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 96%.
^ A/C head pressure is greater than 440 psi (4.6 volts) or less than 32 psi (0.3 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition 1 voltage is below 10 volts.
^ Engine speed is greater then 4400 RPM for any amount of time.
^ ECT is less than 124°C (255°F).
^ IAT is less than 5°C (41°F).

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

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 A/C 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 is greater than 96%.
^ A/C head pressure is greater than 440 psi (4.6 volts) or less than 32 psi (0.3 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition 1 voltage is below 10 volts.
^ Engine speed is greater then 4400 RPM for any amount of time.
^ ECT is greater than 124°C (255°F).
^ IAT is less than 5°C (41°F).

Test Description

Number(s) below refer to circled number(s) on the Diagnostic Chart:

The A/C compressor clutch should not be engaged with the engine running if an A/C mode is not selected at the control head.

2. Stored Diagnostic Trouble Codes (That may not illuminate the Malfunction indicator Lamp) may disable the A/C compressor. This Diagnostic Table may lead to improper diagnosis and replacement of good parts if Diagnostic Trouble Codes are present.
3. The A/C compressor clutch should apply if the conditions listed under the Important are met.
6. Checks for an A/C request signal from the control head to the PCM.
12. Checks the fuse to determine if the ignition feed circuit(s) are open or shorted to ground.
15. The A/C diode (clamping diode) is used to control voltage surges that occur when the A/C clutch coil is cycled from ON to OFF. If the diode is open, the voltage surge may blow the A/C fuse.
16. Determines whether the A/C relay current draw is the reason for the blown A/C fuse.