Air Conditioning System

GENERAL
All cars with ACC and certain cars with a heating and ventilation system are equipped with air conditioning (A/C).

The A/C system is switched on and off with A/C switch 169 or from ACC panel 216 via EDU 210. Compressor 170 and auxiliary A/C radiator fan 172 are activated with relay 156.

The following conditions must be met in order for switch 169 to be supplied with current from fuse 6:
^ Main ventilation fan switch 149 closed, i.e. air distribution control open.
^ Ventilation fan on, i.e. switch 35 in any of positions 1-4.

As soon as switch 169 is depressed, the integrated lamp is supplied with current directly. When the switch is not depressed, the lamp is supplied with current from rheostat 16.

For relay 156 to operate and start the A/C compressor, the following conditions must be met:
^ Contacts H and L in pressure switch 166 must be closed (normal position).
^ Antifrost thermostat 171 must be closed, i.e. the ambient temperature must be about +6 °C (+43 °F) at least.
^ The temperature of the coolant must be lower than 119 °C (247 °F) in a 4-cyl. engine and 116 °C (240 °F) in a V6 engine. The temperature is measured by the EDU control module.

If these conditions are satisfied, operating current will be supplied from pin 4 of the EDU control module, via pressure switch 166 and antifrost thermostat 171, to the engine management system, which then grounds relay 156. Compressor 170 and radiator fan 172 start.

When the evaporator temperature drops towards freezing point, the antifrost thermostat opens and the engine management system disconnects relay 156. The compressor and radiator fan stop.

The temperature of the evaporator starts to rise and when it reaches the antifrost switching point, relay 155 will again be grounded and the compressor and radiator fan will start up once more.

When the system is cycling, the output cooling effect is less than what the A/C system is capable of producing. High fan speed, high outside temperature and the speed of the car are factors which strongly influence the ability of the system to reach the cycling point.

DESCRIPTION OF OPERATION
Pressure Switch 166
Switches L and H are safety switches to protect against too low and too high refrigerant pressure. They are normally closed but switch L opens if the pressure drops below about 2 bar. At outside temperatures below 0 °C (32 °F) the pressure will not be higher than this and switch L deactivates the system.

If the refrigerant pressure exceeds 27 bar, switch H opens and the system is deactivated.

Antifrost Thermostat 171
When the temperature of the evaporator drops to 2.0 +/- 1.1 °C (35.5 +/-2 °F), the thermostat opens and then closes when the temperature rises to 3 °C (5 °F) above its opening temperature.

3-Stage Pressure Switch 166
The regular radiator fan 37 is controlled via contact M in pressure switch 166. The switch is normally open when the system is switched on. When the refrigerant pressure reaches 16 bar, the switch closes and relay 155 operates, thus starting the fan motor.

The switch then goes on and off, depending on the degree of cooling. Switch-off occurs at 12 bar.

For connection of radiator fan 37, see the Cooling System wiring diagram.

Engine Management Systems
To prevent engine speed from dropping when the compressor is engaged, the control module compensates for the increased load by raising the idling speed.

Trionic
In the Trionic system, operating current is supplied from antifrost thermostat 171 to pin 59. Control module 430/589 grounds relay 156 via pin 54.

Motronic M2.8.1
In the Motronic M2.8.1 system, operating current is supplied from antifrost thermostat 171 to pin 40 of control module 510. Pin 25 of the control module grounds relay 156.

Motronic M5.2
In the Motronic M5.2 system, operating current is supplied from antifrost thermostat 171 to pin 59 of control module 587. Pin 54 of the control module grounds relay 156.

Recirculation Motor
When switch 143 is pressed, motor 38 turns the air recirculation flap to the recirculation position, i.e. no fresh air is supplied to the cabin. When the switch is released by being pressed again, the motor returns the flap to the fresh air mode.

The switch is supplied with current via fuse 1. Its integral lamp lights up at full intensity when in the depressed position.