HVAC Control Assembly

The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 3 voltage circuit provides power to the control assembly. An integrated potentiometer controls the air temperature door position. The blower motor speed is controlled by a rotary switch. The integrated vacuum system controls the mode door position. The control assembly supports the following features:

Air Temperature Actuator

The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground, and position signal circuits enable the actuator to operate. The position signal circuit uses a 0-12-volt linear-ramped signal to command the actuator movement. The 0 and 12-volt control values represent the opposite limits of the actuator range of motion. The values in between 0-12 volts correspond to the positions between the limits.

When the HVAC control assembly sets a commanded, or targeted value, the position signal is set to a value between 0-12 volts. The actuator shaft rotates until the commanded position is reached. The assembly will maintain the control value until a new commanded value is needed.

A/C High Pressure Switch

The A/C high pressure switch protects the A/C system. The A/C high pressure switch is a 2-wire normally closed switch. The A/C high pressure switch opens the A/C request signal and disables the A/C compressor when line pressure exceeds 2 965 kPa (430 psi). The switch will then close once the pressure reaches 1 586 kPa (230 psi).

A/C Low Pressure Switch

The A/C low pressure switch protects the A/C system. The A/C low pressure switch is a 2-wire normally closed switch. The A/C low pressure switch opens the A/C low pressure switch signal circuit and disables the A/C compressor when line pressure falls below 152 kPa (22 psi). The switch will then close once the pressure reaches 303 kPa (44 psi).

The HVAC control assembly applies voltage to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:

When the operator requests High speed, the HVAC control assembly applies voltage to the blower motor relay through the high blower motor control circuit. The voltage energizes the blower motor relay, connecting the blower motor to battery positive voltage. The high speed blower motor relay is integral to the blower motor resistor assembly.

Mode Switch

The mode switch is a rotary vacuum valve that directly applies vacuum to the appropriate vacuum actuator. Use the mode switch to change the air delivery mode in the vehicle.

MAX A/C

When the operator selects MAX A/C, the mode actuator has vacuum applied to it through the yellow vacuum line, making the vent door open. The defrost actuator has vacuum applied to it through the green vacuum line, making the defrost door close and the heater door open through mechanical linkage. A solenoid inside the HVAC control assembly connects the recirculation door vacuum actuator to the vacuum source. The recirculation actuator retracts, allowing air to be recirculated. A/C is forced ON.

A/C

When the operator selects A/C, the mode actuator has vacuum applied to it through the Yellow vacuum line, making the vent door open. The defrost actuator has vacuum applied to it through the green vacuum line, making the defrost door close and the heater door open through mechanical linkage. A/C is forced ON.

Bi-Level Mode

When the operator selects Bi-Level, the following occurs:

Vent Mode

When the operator selects VENT, the mode actuator has vacuum applied to it through the yellow vacuum line, making the vent door open. The defrost actuator has vacuum applied to it through the green vacuum line, making the defrost door close and the heater door open through mechanical linkage.

Floor Mode

When the operator selects FLOOR, the mode actuator has vacuum applied to it through the red vacuum line, pushing the vent door closed. The defrost actuator has vacuum applied to it through the green vacuum line, pulling the defrost door closed and opening the heater door through mechanical linkage.

Mix-Blend Mode

When the operator selects Mix-Blend, the following occurs:

Defrost Mode

When the operator selects Defrost, the following occurs:

Recirculation is only available in MAX A/C. When MAX A/C is selected a solenoid inside the HVAC control assembly connects the recirculation door vacuum actuator to the vacuum source. The recirculation actuator retracts, closing the recirculation door.

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The control assembly takes the following actions with a mode operation selected, and an air temperature setting selected:

Ignition 3 voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The Mode switch must be in one of the following positions for an A/C request:

Once the mode switch is in one of these positions, the mode switch applies ignition 3 voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the A/C compressor relay and the engine control module (ECM). The voltage applied to the ECM is an input, this ensures idle quality. For the relay to close its internal contacts to supply the A/C compressor clutch with battery positive voltage, the A/C low pressure must be closed to provide a path to ground for the relay coil through the A/C low pressure switch signal circuit.

Once engaged, the compressor clutch will be disengaged for the following conditions:

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The control assembly takes the following actions with a mode operation selected, and an air temperature setting selected:

Ignition 3 voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The Mode switch must be in one of the following positions for an A/C request:

Once the mode switch is in one of these positions, the mode switch applies ignition 3 voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the powertrain control module (PCM), for the 8.1L (L18), or engine control module (ECM), for the 6.6L (LB7). The voltage applied to the PCM/ECM is an A/C request signal. The PCM/ECM then monitors the voltage on the A/C low pressure switch signal. If the voltage is low near ground, then the PCM/ECM will enable the A/C compressor clutch relay. The PCM/ECM provides a ground for the relay so it can close it internal contacts. The A/C low pressure must be closed to provide a path to ground for input to the PCM/ECM through the A/C low pressure switch signal circuit. The following conditions must be meet in order for the PCM/ECM to turn ON the A/C compressor clutch relay:

Once engaged, the compressor clutch will be disengaged for the following conditions:

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

The purpose of the auxiliary heating system is to supply heat to the rear interior of the vehicle. The heater auxiliary blower motor switch is located on the headliner and controls the auxiliary blower motor speeds for rear heater operation. Placing the heater auxiliary blower motor switch in any position other than OFF, turns on the solenoid allowing vacuum to open the water valve. This action allow coolant to flow through the auxiliary heater core. The location of the hot water shut off valve is under the right rear area of the cab. The only way to regulate the temperature of the air is to regulate the blower speed. The slower the blower speed, the higher the output air temperature. The slower moving air has more time to absorb heat from the heater core.

There are no mode adjustments for the auxiliary system.

Voltage is supplied to the auxiliary heater blower motor switch from the auxiliary heater blower motor relay in the instrument panel fuse block. The auxiliary heater blower motor switch applies voltage to the auxiliary blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the auxiliary heater blower motor per particular speed request:

When the operator requests High speed, voltage is applied directly to the auxiliary heater blower motor.

Engine coolant is the essential element to the heating system. The thermostat controls the normal engine operating coolant. 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 hose, in a pressurized state. Engine coolant flows through the main inlet hose and through the hot water shutoff valve to the auxiliary inlet heater hose at the rear of the vehicle, in a pressurized state. The heater core is located inside of the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculates back through the engine cooling system.