Fant dette på et forum,
A/C Compressor Control The variable displacement A/C compressor is permanently driven by the engine. The flow of refrigerant through the A/C compressor, and the resultant system pressure and evaporator operating temperature, is regulated by the refrigerant solenoid valve. Operation of the refrigerant solenoid valve is controlled by the ATCM using a 400 Hz PWM signal. The duty cycle of the PWM signal is calculated using the following parameters: â€¢ A/C compressor torque â€¢ A/C compressor torque maximum â€¢ A/C cooling status â€¢ A/C demand â€¢ A/C refrigerant pressure â€¢ Ambient air temperature â€¢ Blower speed â€¢ Engine cranking status â€¢ Evaporator temperature â€¢ Transmission gear status. When A/C is selected, the ATCM maintains the evaporator at an operating temperature that varies with the in-vehicle cooling requirement. The ATCM increases the evaporator operating temperature, by reducing the refrigerant flow, as the requirement for air cooling decreases, and vice versa. During an increase of evaporator operating temperature, to avoid compromising the dehumidification function, the ATCM controls the rate of temperature increase, which keeps the cabin humidity at a comfortable level. When the economy mode is selected, the PWM signal holds the refrigerant solenoid valve in the minimum flow position, effectively switching off the A/C function. The ATCM incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high pressure limit, the duty cycle of the PWM signal is progressively reduced until the system pressure decreases. When the system pressure falls below the low pressure limit, the duty cycle of the PWM signal is held at its lowest setting, so that the A/C compressor is maintained at the minimum stroke, to avoid depletion of lubricant from the A/C compressor. The protection algorithm is calculated at a high rate, to enable early detection of the rapid pressure changes possible if a system fault develops. The refrigerant pressure sensor provides the ATCM with a pressure input from the high pressure side of the refrigerant system. The sensor is located in the refrigerant line between the condenser and the thermostatic expansion valve. The ATCM supplies a 5V reference voltage to the refrigerant pressure sensor and receives a return signal voltage, between 0 and 5V, related to system pressure. The ATCM uses the signal from the refrigerant pressure sensor to protect the refrigerant system from extremes of pressure and to calculate A/C compressor load on the engine. The ATCM also transmits the A/C compressor load value to the ECM, via the medium speed CAN bus, instrument cluster and high speed CAN bus, for use in controlling the speed of the engine cooling fan. To protect the system from extremes of pressure, the ATCM sets the A/C compressor to the minimum flow position if the pressure: â€¢ Decreases to 1.9 â€“ 0.2 bar (27.5 â€“ 3 lbf/in2): the A TCM loads the A/C compressor again when the pressure increases to 2.8 â€“ 0.2 bar (40.5 â€“ 3 lbf/in2). â€¢ Increases to 33 â€“ 1 bar (479 â€“ 14.5 lbf/in2): the ATCM loads the A/C compressor again when the pressure decreases to 23.5 â€“ 1 bar (341 â€“ 14.5 lbf/in2).