PCM Controlled A/C Circuit Diagnosis

Schematic:

CIRCUIT DESCRIPTION
When an A/C mode is selected, the HVAC control head supplies a battery positive voltage 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 positive 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 more than 90 percent.
^ A/C head pressure is more than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition voltage is below 10.5 volts.
^ Engine speed is more than 5440 RPM.
^ ECT is more than 125°C (257°F).
^ IAT is less than 5°C (41°F).

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

DIAGNOSTIC AIDS
Inspect for the following:
Inspect for conditions that will disable A/C operation:
^ Throttle angle more than 90 percent.
^ A/C head pressure more than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition voltage less than 10.5 volts.
^ Engine speed more than 5440 RPM.
^ ECT more than 125°C (257°F).
^ IAT less than 5°C (41°F).

TEST DESCRIPTION

Steps 1 - 7:

Steps 8 - 17:

Steps 18 - 26:

Steps 27 - 36:

The numbers below refer to step numbers on the diagnostic table:
2. Stored diagnostic trouble codes 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 engage if the appropriate conditions have been met.
4. 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.
6. Tests for an A/C request signal from the control head to the PCM.
12. Tests the ignition positive voltage circuit to the A/C relay.
13. Determines whether the A/C relay or the circuitry is at fault.
21. Determines if the PCM is turning the A/C relay ON due to a false A/C request.
22. Isolates the problem to the A/C control head or the PCM.
27. Determines if the A/C relay is malfunctioning or the A/C relay control circuit is grounded all the time.
35. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.