The air conditioning system uses a compressor with variable
displacement.
The A/C compressor clutch is controlled by the PCM. The HVAC controller
sends the A/C request to the Powertrain Control Module (PCM). This allows
the PCM to manage idle quality, improve wide open throttle performance,
and to protect the A/C system from excessively high and low pressures.
The PCM controlled air conditioning system uses a variable displacement
compressor. The compressor meets the A/C requirements without cycling. For
a description of the system and an explanation of the components used, refer
to A/C System Description in HVAC.
The A/C electrical system consists of the following components:
| • | A/C refrigerant pressure switch |
| • | A/C compressor clutch relay |
When A/C is selected through the A/C control head a signal
is communicated to the PCM. The A/C compressor clutch relay is controlled
through the PCM. This allows the PCM to increase the
engine idle speed just prior to A/C clutch engagement for better
idle quality. In addition the PCM will command the cooling
fans on during A/C operation. The PCM monitors the A/C
refrigerant pressure. If the A/C refrigerant pressure,
and engine operating conditions are within a specific calibrated
acceptable ranges the PCM will enable the A/C compressor
relay. This is accomplished by providing a ground path
for the A/C relay coil within the PCM. When the A/C compressor
relay is enabled 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 any of the following conditions
are met:
| • | Throttle greater than 90% |
| • | A/C head pressure greater than 427 psi (4.27 volts)
or less than 41 psi (0.35 volt) (as determined by the A/C refrigerant
pressure sensor). |
| • | Ignition voltage below 9.5 volts. |
| • | Engine speed greater than 6900 RPM for 5 seconds. |
| • | ECT greater than 121°C (250°F). |
| • | IAT less than 0°C (32°F) |
The
Air Conditioning (A/C) Compressor Control Circuit Diagnosis
should be used for diagnosing the electrical
portion of the A/C compressor clutch circuit. When diagnosing the refrigerant
portion of the HVAC system, the VDOT A/C System Diagnosis should
be used.
A scan tool will be used in diagnosing the system. The scan tool has
the ability to read the A/C request and refrigerant pressure inputs to the
PCM. The scan tool can display when the PCM has commanded the A/C clutch
on. The scan tool should have the ability to override the A/C request signal
and energize the A/C control relay.
