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 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 than 4392 RPM for less than 1 second.
^ 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).

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 than 4392 RPM for less than 1 seconds.
^ 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 NC 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.