PCM Controlled A/C Circuit Diagnosis

Schematic:

CIRCUIT DESCRIPTION

When an A/C mode is selected, the NC 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 percent.
^ 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 10.5 volts.
^ Engine speed is greater than 4500 RPM for longer than 5 seconds.
^ Engine speed is greater then 5400 RPM for any amount of time.
^ ECT is greater than 125°C (257°F).
^ IAT is less than 5°C (41°F).

Any of the conditions described above will inhibit NC compressor relay operation.

Diagnostic Chart:

DIAGNOSTIC AIDS
Check for the following conditions:
^ Poor connection at the PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
^ Damaged harness. Inspect the wiring harness for damage.
^ If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected to the NC relay control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the A/C relay. A change in voltage will indicate the location of the malfunction.

Check for conditions that will disable A/C operation:
^ Throttle angle greater than 90 percent.
^ A/C head pressure 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 below 10.5 volts.
^ Engine speed greater than 4500 RPM for longer than 5 seconds.
^ Engine speed greater then 5400 RPM for any amount of time.
^ ECT greater than 125°C (257°F).
^ IAT less than 5°C (41°F).