HVAC Control Components
HVAC CONTROL MODULEThe HVAC control module is the interface between the operator and the Instrument Panel Integration Module (IPM). The HVAC control module and the IPM communicate using:
^ HVAC control module data signal
^ HVAC control module clock signal
INSTRUMENT PANEL INTEGRATION MODULE
The purpose of the IPM is to maintain the passenger compartment of the vehicle at a comfortable temperature, to clear fog and frost from the windshield, and to provide ventilation air to the occupants. The IPM processes information provided from:
^ Air temperature sensors
^ Actuators
^ Driver input
The HVAC personalization, if equipped allows two different operators to recall:
^ Air temperature
^ Blower speed
^ Mode
Power is provided to the HVAC control module through the ignition 3 voltage circuit. Two Sources provide power to the IPM:
^ Battery positive voltage, Keep Alive Memory (KAM)
^ Ignition 3 voltage
The IPM is re-calibrated after replacement or an actuator replacement by:
^ Using the scan tool
^ Removing the battery positive voltage circuit fuse
When in automatic mode the system will maintain the air temperature selected unless it is set to full cold or full hot temperature. When at these full extremes the system will position the air temperature actuators to its full cold or hot position depending on the selected temperature.
AIR TEMPERATURE ACTUATOR, 5 WIRE SINGLE CONTROL W/FEEDBACK BACK
The flatpack actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5 volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.
The IPM uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the IPM sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the IPM. Once the position signal and the commanded value are the same, the IPM changes the control signal to 2.5 volts.
AIR TEMPERATURE SENSORS
The ambient temperature sensor is a 2 wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:
^ Ambient Air Temperature Sensor
^ Inside Air Temperature Sensor
^ Upper Left Air Temperature Sensor
^ Upper Right Air Temperature Sensor
^ Lower Left Air Temperature Sensor
^ Lower Right Air Temperature Sensor
The duct sensor temperature response is different from the ambient and inside sensors. A signal and low reference circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases. The sensor signal decreases as the resistance decreases. The sensor signal varies between 0-5 volts. The IPM converts the signal to a range between 0-255 counts.
If the IPM detects a malfunctioning sensor, then the IPM software will use a defaulted air temperature value. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.
Air Temperature Description And Operation:
The HVAC control module displays the ambient air temperature value that it receives from the IPM. The scan tool has the ability to update the displayed ambient air temperature. The ambient air temperature value is displayed or updated under the conditions.
SUNLOAD SENSOR
The sunload sensor is a 2 wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are integrated into the sunload sensor assembly. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor conductance increases. The sensor signal decreases as the conductance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The IPM converts the signal to a range between 0-255 counts.
The sunload sensor provides the IPM a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.
If the IPM detects a malfunctioning sensor, then the IPM software will use a defaulted sunload value. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed.
A/C PRESSURE SENSOR
The A/C refrigerant pressure sensor is used to protect the A/C system. The sensor is mounted on the discharge line of the A/C compressor hose assembly. The sensor output to the powertrain control module (PCM) is varied 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 pressure climbs above 2968 kPa (430 psi), then the PCM will turn off the A/C compressor clutch. If the line pressure falls below 255 kPa (35 psi), then the PCM will turn off the A/C compressor clutch. A 5 volt reference signal is sent from the PCM to the A/C refrigerant pressure sensor over the 5 volt reference circuit. The PCM monitors the A/C pressure by monitoring the 5 volt signal on the A/C refrigerant pressure sensor signal circuit. Ground for the A/C refrigerant pressure sensor is provided by the low reference circuit.
A/C COMPRESSOR TEMPERATURE SWITCH
The A/C compressor is protected from overheating by the temperature switch, which is mounted at the rear of the A/C compressor. The switch is wired internally to the compressor and is not serviceable. The switch opens and disengages the compressor clutch when temperatures reach 155°C (311°F). The switch closes when the compressor temperature cools to 125°C (257°F).
If the compressor temperatures get to high, then the switch opens the ground circuit to the compressor clutch coil, disengaging the A/C compressor clutch. There is no feedback to the PCM that this has occurred. The time frame the switch remains open, depends on how the vehicle is being operated. Conditions when the A/C compressor temperature switch may open:
^ High ambient temperatures
^ Low refrigerant charge
^ Towing a trailer on a long steep grade
^ Descending a grade at high speed in a low gear
DUAL-STAGE ORIFICE
The purpose of the dual stage orifice is to extend the operating range of the A/C system. The Dash Integration Module (DIM) energizes the HVAC solenoid by grounding the A/C orifice relay control circuit. If the DIM energizes the HVAC solenoid, then ignition 3 voltage is supplied to the A/C orifice solenoid through the A/C orifice solenoid supply voltage circuit. The valve allows the compressor to stay on by reducing compressor head pressure during warm to hot ambient drive away conditions. With the valve, a typical head pressure reduction of approximately 276 kPa (40 psi) can be expected during rapid acceleration. The valve reduces compressor cycling under mild ambient conditions. The valve is electronically controlled and the orifice size is determined by the position of a solenoid plunger. The diameters are:
^ Valve energized or ON, the plunger moves to the 1.57 mm (0.062 in) orifice size
^ Valve de-energized or OFF, the plunger moves to 2.03 mm (0.080 in)
The DIM commands the orifice valve relay using the following four class 2 data inputs.
^ A/C compressor clutch state
^ High side pressure
^ Engine RPM
^ Vehicle speed
The valve will remain OFF when the ignition or the A/C system is OFF. Exceeding any one of the following limits will turn the valve to the OFF position.
^ A/C compressor clutch disengagement
^ High side pressure falls below 931 kPa (135 psi)
^ High side pressure exceeds 2620 kPa (380 psi)
^ Engine speed exceeds 3500 RPM
^ Vehicle speed exceeds 101 km/h (63 mph)