A/C Circuit Diagnosis - HVAC System - Automatic

Diagnostic Chart (Part 1 Of 3):

Diagnostic Chart (Part 2 Of 3):

Diagnostic Chart (Part 3 Of 3):

Engine Controls Schematics: Heater-A/C Control And A/C Compressor Clutch Coil:

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 Powertrain Control Module (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:
^ The Throttle angle is more than 96 percent.
^ The A/C head pressure is more than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) as determined by the A/C refrigerant pressure sensor.
^ The ignition 1 voltage is less than 10.5 volts.
^ The engine speed is more than 4300 RPM for more than 1 second.
^ The engine speed is more than 5400 RPM for any amount of time.
^ The Engine Coolant Temperature (ECT) is more than 124°C (255°F).
^ The Intake Air Temperature (IAT) is less than 5°C (41°F).

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

DIAGNOSTIC AIDS
Check for the following conditions:
^ Any poor connection at the PCM - Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connections.
^ A 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 the 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 the A/C operation:
^ The throttle angle is more than 96 percent.
^ The A/C head pressure is more than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) as determined by the A/C refrigerant pressure sensor.
^ The ignition 1 voltage is less than 10.5 volts.
^ The engine speed is more than 4300 RPM for more than 1 second.
^ The engine speed is more than 5400 RPM for any amount of time.
^ The ECT is more than 124°C (255°F).
^ The IAT is less than 5°C (41°F).

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
2. The DTC P1641 diagnostic handles the control portion of the relay circuit. The DTC P0530 diagnostic pertains to the A/C refrigerant pressure sensor. If either DTC is set, perform that diagnostic first.
3. Under certain conditions, the PCM may engage the A/C compressor clutch briefly at start-up. Answer Yes to this step if the A/C compressor clutch remains engaged after the engine is started with all of the A/C controls in the Off position.
If the refrigerant pressure is low, refer to Heating, Ventilation, and Air Conditioning for the diagnostic procedures.
6. Check for poor connections and terminal tension at the fuse. Refer to Testing for Intermittent and Poor Connections in Diagrams.