Compressor Clutch
The compressor clutch assembly consists of a stationary electromagnetic coil with a zener diode, a hub bearing and pulley assembly, and a clutch plate. See Fig 1. The electromagnetic coil unit and the hub bearing and pulley assembly are each retained on the nose of the compressor front housing with snap rings. The clutch plate is keyed or splined to the compressor shaft, and secured with a bolt. The compressor clutch plate and pulley unit, or the clutch coil are available for separate service replacement. The clutch coil zener diode is integral to the clutch coil pigtail wire and connector and, if faulty or damaged, the clutch electromagnetic coil unit must be replaced.
The compressor clutch components provide the means to engage and disengage the compressor from the engine serpentine accessory drive belt. When the clutch coil is energized, it magnetically draws the clutch plate into contact with the clutch pulley and drives the compressor shaft. When the coil is not energized, the pulley freewheels on the clutch hub bearing, which is part of the pulley. A zener diode is connected in parallel with the clutch electromagnetic coil. This diode controls the dissipation of voltage induced into the coil windings by the collapsing of the electromagnetic fields that occurs when the compressor clutch is disengaged. The zener diode dissipates this induced voltage by regulating a current path to ground. This arrangement serves to protect other circuits and components from potentially damaging voltage spikes in the vehicle electrical system that might occur if the voltage induced in the clutch coil windings could not be dissipated. The compressor clutch engagement is controlled by several components: the HVAC controls in the passenger compartment, the A/C pressure transducer on the liquid line, the evaporator temperature sensor on the expansion valve, the Powertrain Control Module (PCM) in the engine compartment, and the compressor clutch relay in the Intelligent Power Module (IPM). The PCM may delay compressor clutch engagement for up to thirty seconds.