Heating, Ventilation & Air Conditioning: Overview GMC Yukon Denali

Heater System Description

The heater system provides heating, ram air, power ventilation, and the windshield defrosting. The blower fan draws outside air from the plenum at the base of the windshield into the heater module. The forward movement of the vehicle forces additional outside air, or ram air, into the heater module. Within the heater module, the air is heated. The heated air is routed through the ducts to the proper outlets for discharge into the passenger compartment. A control assembly in the instrument panel allows the operator to control the blower speed, the mode of operation, and the temperature of the air coming from the heater system.

The heater module contains components that heat the air, and direct the air to the proper ducts and outlets. The components are enclosed inside a two part housing. The front housing mounts to the front of the cowl panel in the engine compartment. The rear housing mounts to the rear of the cowl panel in the passenger compartment under the instrument panel.

Odor Description

Some vehicles may experience odors emitted from the air conditioning system primarily at start up in hot, humid climates. This odor may be the result of debris in the heater and evaporator module or microbial growth on the evaporator core.

A single application of the Goodwrench Air Conditioning System Deodorizer will remove odor from the air conditioning system.

Air Distribution System Description

Within the heater module is a series of valves called the temperature valve, the vent valve and the defroster valve. Valves are hinged components that direct airflow through the various sections of the heater module. The valves provide the proper airflow for the selected operating mode.

Expansion (Orifice) Tube Description

The plastic expansion tube, with the mesh screen and orifice is in the evaporator inlet pipe. The orifice tube provides a restriction to the high-pressure liquid refrigerant in the liquid line, metering the flow of refrigerant to the evaporator as a low-pressure liquid. The filter screens on both the inlet and outlet sides protect the expansion tube and orifice from contamination. Service the tube only as a replacement assembly.

When the engine is turned OFF, after the air conditioning has been operating, the refrigerant in the system flows from the high-pressure side of the expansion tube orifice to the low-pressure side until the pressure equalizes. This is detectable as a faint hissing sound for 30-60 seconds. The hissing is normal.

When system diagnostics indicate a restricted expansion tube, replacement of the restricted expansion tube may not be necessary. Metal chips, flakes, or slivers found on the screen may be removed with compressed air. Reuse the expansion tube under the following conditions

1. The plastic frame is not broken.
2. The expansion tube is not damaged or plugged.
3. The screen material is not torn.
4. The screen is not plugged with fine gritty material.
5. An undamaged o-ring

Evaporator Description

The evaporator cools and dehumidifies air before the air enters the passenger compartment. The following events occur in the evaporator

1. Low-pressure, low temperature liquid/vapor refrigerant enters the evaporator.
2. The refrigerant flows through the evaporator's tubing.
3. The refrigerant evaporates.
4. The refrigerant exits the evaporator as low-pressure, low temperature, mostly vapor refrigerant.
5. As the refrigerant evaporates, the refrigerant absorbs heat from the air flowing over the evaporator.

As the process of heat loss from the air to the evaporator core is taking place, any moisture (humidity) in the air condenses on the outside surface of the evaporator core and the moisture drains off as water.

Condenser Description

The condenser is located in front of the radiator.

The condenser is made up of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. Compressed refrigerant enters the condenser in a high pressure, high temperature vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid. Refrigerant exiting the condenser is in a high-pressure, medium temperature liquid state.

Scheme 1

The sealed accumulator is connected to the evaporator outlet pipe. The accumulator stores the refrigerant (vapor and liquid) and the oil from the evaporator. A desiccant at the accumulator bottom dries any moisture in the system. An oil bleed hole at the accumulator outlet pipe end provides the oil return path to the compressor. The accumulator is not serviceable and should only be replaced when leaking due to the following conditions

1. Perforation
2. A damaged sealing area
3. Damaged fastener threads
4. Outside air has entered the system for extended periods of time