Heating and Air Conditioning: Description and Operation

Fig 1 Common Blend-Air Heater-Air Conditioner System:

Fig 2 Heater-A/C System Air Flow (Front View):

NOTES:
- To maintain the performance level of the heating-air conditioning system, the engine cooling system must be properly maintained.
- The use of a bug screen is not recommended. Any obstructions in front of the radiator or condenser will reduce the performance of the air conditioning and engine cooling systems.

HEATER AND AIR CONDITIONER
All vehicles are equipped with a common heater-A/C housing assembly. The system combines air conditioning, heating, and ventilating capabilities in a single unit housing mounted under the instrument panel. On heater-only systems, the evaporator coil is omitted and replaced with an air restrictor plate.

Outside fresh air enters the vehicle through the cowl top opening at the base of the windshield, and passes through a plenum chamber to the heater-A/C system blower housing. Air flow velocity can then be adjusted with the blower motor speed selector switch on the heater-A/C control panel. The air intake openings must be kept free of snow, ice. leaves, and other obstructions for the heater-A/C system to receive a sufficient volume of outside air.

The heater and optional air conditioner are blend-air type systems. In a blend-air system, a blend-air door controls the amount of unconditioned air (or cooled air from the evaporator on models with air conditioning) is allowed to flow through, or around, the heater core. A temperature control knob on the heater-A/C control panel determines the discharge air temperature by moving a cable, which operates the blend-air door. This allows an almost immediate manual control of the system's output air temperature.

The mode control switch on the heater-A/C control panel is used to direct the conditioned air to the selected system outlets. The mode control switch uses engine vacuum to control the mode doors by operating vacuum actuator motors.

On air conditioned vehicles, the outside air intake can be shut off by selecting the recirculation mode with the mode control switch. This will open a vacuum actuated recirculating air door and recirculate the air that is already inside the vehicle.

The optional air conditioner for all models is designed for the use of non-CFC, R-134a refrigerant. The air conditioning system has an evaporator to cool and dehumidify the incoming fresh or recirculated air prior to blending it with the heated air. This air conditioning system uses a fixed orifice tube in the condenser outlet line to meter refrigerant flow to the evaporator coil. To maintain minimum evaporator temperature and prevent evaporator freezing, a fixed pressure setting switch on the accumulator cycles the compressor clutch.

Fig 3 Heater-Air Conditioner Control Panels:

HEATER AND AIR CONDITIONER CONTROLS
Both the heater and heater-A/C systems use a combination of mechanical, electrical, and vacuum controls. These controls provide the vehicle operator with a number of setting options to help control the climate and comfort within the vehicle. Refer to the owner's manual for more information on the suggested operation and use of these controls.

The heater-only or heater-A/C control panel is located to the right of the instrument cluster on the instrument panel. The control panel contains a rotating-type temperature control knob, a rotating-type mode control switch knob, a rotating-type blower motor switch knob, and the Message Center. For more information on the Message Center.

The heater-only or heater-A/C control panel cannot be repaired. If faulty or damaged, the entire unit must be replaced. The control knobs and Message Center bulbs are available for service replacement.

CIRCUIT OPERATION
This information of the wiring diagrams covers the operation of the Heater only and the Heater-A/C systems.

Several fuses supply power for the A/C-Heater system. Fuse B, a 20 amp, from the Power Distribution Center (PDC) supplies battery voltage on circuit F31 to the contact side of the A/C compressor clutch relay. Circuit F31 is HOT at all times.

In the START or RUN position, the ignition switch connects circuit A1 from fuse 3 in the PDC to circuit A21. Circuit A21 feeds fuse 9, a 10 amp, in the fuse block. Circuit F18 connects from fuse 9 to the coil side of the A/C compressor clutch relay

In the RUN position only, the ignition switch connects circuit A2 from fuse 2, a 40 amp, in the PDC to circuit A22. Circuit A22 supplies voltage to the coil side of the blower motor relay.

Circuit E2 from fuse 13 in the fuse block powers the illumination lamps in the A/C heater control switch.

When the A/C-Heater control switch is moved to an A/C position or the defrost position, the Powertrain Control Module (PCM) receives the A/C select signal on circuit C90. Circuit C90 connects to PCM cavity C23. If the A/C low pressure and high pressure switches are CLOSED, the Powertrain Control Module (PCM) receives the A/C request signal on circuit C20. Circuit C20 connects to cavity C22 of the PCM.

After receiving the A/C request signal, the PCM supplies ground for the coil side of the A/C clutch relay on circuit C13. Circuit C13 connects to cavity C1 of the PCM. Circuit F18 supplies battery voltage to the coil side of the relay. Circuit F31 from fuse B in the Power Distribution Center (PDC) supplies voltage to the contact side of the relay

When the PCM grounds the A/C compressor clutch relay, the contacts CLOSE and connect circuits F31 and C3. Circuit C3 supplies voltage to the A/C compressor clutch. Ground for the compressor clutch is provided on circuit Z11.

The A/C compressor clutch connector has a built-in diode. The diode controls the induced voltage that results from the magnetic field collapsing when the clutch disengages. The diode provides a current path to protect other components and systems.