I had the system charged with r134 and I have a compressor off of a late 90's pickup. The problem I have is that the compressor will not turn on. Is there some switch that needs to be replaced? I read in the service manual that the compressor should be on whenever the temp is higher than 40 but it isn't. Any ideas as to why?

How did you get to charge the system with out the compressor running?even if you draw vac its still not full till you run the compressor to draw the freon.if the system has a low pressior switch take the plug off and jump the two wires to see if the compressor comes on?if it does put a set of gages on to see what you have 25to30 on the low and 150 to 190 on hight.if not put more freon in the system.hope that helps nick

when I had it charged he jumped it I assume how you said (?), so I know it must be in the electronics somewhere. Any ideas?

when you got your car back from the shop was the air working?what year and make of car is it?

no, it was a friend that charged it, and he told me I needed a switch to make it work. The fan control switch. I got that and it is yet to go.
The car is a 1968 Camaro.

The fan control switch has nothing to do with the A/C system. All it does is control the speed of the fan. There is another switch (lo-blo switch)on the back of the A/C control levers that energizes the compressor clutch, and a second switch on top of the inner heater core box that is operated by one of the doors in the box. The clutch is only energized with the control lever in MAX or Bi-Level and the temperature control switch in Cold.

Put the levers in those positions and with the key on check for 12V at the compressor clutch, if there isn't any there move back to the lo temperature switch on the side of the evaporator case on the firewall. There are two wires in this connector a tan and a green The tan wire runs back into the dash from there, if you have 12V there, but not on the green wire (which goes to the compressor clutch) then the lo temp switch is bad. If there is no voltage on either wire you have to move back into the car and check under tha dash for power.