Compressor Regulation - Cooling

The E39, 9/97 E38 750iL, E46 - IHKA uses a new variable displacement A/C compressor.

The swash plate of the compressor is hinged so that it can vary the piston travel based on the output requirements of the system. The swash plate position is controlled by the control valve located in the compressor.

The control valve regulation is based on the low and high side pressures of the system. A "high" low side pressure (high load) will cause the control valve to close and block discharge pressure from entering into the crankcase of the compressor.

Fig 1: Identifying Compressor Regulation - Cooling Components (1 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

When the low side pressure decreases, the control valve opens. The swash plate moves to a position of minimum travel and consequently reduces the compressor output.

The compressor output varies continually based on the constant change in the contributing pressures.

Fig 2: Identifying Compressor Regulation - Cooling Components (2 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

At low engine RPMs and/or high temperature loads, the piston travel (displacement) of the compressor pistons are at the maximum point. This allows the compressor to provide maximum cooling efficiency at idle speeds and when high output is required (heavy demand for cooling).

Fig 3: Identifying Low Side Pressure (High)

Courtesy of BMW OF NORTH AMERICA, INC.

At higher engine RPMs and when the load on the system is low, the swash plate moves so that the piston travel is shortened.

This reduces the constant high load output of the compressor any time the A/C system is on. It also reduces the cycling of the compressor due to the low temperature of the evaporator (evaporator temperature sensor causing the system to cycle at 3°C). An overall effect of this is improved fuel economy.

Fig 4: Identifying Low Side Pressure (Low)

Courtesy of BMW OF NORTH AMERICA, INC.