Cadillac Repair: 1991 Deville air conditioning? 4.9L, climate control, computor

Question
My ac pressure is low and the compressor will not engage so I can add freon......

?...where is the switch/ plug located to manually run the compressor til I get the pressure up?

I found 3?.....one of them, when unplugged, turns the cooling fan on, but I couldn't get the compressor to engage off it...was that the correct one and I'm just not getting contact?
Thanks in advance.....
(Problem/ reason----a 'friend' was gonna change it over to 134, and bled the system with the valve.....LOL!.....friend indeed! Claimed to know ac!!?)

Live and learn......and at my age too.....DB

Answer
Hi DeWayne, I'm sure your friend ment well but this car is very high tech and requires knowledge of the a/c and BCM computor systems. If you set a code for the a/c being low then the code will need to be cleared before the compressor will engage. Usually with 1lb in the system it is enough to engage the comp. The a/c relay might be bad even though there aren't any codes. It is under the black plastic cover at the firewall and the first one from the passengers side I believe. tapping on it will get the compressor to engage/disengage if the relay is bad or weak. To clear the codes with the key on press off and warmer on the climate control panel for 5 seconds and then it will go into diagnostics. After it displays 70 on the fuel data panel push and hold off and low fan until it shows F00. Then the a/c should engage when the engine is started and put on auto. Hope that makes sence. Bill