Air Conditioning (A/C) Compressor Control

WARNING: This page is about a different car, the 2006 Land Rover Range Rover Sport. However, it is still accessible from the selected car via links, so may be relevant.

The variable displacement air conditioning (A/C) compressor is permanently driven by the engine. The flow of refrigerant through the air conditioning (A/C) compressor, and the resultant system pressure and evaporator operating temperature, is regulated by the refrigerant solenoid valve. Operation of the refrigerant solenoid valve is controlled by the automatic temperature control (ATC) module using a 400 Hz pulse width modulation (PWM) signal. The duty cycle of the pulse width modulation (PWM) signal is calculated using the following parameters:

A/C compressor torque.
A/C compressor torque maximum.
A/C cooling status.
A/C demand.
A/C refrigerant pressure.
Ambient air temperature.
Blower speed.
Engine cranking status.
Evaporator temperature.
Transmission gear status.

When A/C is selected, the ATC module maintains the evaporator at an operating temperature that varies with the in-vehicle cooling requirement. The ATC module increases the evaporator operating temperature, by reducing the refrigerant flow, as the requirement for air cooling decreases, and vice versa. During an increase of evaporator operating temperature, to avoid compromising the dehumidification function, the ATC module controls the rate of temperature increase, which keeps the cabin humidity at a comfortable level.

When the economy mode is selected, the PWM signal holds the refrigerant solenoid valve in the minimum flow position, effectively switching off the A/C function.

The ATC module incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high pressure limit, the duty cycle of the PWM signal is progressively reduced until the system pressure decreases. When the system pressure falls below the low pressure limit, the duty cycle of the PWM signal is held at its lowest setting, so that the A/C compressor is maintained at the minimum stroke, to avoid depletion of lubricant from the A/C compressor. The protection algorithm is calculated at a high rate, to enable early detection of the rapid pressure changes possible if a system fault develops.