Air Conditioning Control System
Description
Air Conditioning Control System
In conjunction with the Engine Control Module (ECM), the air conditioning control system operates the cooling/condenser fans and the compressor clutch to control the flow of refrigerant through the system.
The air conditioning control system comprises of a compressor clutch relay, an evaporator temperature sensor, a refrigerant pressure sensor, a cooling fan control module and control switches. These controls, in conjunction with the cooling fans, compressor clutch, blower and heater distribution and blend unit, maintain the required environment inside the vehicle with minimal input from the driver.
When air conditioning is not selected, air is supplied by ram effect or blower operation to the areas selected by the air distribution control. The air mix flap on the heater assembly blend unit controls the temperature of the air being delivered. No cooled air is available.
Selecting air conditioning provides the added facility of cooled air available to be mixed with heated air in the blend unit. When required, a fully cold condition can be selected by turning the temperature control selector to the cold position, this automatically closes the path of inlet air through the heater matrix.
Mixtures of cooled, fresh, and hot air can be selected to give the required interior environmental conditions by selection at the control panel.
Evaporator Temperature Sensor
The evaporator temperature sensor is a Negative Temperature Coefficient (NTC) encapsulated thermistor installed in the air outlet side of the evaporator. On K1.8 and non NAS KV6 models, the evaporator temperature sensor signal is fed directly to the ECM. On Td4 and NAS KV6 models, the evaporator temperature sensor is connected to the instrument pack, and the signal is relayed to the ECM from the instrument pack via the CAN Bus.
The temperature signal is used to prevent the air conditioning system from operating when the evaporator is frozen. Frosting of the evaporator cooling fins will cause a reduction in the effectiveness of the cooling system.
If the temperature at the evaporator falls low enough for ice to form on the fins, the ECM withholds or discontinues engagement of the compressor clutch. When the temperature at the evaporator rises sufficiently, the ECM engages the compressor clutch.
The evaporator temperature sensor is also used in conjunction with the refrigerant pressure sensor to facilitate compressor load prediction for optimum idle speed control and load management. The A/C system places an extra load on the engine when the compressor is operating, so the ECM automatically adjusts the idle speed to compensate for the additional load.
Refrigerant Pressure Sensor
The refrigerant pressure sensor is located in the refrigerant lines. On LHD vehicles with KV6 engines it is located at the RH side of the engine compartment close to the outlet from the condenser in the refrigerant line leading to the thermostatic expansion valve. On all other engine/vehicle derivatives the sensor is located in the same refrigerant line at the LH side of the engine compartment. The refrigerant pressure sensor provides the ECM with a pressure input from the high pressure side of the refrigerant system.
The ECM uses the signal from the refrigerant pressure sensor to protect the system from extremes of pressure, by disengaging the compressor clutch. The signal is also used for cooling fan control. The temperature sensor used has a low pressure range of 0 - 600 psi and provides the following functions:
- Provide a safety cut-out function if the refrigerant pressure goes either too high or too low.
- Indicate when the refrigerant pressure reaches such a point that additional cooling is required - if the pressure reaches the medium point, the cooling fans will be switched to high speed.
- The pressure sensor is used in conjunction with the evaporator temperature sensor to predict compressor load for load management at idle/part throttle.
On K1.8 and non NAS KV6 models, the refrigerant pressure sensor signal is fed directly to the ECM. On Td4 and NAS KV6 models, the refrigerant pressure sensor is connected to the instrument pack, and the signal is relayed to the ECM from the instrument pack via the CAN Bus.
Because the compressor is lubricated by oil suspended in the refrigerant, a low pressure signal from the sensor is used by the ECM to prevent operation of the compressor unless there is a minimum refrigerant pressure, and thus refrigerant and oil in the system.
Control Switches
The control switches consist of two latching push switches installed in the centre console, an air conditioning switch and a fresh/recirculated air switch. Each switch contains an amber indicator lamp which illuminates when air conditioning or recirculated air is selected, as applicable.
The A/C switch is supplied with a positive feed through the blower motor switch when one of the four blower speeds is selected. When the A/C switch is selected, a signal is supplied to the instrument pack through a hard wire connection. The instrument pack interprets the A/C request signal and transfers the request to the ECM via the CAN bus.
On vehicles from 2003 model year, an automatic A/C on feature is introduced. This feature automatically selects A/C on when the air distribution control is moved to the feet/screen or screen positions. A microswitch, which is operated by the distribution control knob, is wired in parallel with the A/C switch and has the same functionality. The automatic on function cannot be overridden and there is no indicator lamp to display when the A/C is active. The blower motor switch must be in positions 1 to 4 before the automatic on function will operate.
Compressor Clutch Relay
The compressor clutch relay switches power to the compressor clutch under the control of the ECM. The relay is located in the engine compartment fusebox. The compressor clutch is energized to engage and de-energized to disengage.
Compressor
Operation of the clutch is controlled by the engine control module (ECM). To protect the refrigerant system from unacceptably high pressure, a pressure relief valve is installed in the outlet side of the compressor. The pressure relief valve is set to operate at 3430 kPa (497.5 lbf.in) and vents excess pressure into the engine compartment.
The ECM controls the operation of the compressor via the compressor clutch relay in the engine compartment fuse box. When the A/C switch is used to request air conditioning, the ECM energises the compressor clutch relay to supply a power feed to the compressor clutch. Engagement of the compressor clutch is withheld, or discontinued, if refrigerant pressure exceeds upper or lower pressure limits:
- The upper pressure limit is 29 bar (421 sq.lbf/in), e.g. due to a blockage. Compressor engagement is reenabled when the pressure decreases to 23 bar (334 sq.lbf.in).
- The lower pressure limit is 1.6 bar (23.2 sq.lbf/in), e.g. due to a leak. Compressor engagement is re-enabled when the pressure increases to 2.0 bar (29.0 sq.lbf/in).
Cooling and Condenser Fans
The cooling fans operate automatically whenever the air conditioning system is switched on, provided the system pressure is correct. The motors of the two fans are variable speed and are switched together in parallel.
The ECM controls the operation of the variable speed cooling fans via a cooling fan controller installed on the back of the cooling fan assembly. The control signal from the ECM to the fan controller is a pulse width modulated (PWM) signal. The output from the ECM runs from 10% to 90%, corresponding to 0% to 100% fan current output from the interface. PWM signals below 10% or greater than 90% correspond to open and closed loop circuit failures, which can be diagnosed using T4. The cooling fan controller receives the PWM signal and regulates the power feeds to the cooling fans accordingly to produce the required fan speed.
When air conditioning is selected, the cooling fans initially operate at minimum speed. If the pressure increases to 16 bar (232 sq.lbf/in), the speed of the fans progressively increase up to a maximum speed at 27 bar (392 sq.lbf/in). When the fans are at maximum speed, if the refrigerant pressure decreases, the speed of the fans progressively decrease down to a minimum speed at 15 bar (218 sq.lbf/in). Operation of the cooling fans is also affected by coolant temperature.