The air temperature controls are divided into four areas:

HVAC Control Assembly

The HVAC control assembly is located in the instrument panel and contains the following controls:

The A/C switch and the Fresh/Recirculation switch are located in the instrument panel. Each switch contains its own ON indicator.

Air Temperature Control

The temperature control determines the amount of air flow to the heater core which regulates the temperature of the air entering the passenger compartment. A cable connects the control to the air flow door that is located in the heater case. When the temperature control is moved to the coldest position, the air mix door closes blocking the flow of air to the heater core. When the temperature control is moved to the hottest position, the air mix door opens fully allowing the air to flow through the heater core. Moving the temperature control between the coldest and the hottest positions allows the operator to adjust the passenger compartment temperature to the desired level.

Evaporator Temperature Sensor

The engine control module (ECM) monitors the evaporator temperature sensor in order to detect a possible evaporator icing condition and to determine in-vehicle temperature. The ECM disables the compressor clutch if the temperature input does not correspond to a predetermined value.

A/C Refrigerant Pressure Switch

The A/C refrigerant pressure switch prevents the A/C system from operating when an excessively high or excessively low pressure condition exists. The ECM enables the compressor clutch when the A/C pressure is between 196-3140 kPa (28-455 psi).

The A/C refrigerant pressure switch can also ground the control circuit of the fan 2 relay, thereby switching the cooling fan from low speed to high speed. The switch is open below 1226 kPa (178 psi) and closed above 1520 kPa (220 psi). The change in fan speed improves the condenser's ability to lower refrigerant temperatures and pressures.

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. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

Moving the air temperature control to the warmest position diverts most of the airflow through the heater core, which increases the outlet air temperature. Moving the air temperature control to the coldest position diverts most of the airflow around the heater core, which decreases the outlet air temperature.

Pressing the A/C button enables the ECM to request A/C compressor engagement and turn on the A/C indicator. The A/C button sends a signal to the ECM for A/C compressor engagement. The ECM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged. Defrost and Defog mode selections will request A/C operation but not turn on the A/C indicator. The blower switch must be in any position except OFF for the A/C system to operate. The A/C system can operate regardless of the temperature control setting.

The following conditions must be met in order for the A/C compressor clutch to turn on:

The ECM monitors the A/C refrigerant pressure switch signal circuit. If the ECM detects a pressure signal out of parameters, it will disable the A/C request. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The sensor information is used by the ECM to determine the following:

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

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 case. Heated air is distributed to the passenger compartment, through the HVAC case, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC case temperature door. The coolant exits the heater core through the return heater hose and recirculates back through the engine cooling system.

The engine cooling fan system consists of an electric cooling fan, two fan relays and a fan resistor. The fan 1 relay controls power to the fan motor. The fan 2 relay controls the ground path of the fan motor. The gauge fuse supplies ignition voltage to the coils of both the fan 1, and the fan 2 relays. The RDI fuse supplies battery voltage to the switch side of the fan 1 relay. The ECM controls the ground for the coils of both relays. The ECM controls low and high speed fan operation by energizing and de-energizing the fan 2 relay which changes the ground path of the fan motor.

During low speed operation, which is when the A/C is operating and the engine coolant temperature (ECT) is below 83°C (181°F), the ECM supplies the ground path for the fan 1 relay through the cooling fan 1 relay control circuit. This energizes the relay, closes the fan 1 relay contacts, and supplies battery voltage from the RDI fuse through the cooling fan motor supply voltage circuit to the fan motor. The ground path for the fan motor is through the closed contacts of the de-energized fan 2 relay, through the fan resistor to G103. The result is a series circuit with the fan running at low speed.

During high speed operation, which is when the ECT reaches 93°C (199°F) or the A/C system pressure exceeds 1520 kPa (220 psi), the ECM supplies the ground path for the fan 1 relay through the cooling fan 1 relay control circuit. This energizes the relay, closes the fan 1 relay contacts, and supplies battery voltage from the RDI fuse through the cooling fan motor supply voltage circuit to the fan motor. The ECM also supplies the ground path for the fan 2 relay. This energizes the relay, switches the fan 2 relay contacts, and supplies a ground for the fan motor directly to G103. The result is a series circuit with the fan running at high speed.

The A/C refrigerant pressure switch is in parallel with the ECM controlled ground for the coil of the fan 2 relay. If the A/C system pressure exceeds 1520 kPa (220 psi), the pressure switch closes the ground circuit to the coil of the fan 2 relay, initiating high speed fan operation.

The ECM commands low speed fan operation when all of the following conditions occur:

The ECM commands high speed fan operation when either of the following conditions occur: