Does anybody know why my A/C does not work above1800 rpm,it works great at idle and up to 1800rpm.It was 1/2 lb.low,evac system and re charged and still doing same thing.Is it the compressor?Thanks for any help....

 

Chances are the system is still a little low. The compressor pumps a larger volume as the RPMs increase. The system is probably shutting down on low pressure. Try pulling the connector on the low pressure switch and jumper the two wires and see what it does. The low pressure switch is located on the suction line by the firewall (in the engine compartment) close to the accumulator (the round can like thing).
Tom