Using inputs to both the powertrain control module (PCM)
and to the heater and A/C programmer, the PCM controls the compressor clutch.
The following components are the inputs to the PCM:
| • | The heater and A/C programmer request for the compressor |
| • | The heater and A/C programmer's A/C possible flag |
The following components are the inputs to the heater and A/C programmer:
| • | The low refrigerant pressure switch |
| • | The high refrigerant pressure switch |
| • | The A/C high side temperature |
| • | The A/C low side temperature |
| • | The rear defogger switch (from the instrument panel cluster (IPC)) |
| • | The ambient air temperature |
The voltage source to the compressor clutch is from the 10 A
A/C fuse through the A/C compressor clutch relay which is under PCM control.
When the following inputs to the heater and A/C programmer are at the
indicated values, the heater and A/C programmer will signal the PCM that compressor
operation is possible:
| • | The climate control setting is not OFF or ECON. |
| • | The outside temperature is above 4°C (40°F). |
| • | No compressor disabling DTCs are set. |
Once engagement is possible, 2 modes of compressor clutch control
are available for the minimum time the clutch will stay on. These modes of
clutch control are based upon vehicle speed.
Extended Compressor at Idle
This mode allows for smoother idle conditions by increasing the minimum
clutch ON time to a maximum of 45 seconds whenever the vehicle speed
drops below 24 km/h (15 mph).
Minimum Compressor on Time
Vehicles operating above 32 km/h (20 mph) are controlled
to a minimum ON time of 4-6 seconds before the heater and A/C
programmer will allow the clutch to disengage.
After the minimum ON time has expired, the heater and A/C programmer
will request the powertrain control module (PCM) to disengage the compressor
clutch only if the low side temperature is below -2°C (28°F).
After reaching this turn-off temperature, the heater and A/C programmer
waits for the low side to reach a turn-on temperature of 10°C (50°F).
The cycle then repeats. During the compressor ON cycle any of the following
conditions will force the PCM to disengage the clutch:
| • | DTC B1347--Repeated occurrences of an A/C low side
temperature below -7°C (20°F) and/or the A/C low pressure switch
is open. |
| • | DTC B1348--The A/C low pressure switch is open due
to a catastrophic loss of refrigerant. |
| • | DTC B1350--The coolant temperature is above 126°C
(259°F). |
Except for the wide open throttle, any one of the above conditions will
keep the compressor OFF until the condition no longer exists. The wide open
throttle condition will keep the compressor OFF for a maximum of 20 seconds.
On the initial crank, the heater and A/C programmer will search for
indications of liquid refrigerant which may have collected at the compressor.
If liquid refrigerant is found, the A/C clutch engages. Shortly after
the engine starts, the clutch disengages and waits for the normal calibrations
and parameters to be met before signaling the clutch.
Idle Speed Control
In order to minimize the effects of the compressor clutch operation
on the engine idle speed, the powertrain control module (PCM) can modify the
engine revolutions per minute (RPM) during the A/C operation. When the
PCM receives the clutch engagement signal from the heater and A/C programmer
in anticipation of the engine load, the PCM makes an adjustment in the
idle speed control motor in order to compensate.
Low Refrigerant Message
In the process of controlling the compressor clutch, the heater and
A/C programmer monitors certain system inputs for an indication of a low refrigerant
charge. If the system's refrigerant state of charge should fall below approximately
1/4 of capacity, the following conditions are present:
| • | The VERY LOW A/C REFRIGERANT -- COMPRESSOR OFF message displays. |
| • | The heater and A/C programmer will store DTC B1347 and/or
DTC B1348 in memory. |
