the a/c switch works in the car but the compressor doesn't come on,there is a charge in the system,when I jump relay compressor comes on and its plenty cold..is there a ground someplace for the fuse block under the hood?..seems like the relay isn't getting a ground...

Maybe not enough pressure in the system to trip the low pressure switch. Have gauges put on it.

yea I put gauges on and it was low..i put in a can yesterday and guage read 45..today I put real gauges on it and it was 24..I use the 12oz cans with the cable and built in guage,it read 24 today too..weird.

You probably have a leak somewhere.

I have a '97 Chevy Silverado that was leaking at the shraeder valve on the low side.

Yes, I have seen that happen, all too often.....you have a leak, find it, repair it, evacuate the system and refill, and the vehicle comes back a few days later with a leak......either the low or high side service valve.....GM must of went to the low bidder on these....

I will check the valve..friend of mine reversed the lines on the condenser on his truck,a/c didn't work..could not figure it out for a little while,didnt know you could reverse them.