Heater Mode

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by toggling the temperature control switch, located on the HVAC control module, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C. The vehicle passenger can adjust their temperature by adjusting the right air temperature switch assembly. Passenger temperatures can be offset 4°C (8°F) cooler or warmer than the drivers setting.

The HVAC control module is the interface between the vehicle operator and the Instrument Panel Integration Module (IPM). The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit. The IPM is also powered from the rear fuse block through the ignition 3 voltage circuit along with the battery positive voltage circuit. Both modules are grounded by the ground circuit.

Air Temperature Actuators
When the HVAC system is in the OFF mode, toggling the temperature switch up or down will enable the automatic HVAC system. If the HVAC control module displays any temperature between 19-29°C (66-84°F), toggling the temperature button up/down will increase/decrease the set temperature by 1 degree. If the temperature setting is 30°C (85°F), toggling the temperature switch up increases the set temperature to 32°C (90°F).

The rear fuse block provides power to the left and right air temperature actuators through the ignition 3 voltage circuit. Ground is provided by the low reference circuit, the IPM and ground circuit.

The air temperature actuators are electronic stepper motors with feedback potentiometers. Zero volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the IPM, to the left air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the logic circuit is used to determine the left air temperature door position sensor signals value. A separate 5 volt reference is sent from the IPM to the logic circuit. The IPM software uses this reference voltage to determine the left air temperature actuator position through the left air temperature door position signal circuit. The motor opens the left air temperature door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. Ground is provided to the left air temperature actuator by the ground circuits and IPM.

The right air temperature actuator operates the same as the left side. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the IPM, to the right air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the logic circuit is used to determine the right air temperature door position sensor signals value. A separate 5 volt reference is sent from the IPM to the logic circuit. The IPM software uses this reference voltage to determine the right air temperature actuator position through the right air temperature door position signal circuit. The motor opens the right air temperature door to a position to divert sufficient air past the heater core to achieve the desired passenger temperature. Ground is provided to the right air temperature actuator by the ground circuits and IPM.

Temperature Sensors
The automatic system uses multiple sensors to achieve and maintain the desired temperature. The inside air temperature sensor provides the IPM software with the temperatures of the air drawn through the aspirator. A 5 volt reference signal is sent from the IPM to the inside air temperature sensor over the inside air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. As the air temperature increases, resistance decreases. Remaining voltage from the inside air temperature sensor is sent back to the IPM ground through the low reference circuit.

The lower left air temperature sensor provides the IPM software with the temperature of the air exiting from the left closeout/insulator panel. A 5 volt reference signal is sent from the IPM to the lower left air temperature sensor over the lower left air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. Remaining voltage from the lower left air temperature sensor is sent back to the IPM ground through the low reference circuit.

The lower left air temperature sensor provides the IPM software with the temperature of the air exiting from the right closeout/insulator panel. A 5 volt reference signal is sent from the IPM to the lower right air temperature sensor over the lower right air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. Remaining voltage from the lower right air temperature sensor is sent back to the IPM ground through the low reference circuit.

The upper left air temperature sensor provides the IPM software with the temperature of the air exiting from the left A/C duct. A 5 volt reference signal is sent from the IPM to the upper left air temperature sensor over the upper left air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. Remaining voltage from the upper left air temperature sensor is sent back to the IPM ground through the low reference circuit.

The upper right air temperature sensor provides the IPM software with the temperature of the air exiting from the right A/C duct. A 5 volt reference signal is sent from the IPM to the upper right air temperature sensor over the upper right air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. Remaining voltage from the upper right air temperature sensor is sent back to the IPM ground through the low reference circuit.

The ambient air temperature sensor provides the IPM software with the temperature of the air outside the vehicle and displays that temperature on the HVAC control module. A 5 volt reference signal is sent from the IPM to the ambient air temperature sensor over the ambient air temperature sensor signal circuit. A thermister Inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the IPM. As the air temperature increases, resistance decreases. Remaining voltage from the ambient air temperature sensor is sent back to the IPM ground through the low reference circuit.

Since the sensor is mounted underhood, it can be affected by city traffic, idling, and hot engine restarts. A temperature memory feature is used in the IPM ambient air temperature programming to help provide greater accuracy under engine restart conditions. If the engine coolant temperature is not more than 10°C (18°F) above the ambient air temperature sensor reading, or if the engine has not been started in 2 hours, then the actual ambient air temperature sensor reading is displayed. However, if the engine coolant is more than 10°C (18°F) above the sensor reading, the memorized ambient air temperature is displayed. This is the last displayed temperature sensed when the engine was operating. At vehicle speeds greater than 16 km/h (10 mph), the ambient air temperature displayed may be allowed to increase, but only after a built-in 300 second time delay which allows for ambient air to cool the sensor. The time delay starts when vehicle speed reaches or maintains at least 16 km/h (10 mph). If the sensor reading is ever less than the displayed value, then the ambient air temperature changes are displayed as rapidly as possible.

Sunload Sensor
The sunload sensor assembly houses both the driver and passenger side sensors. The sunload sensor assembly provides the IPM software with the amount of sun light entering the drivers and passengers side through the windshield. A 5 volt reference signal is sent from the IPM to the sunload sensor over the driver or passenger sunload sensor signal circuit. A variable photo-diode resistor inside the sensor varies the voltage. That varied voltage provides a separate driver and passenger signal to the software inside the IPM. The sensor also provides an average reading on the ambient light sensor signal circuit to the IPM for the twilight sentinel feature. Remaining voltage from the sunload sensor assembly is sent back to the IPM ground through the low reference circuit.

Dual Zone Control Switch
The right air temperature switch assembly is provided to allow the passenger to offset air discharge temperatures on the right side of the vehicle. Passenger temperatures can be set 4°C (8°F) cooler or warmer than the drivers setting. The switch assembly has LED's that will illuminate to inform the passenger of the intensity of their offset. To activate the dual zone, the passenger presses either the red or blue switch to the desired offset. Temperature offset will be allowed as long as the set temperature is not in the maximum hot, 32°C (90°F), or cold, 16°C (60°F) settings. If the right air temperature switch assembly has been turned on, it can be turned off by pressing the OFF switch once on the HVAC control module. Pressing the OFF switch a second time will turn off the entire HVAC system. Greater sunload on one side of the vehicle may cause the discharge air temperatures to be different, even when the HVAC system is not operating in dual zone mode.

The rear fuse block provides power to the passenger door module and right air temperature switch assembly through the battery positive voltage circuit. Ground is provided by the ground circuits.

A 5 volt reference is sent to the right air temperature switch assembly through the right air temperature switch signal circuit. The voltage is varied by a series of in-line resistors when a temperature switch is pressed. That varied voltage is sent back to the passenger door module on the low reference circuit and sends a response back to the driver door module to illuminate the red and blue LED's. The driver door module communicates this message to the passenger door module on 4 different passenger temperature signal circuits.

Once the temperature offset request is made to the passenger door module, the signal is sent to the driver door module. The driver door module is connected to the passenger door module through the power door serial data circuit. The driver door module will make the request to the IPM through the class 2 serial data circuit. The IPM will move the right air temperature actuator to the desired offset.

Steering Wheel Temperature Control Switch
A separate temperature switch is mounted on the steering wheel to allow the driver to adjust the HVAC temperature. Power to the right steering wheel control is delivered from the dash integration module, through the steering column fuse holder and inflatable restraint steering wheel module coil on the remote radio control supply voltage circuit. When the driver toggles the temperature switch up or down, the voltage is sent through a series of resistors. That varied voltage is sent back through the inflatable restraint steering wheel module coil to the dash integration module through the remote radio control signal circuit. Once the dash integration module receives the varied voltage signal, the information is sent out over the class 2 serial data circuit, to the IPM where the air temperature actuator is adjusted.

Engine Coolant
Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.