24 - Heating and Air Conditioning / Plumbing / COMPRESSOR, A/C / Operation
OPERATION - COMPRESSOR
VARIABLE DISPLACEMENT A/C COMPRESSOR
NOTE:
Typical variable displacement A/C compressor shown.
The Denso 7SBH17C A/C compressor is a Variable Displacement Compressor (VDC) which is driven by the engine through a LE40 Clutch (1) and pulley (2). This compressor uses an integral control solenoid (3) to reduce or increase the stroke of the pistons (4) within the compressor by automatically adjusting the angle of the swashplate (5).
The A/C compressor draws in low-pressure refrigerant vapor from the A/C evaporator through its suction port. It then compresses the refrigerant into a high-pressure, high-temperature refrigerant vapor, which is then pumped to the A/C condenser through the compressor discharge port. The A/C compressor is lubricated by refrigerant oil that is circulated throughout the refrigerant system with the refrigerant (Refer to 24 - Heating and Air Conditioning/Plumbing/OIL, Refrigerant - Standard Procedure) .
The A/C compressor is controlled by the HVAC module. The HVAC module calculates compressor displacement required by A/C system load and demand by monitoring vehicle speed, A/C high side pressure, engine speed, evaporator temperature, accelerator pedal position, ambient temperature and A/C heater request signals. The HVAC module then sends a Pulse Width Modulated (PWM) signal to the A/C compressor control solenoid to increase or decrease refrigerant flow. The amount of refrigerant allowed to pass through the compressor determines the head pressure which controls the angle of the swashplate, which in turn, determines the amount of compressor displacement. When there is no demand for A/C, the swashplate is adjusted to nearly a zero degree angle, which removes compressor torque drag from the engine. When a greater displacement is needed, the PWM duty cycle is higher and when a lesser displacement is needed, the PWM duty cycle is lower. Maximum duty cycle is 100% making the displacement the maximum the compressor is capable of producing. Conversely, minimum duty cycle is limited to approximately 11% of compressor displacement capacity.
This A/C compressor will reduce the displacement of the compressor based on ambient air temperature. For example; as the outside temperature falls from 27° C (80° F) to 19° C (66° F), the amount of refrigerant flow required through the A/C orifice tube to keep the A/C evaporator cool becomes less, and the swashplate is adjusted to a lower angle, which reduces compressor displacement. However, due to the lower outside air temperature, panel air outlet temperature will not noticeably change.
The A/C compressor control system is diagnosed using a scan tool (Refer to 24 - Heating and Air Conditioning - Diagnosis and Testing) .
CAUTION:
Do NOT run the engine with a vacuum pump in operation or with a vacuum present within the A/C system when equipped with a variable displacement A/C compressor. Failure to follow this caution will result in serious A/C compressor damage.
CAUTION:
Be certain to adjust the refrigerant system oil level when replacing an A/C compressor . Failure to properly adjust the refrigerant oil level can prevent the A/C system from operating as designed and can cause serious A/C compressor damage.
