AC compressor will not engage

[Flying Dutchman]

This is on a 97. I cannot get the AC compressor to engage. Even when the AC is off, I heard that the compressor should engage when the defrost cycle is selected.
So far, the compressor itself works. I can get it to turn on if I jump the relay switch under the hood. But i have tried swapping out the relay with a new one but that doesn't fix the problem.
Could there be a safety mechanism or switch that will not let the comp engage in case there is something wrong with the whole system? Eg: low fluid or ??? (just guessing).
I don't use this a lot but it did blow cold last year. Plus will something else get damaged if the compressor doesn't circulate on the defrost mode?
Any ideas are welcome

[dawnnewsome3]

It may be low of freon check and see if it has pressure easy way is to remove the cap from the suction line the larger of the two lines . If it does get a can of freon with attachment hose and refill , it may be just low enough to not activate the pressure switch . As for it not working on defrost it doesn't hurt anything , the reason it cycles on and off during defrost has to do with changing the airs humidity to help defrost the windshield easier.

[LastOne]

I agree on the low freon as being the problem. I had a leaking schrader valve on mine. It is best to have a set of gauges to troubleshoot. You can get a set at parts houses or Harbor Freight for as low as 39.00 Also know for about $8 you can get a kit that has 3-4 valves, caps etc to replace the leaking ones. Much cheaper than buying the hose. They come out just like a tire valve core. Go get some gauges, they will pay for themselves quickly. Sam's Club has a 30lb jug of R134a for $85. You and a buddy or two can go in on it and save over the little cans. A better fit and no waste. How many times is you system full and you loose half a can of the small 134a to the great outdoors.....