(VDOT) Refrigeration System
Refrigerant System ComponentsThe following are the major components of the air conditioning refrigerant system:
- Refrigerant
- Compressor
- Condenser
- Expansion (orifice) tube
- Evaporator core
- Accumulator
- Refrigerant tubes and hoses
- Sealing washers and O-rings Refrigerant pressure sensor
VDOT A/C Refrigerant System
The basic operation of the variable displacement orifice tube (VDOT) refrigeration system is the same as for conventional A/C systems. The VDOT refrigeration system utilizes a variable displacement compressor which contains seven cylinders; a V7 compressor that can maintain the automotive air conditioning demand under all conditions without cycling the compressor.
The basic compressor mechanism is a variable angle wobble-plate with axially oriented cylinders. The control of the compressor displacement is a bellows actuated control valve located in the rear head of the compressor which senses suction pressure. The wobble-plate angle and compressor displacement are controlled by the crankcase-suction pressure differential. When the A/C capacity demand is high, the suction pressure will be above the control point; the control valve will maintain a bleed from the crankcase to suction; no crankcase-suction pressure differential; and the compressor will have maximum displacement. When the A/C capacity demand is lower and the suction pressure reaches the control point, the control valve will bleed discharge gas into the crankcase and close off a passage from the crankcase to the suction plenum. The angle of the wobble-plate is controlled by a force balance on the seven pistons. A slight elevation of the crankcase-suction pressure differential creates a total force on the pistons resulting in a movement about the wobble-plate pivot pin that reduces the plate angle.
The compressor has a unique lubrication system. The crankcase-suction bleed is routed through the rotating wobble-plate for lubrication of the wobble-plate bearing. The rotation acts as an oil separator, which removes some of the oil from the crankcase-suction bleed, rerouting it to the crankcase where it can lubricate the compressor mechanism.
The compressor, depending upon engine usage, is cut-off under certain conditions, such as wide-open throttle, low idle speed, low ambient air temperature, high power steering loads and high engine oil or engine coolant temperatures. The V7 compressor has a mechanical destroker that will destroke during engine speeds of 3000-3800 RPM.
When the air conditioning system is turned OFF, the refrigerant in the system will flow from the high pressure side of the evaporator tube orifice to the low pressure side until the pressure is equalized. When the engine is turned to OFF without first turning OFF the A/C system, this may be detected as a sound of liquid flowing (a hissing sound) for 30-60 seconds and is a normal condition.
A/C Accumulator
The accumulator is downstream of the evaporator core. Refrigerant vapor and refrigerant oil are received by the accumulator from the evaporator. The accumulator separates moisture from refrigerant vapor - a desiccant located inside the bottom of the accumulator is used to dry the refrigerant -. and stores the air conditioning refrigerant as both a vapor and a liquid, along with refrigerant oil. An internal oil bleed hole at the outlet of the accumulator internal tube provides the oil return path to the compressor. The accumulator is sealed and non-serviceable. Replace the accumulator only when leaking due to any of the following conditions:
- Perforation
- A damaged seal
- Damaged threads
- Outside air has entered the system for extended periods of time
A/C Compressor
V7 A/C Compressor Components:
The air conditioning compressor is driven by a belt from the engine crankshaft through the compressor clutch pulley. The compressor pulley rotates freely, without turning the compressor shaft, until an electromagnetic clutch coil is energized. When voltage is applied to energize the clutch coil, the clutch plate and hub is drawn rearward toward the pulley. The magnetic force locks the clutch plate and pulley together as one unit in order to drive the compressor shaft.
A/C Condenser
The condenser is located in front of the radiator. The condenser is made up of coils, which carry the refrigerant and cooling fins used to provide rapid transfer of heat. The air passing through the condenser cools the high pressure refrigerant vapor, causing it to condense into a high pressure liquid.
Engine Electric Cooling Fans
Engine cooling fan operation is crucial to the proper operation of the air conditioning system. The cooling fans ensure the proper amount of air flow across the condenser throughout the vehicle operating range. Fan operation should be checked during any air conditioning diagnosis procedure. Special attention should be given to the fan whenever excessive high side pressures are encountered.
A/C Expansion (Orifice) Tube
The plastic expansion (orifice) tube, with its mesh screen and orifice is located in the forward end of the evaporator tube - rear. The expansion (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 expansion tube orifice is protected from contamination by filter screens on both the inlet and outlet sides.
A/C Evaporator
Evaporator Core:
The evaporator core is used to cool and dehumidify the air before the air enters the passenger compartment.
High pressure liquid refrigerant flows through the expansion (orifice) tube and becomes low pressure refrigerant liquid before entering the evaporator. The heat in the air passing through the evaporator core is lost to the cooler surface of the core, thereby cooling the air. As a process of heat loss from the air to the evaporator core surface is taking place, any moisture (humidity) in the air condenses on the outside of the evaporator core and is drained off as water. As the air passes through the core, the cooler surfaces absorb the heat, cooling the air. As the air is being cooled, any humidity in the air condenses on the outside of the evaporator core and is drained off as water.
Refrigerant R-134a
Caution: Avoid breathing the A/C Refrigerant 134a (R-134a) and the lubricant vapor or the mist. Exposure may irritate the eyes, nose, and throat. Work in a well ventilated area. In order to remove R-134a from the A/C system, use service equipment that is certified to meet the requirements of SAE J 2210 (R-134a recycling equipment). If an accidental system discharge occurs, ventilate the work area before continuing service. Additional health and safety information may be obtained from the refrigerant and lubricant manufacturers.
Notice: R-134a is the only approved refrigerant for use in this vehicle.
Notice: Use only Polyalkylene Glycol Synthetic Refrigerant Oil (PAG) for internal circulation through the R-134a A/C system and only 525 viscosity mineral oil on fitting threads and O-rings. If lubricants other than those specified are used, compressor failure and/or fitting seizure may result.
Notice: R-12 refrigerant and R-134a refrigerant must never be mixed, even in the smallest of amounts, as they are incompatible with each other. If the refrigerants are mixed, compressor failure is likely to occur. Refer to the manufacturer instructions included with the service equipment before servicing.
Similar in function to the engine coolant, the refrigerant is the medium within the A/C system that absorbs, carries, and releases heat. This vehicle uses Refrigerant-134a (R-134a), a non-toxic, non-flammable, clear, colorless, and liquefied gas.
While the R-134a A/C system is very similar to an R-12 A/C system, the differences in the lubricants and in the service equipment are important. R-134a carries a charge of a special lubricating oil, Poly-Alkylene Glycol (PAG) refrigerant oil. The GM PAG refrigerant oil has a slight blue tint.
The oil is hydroscopic (water-absorbing) and required storage in a closed container.