AC compressor will not engage

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?:screwy:
Any ideas are welcome

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.

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.....