PCM Controlled A/C Circuit Diagnosis

Circuit Description

When an A/C mode is selected, the A/C control head or the A/C programmer supplies a 12 volt signal to the PCM A/C request input. The A/C compressor clutch relay is controlled by the PCM. The PCM provides a ground path for the A/C relay coil. When the A/C compressor relay is engaged battery voltage is supplied to the compressor clutch coil. The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested unless one or more of the following conditions are present:

^ Throttle angle is greater than 90%.

^ A/C head pressure is greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition 1 voltage is below 10 volts.

^ Engine speed is greater than 4225 RPM for longer than 5 second.

^ Engine speed is greater then 5280 RPM for any amount of time.

^ ECT is greater than 125°C (257°F).

^ IAT is less than 0°C (32°F).

Any of the conditions described above will inhibit A/C compressor relay operation.