# Heat and Air Conditioning system description

## Heating

The heating system utilizes engine coolant via a heater core to heat the cabin. Fresh air is brought into the car from intake vents below the windshield. The air is then directed through the vents in the dash. Temperature is adjusted via a mixing flap that allows a certain amount of air to pass over the heater core. The heater core can be closed off via the heater valve to prevent heat leakage during hot weather operation.

During high heat conditions, intake air in an air conditioned vehicle can be shut off via the Recirculation switch, the system then retrieves air from the inside of the car, allowing the already cool air to be passed back over the air conditioning evaporator. This increases the efficiency of the air conditioning system, but can result on fogged windows on high humidity days.

## Air conditioning

Switching on the Air Conditioning system engages an electromagnetic clutch located on the compressor. This brings the compressor to engine speed.

The compressor raises the pressure of the refrigerant , which , in turn, raises the temperature of the refrigerant.

The refrigerant then passes through the condenser which turns it into a liquid and draws off heat into the airflow from the vehicles motion.

The liquid refrigerant then passes through the receiver drier which extracts any water from the refrigerant.

The receiver drier also contains 2 pressure switches, ( one dual purpose switch on some vehicles ) that shut off the compressor if the refrigerant pressure exceeds or drops below the threshold specified in the table below. This system prevents damage to the AC components from excess pressure, or from wear from low pressure, as the system uses refrigerant transported oil for lubrication.

##### High Pressure switch :

Switch off | 25.5 - 27.7 bar (363 - 394 psi)
Switch back on | 20.0 - 22.5 bar (284 - 320 psi)
Operating pressure | 0.034 - 35 bar (0.5 - 498 psi)

##### Low Pressure switch :

Switch off | 1.72 - 2.22 bar (24.5 - 31.5 psi)
Switch back on | 2.0 -2.4 bar (28.5 - 34 psi)
Operating pressure | 0.034 - 35 bar (0.5 - 498 psi)

The refrigerant then passes into the evaporator, located inside the car, through the expansion valve, which limits the amount of refrigerant that can pass, creating a low pressure point within the evaporator, allowing the refrigerant to revert to a gaseous state. This procedure creates a cooling effect. The blower passes air over and around the evaporator, cooling the air as it goes by.

There is a temperature switch in the evaporator that will shut off the compressor as necessary to prevent freezing.

The temperature change of the air passing over the evaporator causes condensation which is removed via drain tubes extending through the transmission tunnel, allowing the water to escape the vehicle.

Activating the air conditioning also turns on the auxiliary fan mounted to the condenser. This fan will always run on low speed when the AC is active, or if coolant temperature in the radiator exceeds 89 to 93 C (192 - 199 F) the fan will switch off again, unless the AC is on, at 82 - 86C (180 -187F) The fan will switch to high speed, regardless of AC setting, if the coolant temperature in the radiator exceeds 97 - 101C ( 207-214F) and shuts back to low speed when the coolant temp drops to 90-94C (194-201F)

## Controls

Temperature control opens the heater valve when turned beyond 20° of its full cold position.

Fan switch supplies power to the fan for the first 3 speeds via fixed resistors located at the blower motor. The 4th speed is from unrestricted power supplied directly to the fan motor.

Distribution levers supply air flow according to driver request. Any lever in the far left position closes the appropriate vents.
Moving the upper lever to the right opens the defrost vents, directing air to the windshield.
Moving the center lever to the right opens the main dash vents. These vents allow fresh or cooled air only, it cannot be heated.
Moving the lower lever to the right opens the footwell vents directing air to the feet.

In air condition equipped vehicles, the center and lower levers contain micro-switches and must be placed beyond 20 % open in order for the air conditioning compressor to engage. This is done to ensure airflow over the evaporator and prevent potentially serious problems occurring in the air conditioning system. These switches were eliminated as of 1/88 production.

AC button is used to switch the compressor on and off, when AC is selected, the blower motor will always run in , at least, the lowest speed, despite the setting of the fan switch. The auxiliary fan mounted to the condenser will also run on low speed. The AC compressor cycles normally during operation, when the compressor shuts off , so does the auxiliary fan, but the blower motor continues to run on , at least, low speed.

Recirculation button is used to open or close the fresh air flaps. When this switch is activated the blower will come on low speed automatically. When the fresh air flaps are open, there is natural airflow through the vehicle, the blower activation during recirculation operation is to allow continued air flow, only now it is internal air being re-circulated rather than fresh air flowing in from outside.

The car has vents below the rear window that allows air to enter the trunk, where, in turn, it exits the vehicle from vents located behind each rear wheel. This prevents problems associated with air pressure inside the vehicle. |

E30 M3 Repair Manual V2.7. Copyright ©1999-2001 Koala Motorsport & Brett Anderson