The Global Parts Distributors (GPD) External Control Valve Compressor Tester, No. 5811584 allows the technician to diagnose problems with external control valve compressors with three easy steps. This battery operated unit allows the technician to check the resistance of the control valve, test the input signal from the ECU, and manually drive the compressor control valve with multiple commands to ensure that the compressor is working properly. This unit provides digital readings of the resistance of the control valve and the percent of duty cycle that is being received from the ECU. The kit comes with 13 specific control valve plug adapters.