The compressor is driven by a drive belt from the engine crankshaft through the compressor clutch pulley. The compressor pulley rotates freely, without turning the compressor shaft, until an electromagnetic clutch coil is energized. When voltage is applied to energize the clutch coil, a clutch plate and hub assembly is drawn rearward toward the pulley. The magnetic field locks the clutch plate and pulley together as one unit to drive the compressor shaft.
As the compressor shaft turns, the compressor performs two main functions: it compresses the low-pressure refrigerant vapor from the evaporator into a high-pressure, high-temperature vapor. The compressor pumps refrigerant and refrigerant oil throughout the air conditioning system.
The compressor is equipped with a pressure relief valve which is placed in the system as a safety factor. Under certain conditions, the refrigerant on the discharge side may exceed the designed operating pressure. To prevent system damage, the valve is designed to open automatically when the high side pressure exceeds a specified amount. Conditions that might cause this valve to open (defective A/C pressure sensor, inoperative cooling fan, etc.) should be corrected, and the refrigerant oil and refrigerant should be replaced as necessary.
Important: Before any work is performed on the air conditioning system, review precautions on handling Refrigerant 134a (R 134a) along with the discharging, recovery and recycling procedures located in this section.
The Variable Displacement Orifice Tube (VDOT) Refrigerant system employs the constant run V-5 compressor. The V-5 is a variable displacement compressor that can match the automotive air conditioning demand under all conditions without cycling. The basic compressor mechanism is a variable angle wobble-plate with 5 axially oriented cylinders. The center of control of the compressor displacement is a bellows actuated control valve located in the rear head of the compressor that senses compressor suction pressure. The wobble-plate angle and compressor displacement are controlled by the crankcase-suction pressure differential. When the A/C capacity demand is high, the suction pressure will be above the control point: the valve will maintain a bleed from the crankcase to suction; no crankcase-suction pressure differential; and the compressor will have maximum displacement. When the A/C capacity demand is lower and the suction pressure reaches the control point, the valve will bleed discharge gas into the crankcase and close off a passage from the crankcase to the suction plenum. The angle of the wobble-plate is controlled by a force balance on the 5 pistons. A slight elevation of the crankcase-suction pressure differential creates a total force on the pistons resulting in a movement about the wobble-plate pivot pin that reduces the plate angle.
When the engine is turned OFF with the A/C system operating, the refrigerant in the system will flow from the high pressure side of the expansion tube (orifice) to the low pressure side until the pressure is equalized. This may be detected as a faint sound of liquid flowing (hissing) for 30 to 60 seconds and is a normal condition.
Refer to VDOT A/C System Diagnosis , HVAC Compressor Clutch Does Not Engage or A/C Idle Circuit Diagnosis in Engine Controls for diagnosis.
