Climate Control: Condenser

The compressor pumps the refrigerant to the top of the condenser. Almost all of it is a high-pressure vapor, at this point. Because of its high pressure, the temperature at which it can condense is much higher. The high pressure allows the refrigerant to change from a vapor to a liquid, when ambient air, passing over the condenser, carries some of its heat away. Most of the refrigerant is a high-pressure liquid by the time it reaches the bottom of the condenser. The condenser (like the radiator and the heater core) is also a "heat exchanger."

Fig 1: Identifying Condenser Components

Courtesy of BMW OF NORTH AMERICA, INC.

The condenser on BMW A/C systems is equipped with an auxiliary fan that provides additional air flow through the radiator and condenser, when needed.

Auxiliary fan control systems vary from vehicle to vehicle. The following is a typical "basic" example of how an auxiliary fan is controlled.

The auxiliary fan is controlled by two normally open relays, a normal-speed relay, which runs the fan at the "normal" speed; and a high-speed relay, which runs the fan at the "high" speed.

The A/C control module grounds the normal-speed relay whenever the A/C system is turned on. This causes the fan to run at the normal speed.

The relays are also energized by a (normally open) double temperature switch, which senses coolant temperature in the radiator. When coolant temperature rises above 180°F (82°C), the normal-speed half of the switch closes, powering the normal-speed relay, and the auxiliary fan runs at the normal speed, whether or not the snowflake button is depressed.

When the temperature rises above 190°F (88°C), the high-speed half of the switch closes, powering the high-speed relay, and the auxiliary fan runs at high speed.

There is also an intermediate pressure switch fitted to the receiver/dryer. This switch, which is normally open, closes when refrigerant pressure exceeds 260 psi. This energizes the high-speed relay and runs the auxiliary fan at high speed.