's faulty.12yrs on the clutch and compressor is a good life-span. You might start saving for a replacement unit. New w/clutch $350. Reman w/clutch $250.

RE: Service AC System / AC Compressor Off
 

Does this car use refrigerant temperature sensors (thermisters)?

If so I would replace the Low side sensor. When these sensors start to wear out there resistance (ohms) starts to increase. and since the cars computer reads the change in voltage (variable resistance) to determine a temp. value. It therefor thinks you have a problem when in fact you don't because of the erroneous data it is receiving.

So, whats the function of the low side temp sensor?

It tells the computer when to engage and disengage the compressor based on the refrigerant TEMPERATURE not pressure. It has a set temperature range to do so which if I recall corectly is -1C - 11C (30F to 51F). So if it hit -1 it will disengage the compressor because it thinks the evaporator core is about to freeze up. Now remember the part about increasing resistance? Well It just so happens in these cadillac systems higher resistance means a LOWER temp reading. So as soon as then compressor engages I would bet money that the temperature value falls BELOW -1C therfore the computer shuts it off.

I would replace a $30 sensor before go any further.