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.

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 temperature control, located on the HVAC control unit, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting; heater or A/C.

The air temperature is controlled by moving the temperature control dial. Moving the air temperature control mechanically moves the air temperature door. They are linked together by the air temperature cable. The air temperature door position determines the amount of air directed to flow across the heater core.

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 recirculated back through the engine cooling system.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The vehicle operator can activate the A/C system by depressing the A/C switch or by placing the mode control in the defrost or the heat/defrost position. 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 setting.

Regardless of the selected A/C mode setting, a request is made to the A/C compressor control module to turn on the A/C compressor. The request is sent to the A/C compressor control module through the A/C request signal circuit from the A/C switch. The A/C switch receives ignition voltage through the A/C fuse when the heater relay is energized. The A/C compressor control module is grounded at G203.

Ignition voltage is applied to the coil of the heater relay through the GAUGE fuse. With the blower switch in any position except OFF, the heater relay coil is provided a ground through the blower switch at G204 and energizes. Battery voltage is applied through the closed contacts of the heater relay to the coil and the switch sides of the A/C MG relay, to the A/C switch, and to the A/C compressor control module through the refrigerant pressure switch.

The A/C compressor control module turns on the A/C compressor by providing a path to ground through the A/C compressor clutch (MG) relay control circuit. Once the A/C compressor clutch 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. Whenever the compressor is turned on, the A/C compressor clutch diode prevents a voltage spike from burning up the compressor clutch coil. The compressor clutch is grounded at G103. The A/C compressor clutch relay control circuit is grounded internally within the A/C compressor control module.

A/C Pressure Switches

The A/C system is protected by two pressure switches. The A/C refrigerant pressure switch acts to cycle the compressor on and off under conditions of abnormally low or abnormally high refrigerant pressure. The A/C refrigerant pressure switch opens when the A/C line pressure falls below or exceeds a predetermined value (196-3140 kPa [28-455 psi]). An open in this circuit signals the A/C compressor control module to remove the ground for the A/C compressor clutch relay control circuit, thus shutting off the compressor.

The A/C high pressure switch acts to cycle the cooling fans from low to high speed operation. With the A/C on and refrigerant pressure below 1226 kPa (178 psi), indicating low refrigerant temperature, the A/C high pressure switch is closed, allowing both cooling fans to operate at low speed. If the refrigerant pressure exceeds 1520 kPa (220 psi), indicating high refrigerant temperature, the A/C high pressure switch opens, allowing both cooling fans to operate at high speed. When the refrigerant pressure and temperature return to normal, the fan speed is reduced to low. The change in fan speed improves the condenser's ability to lower refrigerant temperatures and pressures. The A/C refrigerant pressure switch incorporates both the high and the low pressure switches in one unit.

When the A/C switch is depressed, and the blower switch is in any position except OFF, a light emitting diode (LED) within the A/C switch illuminates by grounding at G204. When the A/C compressor control module receives an A/C request, the A/C compressor control module provides a signal to the powertrain control module (PCM) for idle control.

Temperature Control

The vehicle operator can determine the temperature of the A/C air by using the temperature control. The temperature control cable opens the air mixture door to a position to divert sufficient air past the evaporator to achieve the desired vehicle temperature. Warm heater air is mixed with cool A/C air at the air mixture door. When the coldest temperature setting is selected, no warm heater air is mixed with the cool A/C air.

The main and auxiliary fan motors run at half speed in series configuration when all of the following conditions occur:

The main and auxiliary fan motors run at full speed in parallel configuration when either of the following conditions occur:

The engine cooling fan system consists of two electric cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the powertrain control module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage on two individual power circuits, from fuse and relay block 1 and from junction block 2. The ground path is provided at G103 and G104.

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 PCM supplies the ground path for the fan 1 and the fan 2 relays through the cooling fan relay control circuit. This energizes both relays, opens the fan 1 relay contacts and switches contacts in the fan 2 relay. The engine main relay supplies battery positive voltage through the cooling fan motor supply voltage circuit to the auxiliary fan motor. When the A/C is operating, the compressor clutch (MG) relay energizes the fan 3 relay. The ground path for the auxiliary fan motor is through the switched contacts in the fan 2 relay, through the closed contacts of the fan 3 relay and through the main fan motor to G103. The result is a series circuit with both fans 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 PCM removes the ground for the fan 1 and the fan 2 relays through the cooling fan relay control circuit. This de-energizes the fan 2 relay which switches the relay contacts and provides a ground path for the auxiliary fan motor at G104. At the same time, the fan 1 relay is de-energized closing the relay contacts and providing battery positive voltage on the main fan motor supply voltage circuit to the main fan motor. During high speed fan operation, both engine cooling fans have their own power and ground path. The result is a parallel circuit with both fans running at high speed.

The A/C high pressure switch is in series with the PCM controlled ground for the coils of the fan 1 and fan 2 relays. If the A/C system pressure exceeds 1520 kPa (220 psi), the pressure switch opens the ground circuit to the coils of the fan no. 1 and no. 2 relays. This has the same effect as if the PCM had removed the ground for the relays.

The PCM will control the ground for the fan relays based on input from the ECT sensor in order to maintain the cooling system at normal operating temperature.

The PCM can also interrupt A/C compressor clutch operation. During high engine loads, the PCM sends a voltage signal to the A/C compressor control module, causing the ground for the A/C MG relay to be removed. The A/C compressor will re-engage when the PCM has determined that the increased load from the compressor will not affect engine performance.