A/C Mode

The purpose of the Air Conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The A/C system is engaged when the HVAC control module is in any mode except VENT and HTR. The A/C system will operate regardless of the temperature setting, as long as A/C line pressures are between 255-2968 kPa (35-430 psi).

The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit and battery positive voltage circuit. The ground circuits provide ground for the module.

When an A/C request is selected, the HVAC control module sends a class 2 serial data circuit to the DIM. In order for the Powertrain Control Module (PCM) to internally ground the A/C compressor clutch relay control circuit, the DIM and PCM must communicate to each other over the class 2 serial data circuits. The PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on the class 2 serial data circuit and is monitored by the DIM. The DIM looks at the A/C line pressure, engine coolant temperature and battery voltage along with the A/C request input from the HVAC control module. The DIM will request A/C operation from the PCM if these parameters are within normal operating limits and the HVAC control module is requesting A/C operation.

The PCM turns ON the A/C compressor by providing a path to ground through the A/C compressor clutch relay control circuit for the A/C compressor clutch relay. Power is provided to the A/C compressor clutch relay internally of the underhood fuse block. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned OFF, the A/C compressor clutch diode provides a path for the voltage spike resulting from the collapsing magnetic field of the compressor clutch coil. The ground circuit provides a pathway to ground for the compressor. The A/C compressor clutch relay control circuit is grounded internally within the PCM. The A/C compressor clutch will be shut down under high electrical loads. The blower motor speeds will be affected first, then the compressor will be turned OFF, unless system is in defrost mode. The HVAC system will operate as if the OFF switch has been pressed. When this happens, the driver will still have the ability to override this mode.

The PCM will engage the A/C compressor clutch any time the engine speed is below 5000 RPM and the A/C is requested unless any of the following conditions exist:
^ Throttle angle is at 100 percent (WOT).
^ Vehicle launch, acceleration from a stop.
^ Idle quality
^ The A/C refrigerant pressure sensor is more than 2968 kPa (430 psi) or is less than 255 kPa (35 psi).
^ Engine speed is more than 5500 RPM.
^ Engine Coolant Temperature (ECT) is more than 121°C (250°F).
^ Transmission shift
^ Engine torque load

Engine Coolant Temperature Sensor
The engine coolant temperature sensor provides the PCM with the temperature of the engine coolant. A 5 volt reference signal is sent from the PCM to the engine coolant temperature sensor over the ECT sensor signal circuit. A variable resistor inside the sensor varies the voltage. That varied voltage provides a signal back to the PCM. Remaining voltage from the engine coolant temperature sensor is sent back to the PCM through the low reference circuit.

A/C Refrigerant Pressure Sensor
The A/C system is protected by the A/C refrigerant pressure sensor. The sensor is mounted on the discharge line of the A/C compressor hose assembly. The sensors' output to the PCM is variable and is dependent upon pressure inside the line. A higher pressure results in a higher voltage output. The A/C pressure is constantly monitored in order to allow the A/C compressor clutch to disengage as needed. If line pressures climb above 2968 kPa (430 psi) or fall below 255 kPa (35 psi), the PCM will turn OFF the A/C compressor clutch.

A 5 volt reference signal is sent out over the 5 volt reference circuit, from the PCM, to the A/C refrigerant pressure sensor. The PCM monitors the A/C pressure by sending out a separate 5 volt signal on the A/C refrigerant pressure sensor signal circuit. This circuit is how the PCM monitors HVAC pressures. Ground for the A/C refrigerant pressure sensor is provided by the low reference circuit.