I had not had the a/c on this car work in 3 years. I checked the shraeder valves and no gas vented out. Upon adding a can of refrigerant, the compressor kicked on. The vents were blowing cool not cold air. Then the compressor kicked off and has stayed off since. The low side pressure reads now 30psi with an additional can of refrigerant hooked up to the manifold gauge set. I guess the compressor needs to kick on to draw in the 2nd can? But I can not seem to recreate the condition that allowed the compressor to kick on. I would check the compressor relay if I knew where it was. So, where is this relay. Any advice is appreciated.

 

High side pressure maybe dropped low enough for the high side low pressure switch to trip shutting off the compressor. You can check voltage at the compressor and/or compressor relay with a DVOM to determine presence of 12 volts.

 

On the firewall, passenger side, the one with the green base.

Larry