The compressor for the air conditioning system is belt-driven
by the engine through the A/C compressor clutch. The clutch controls the compressor
in the following ways:
| • | Disengages when air conditioning is not requested |
| • | Disengages when the engine load must be reduced |
The heater and A/C control communicates the mode selection to the heater
and A/C programmer (HCM) over the keyboard display driver (KDD) data bus.
This communication link is only used by the HCM and heater and A/C control.
The HCM requests A/C compressor clutch engagement with the system operating
in the following modes:
Under normal operating conditions, the following events occur:
| • | The HCM sends an A/C operation desired message to the body control
module (BCM) |
| • | The BCM sends a request for A/C compressor clutch engagement to
the powertrain control module (PCM) |
| • | The PCM supplies a ground to the A/C compressor control relay
coil |
| • | The A/C CLU relay energizes |
| • | Voltage is supplied to the A/C compressor clutch through the A/C
CLU fuse |
| • | The A/C compressor clutch engages |
The PCM monitors A/C refrigerant line pressure and engine coolant temperature.
This information is placed on the Class 2 communication link and is monitored
by the BCM. The BCM looks at A/C line pressure, engine coolant temperature
and battery voltage. The BCM will request A/C operation from the PCM if
these parameters are within normal operation limits and the BCM is requesting
A/C operation.
If the PCM determines that the engine load should be reduced, such as
during full throttle operation, the PCM will de-energize the A/C compressor
control relay. This will disengage the compressor, even though the request
for A/C compressor clutch engagement from the BCM is still present.
The A/C compressor clutch diode is connected across the A/C compressor
clutch terminals. This diode provides a path for current resulting from voltage
spikes. The collapsing magnetic field of the A/C compressor clutch coil generate
these voltage spikes. Voltage spikes occur every time the coil is de-energized.
