The air temperature controls are divided into three primary areas. The first, Heater Mode, is related to how the heater system responds when a heater mode is selected, and how the HVAC system provides the desired temperature for each setting. The second, A/C Mode, is related to how the A/C system responds when an A/C mode is selected by the vehicle operator, and how the HVAC system provides the desired temperature for each setting. The third, A/C Cycle, describes the complete A/C cycle.
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 turning the left temperature switch, located on the heater and A/C control, 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. Passenger temperatures can be set anywhere between 16° (60°F) and 32°C (90°F). The dual zone temperature control system does not have any temperature limits, one zone could be full hot and the other zone can be full cold.
The heater and A/C control provides power to the LH and RH temperature valve actuators through the battery positive power supply circuit. Ground is provided to the stepper motors through the control circuits. Power and ground are provided to the heater and A/C control from the fuse block, through the battery positive voltage, ignition positive voltage and ground circuit.
Temperature Valve Actuators
When the HVAC system is in the OFF mode, pressing any HVAC switch will actuate the automatic HVAC system. If the heater and A/C control displays any temperature between 19°-29°C (66°-84°F), turning the temperature switch will increase/decrease the set temperature by 1 degree. If the temperature is setting is 30°C (85°F), turning the temperature switch up increases the set temperature to 32°C (90°F).
The left hand temperature valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position, through the battery positive power supply circuit. Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.
The right hand temperature valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position, through the battery positive power supply circuit. Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.
Temperature Sensors
When the vehicle operator determines what temperature is desired, the automatic system uses multiple sensors to achieve and maintain the desired temperature. The heater and A/C control software receives an input of the inside air temperature from the inside air temperature sensor. The inside air temperature sensor is an internal component of the heater and A/C control.
The left air duct temperature sensor provides the heater and A/C control software with the temperature of the heat in the duct work. A 5-volt reference signal is sent from the heater and A/C control to the left air duct temperature sensor over the left outblow sensor circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the heater and A/C control. The left air duct temperature sensor is grounded through the sensor ground circuit.
The right air duct temperature sensor provides the heater and A/C control software with the temperature of the heat in the duct work. A 5-volt reference signal is sent from the heater and A/C control to the right air duct temperature sensor over the right outblow sensor circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the heater and A/C control. The right air duct temperature sensor is grounded through the sensor ground circuit.
The ambient air outside temperature sensor provides the heater and A/C control software with the temperature outside the vehicle and displays that temperature on the heater and A/C control. As temperature increases, resistance decreases. A 5-volt reference signal is sent from the heater and A/C control to the ambient air outside temperature sensor over the outside temperature sensor circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the heater and A/C control.
The outside air temperature sensor is located in the front grill area under the front bumper of the vehicle. The heater and A/C control uses this sensor to receive information on ambient air temperature. With this information, the heater and A/C control will give the driver the outside air temperature on the digital display. If the outside temperature increases, the displayed temperature will not change until the vehicle's speed is above 32 km/h (20 mph) for approximately one and a half minutes or the vehicles speed is above 72 km/h (45 mph) for approximately one minute.
These delays help prevent false readings. If the displayed temperature goes down, the outside temperature display is updated immediately. If the vehicle has been turned off for more than three hours, the current outside temperature will be shown when the vehicle is started. If it has been turned off for less than three hours, the temperature will be recalled from the previous vehicle operation.
Sunload Sensors
The headlamp automatic control ambient light sensor is located on the center of the defroster grill. The defroster grill is on the top of the instrument panel. The headlamp automatic control ambient light sensor monitors the sun's solar radiation and uses this information as part of the automatic climate control system when operating in the AUTO mode. This sensor must not be covered if it is to function properly. Covering the sensor will cause it to not be able to compensate for changes in the amount of heat the vehicle is absorbing from the sun.
A 5-volt reference signal is sent from the heater and A/C control to the headlamp automatic control ambient light sensor over the 5 volt referance circuit. A variable photo-diode resistor inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the heater and A/C control through the sun sensor input circuits. The headlamp automatic control ambient light sensor is grounded through the ground circuit.
Dual Zone Control Switch
Passenger temperatures can be set anywhere between 16°C (60°F) to 32°C (90°F). To activate the dual zone, the passenger presses the right air temperature switch, then turns it to the desired temperature offset. The passenger side temperature will appear on the VF display to show the desired temperature. There are two temperature doors, driver (LH) and passenger (RH). Each temperature door is controlled by a corresponding temperature valve actuator, LH or RH respectively. The temperature valve actuators move the associated temperature door to the desired positions which in turn produce the proper temperature of the air delivered to each side of the passenger compartment. The actuators are controlled by the temperature control knobs on the heater and A/C control. The LH temperature control knob is the primary control for both of the actuators. When the MODE button on the heater and A/C control is depressed, the LH temperature control knob will operate both of the actuators to obtain the same temperature to both the LH and RH sides of the passenger compartment. When the AUTO button is depressed, the RH temperature control knob will control the RH actuator to provide a different temperature to the RH side of the passenger compartment, unless the LH temperature control knob is set to the maximum cold or the maximum hot setting. If the LH temperature control knob is set to the maximum cold or the maximum hot setting, the LH temperature control knob will control both the LH and RH actuators. When the LH temperature control knob is in any other position, the RH temperature control knob may be used to control the RH actuator. When the LH temperature control knob is in the full cold position, the actuators position the temperature valves such that the air flow will by-pass the heater core. With the knob in any other position but full cold, the actuators position the temperature valves to allow air to pass through the heater core. The hotter the control knob setting, the more air passes through the heater core, increasing the air temperature entering the passenger compartment.
The left hand temperature valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position, through the battery positive power supply circuit. Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.
The right hand temperature valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position, through the battery positive power supply circuit. Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door. The motor opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.
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.
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 heater and A/C control is in any mode. The A/C system can operate regardless of the temperature setting, as long as outside ambient temperature is above 4°C (40°F).
Power and ground are provided to the heater and A/C control by the fuse block on the battery positive voltage, ignition positive voltage and ground circuit.
When the A/C request is selected, a signal is sent from the heater and A/C control through the high and low pressure switches to the engine control module (ECM). Both pressure switches must be closed for the request signal to complete it's path to the ECM.
The ECM turns on the A/C compressor by providing a path to ground through the A/C compressor relay control circuit for the A/C compressor relay. Power is provided to the A/C compressor relay coil through the ignition voltage circuit from the 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. Ground to the A/C compressor is supplied from the secondary cooling fan temperature switch. The secondary cooling fan temperature switch is closed until the temperature exceeds 120°C (248°F). When the secondary cooling fan temperature switch is open a path for ground will not present at the A/C compressor clutch. The secondary cooling fan temperature switch is grounded through the ground circuit. When A/C is requested the water cut off vacuum solenoid valve is engaged through the vacuum solenoid valve control circuit. When the water cut off vacuum solenoid valve is engaged, vacuum is supplied to the water cut off valve and coolant flow is bypasses the heater core. The A/C compressor diode prevents a voltage spike from entering the vehicles electrical system. The A/C clutch relay control circuit is grounded internally within the ECM. When the ECM reads wide open throttle, the ECM will shut off the compressor for 20 seconds.
