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 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 4,500 RPM for longer than 5 seconds. |
| • | Engine speed is greater then 5,400 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 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 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 4,500 RPM for longer than 5 seconds. |
| • | Engine speed greater then 5,400 RPM for any amount of time. |
| • | ECT greater than 125°C (257°F). |
| • | IAT less than 5°C (41°F). |
