Refrigeration System

Refrigerant R-134a

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-Alkaline 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.

Use only PAG synthetic refrigerant oil for internal circulation through the R-134a A/C system: use only mineral-based 525 viscosity refrigerant oil on the fined threads and the O-rings. Use of any other lubricant may cause compressor or fitting failure.

Thermal Expansion Valve Description

The thermostatic expansion valve (TXV) "bleeds" regulated amounts of liquid refrigerant into the evaporator, thus, changing the state of the refrigerant from a liquid to a gas. The refrigerant absorbs the heat from the passenger compartment air causing it to evaporate into a low pressure vapor before returning to the compressor.

The TXV has a shaft that moves a ball valve from it's seat to control refrigerant flow. As the ball is unseated, more refrigerant will flow through the evaporator assisting in meeting the demand of the A/C system.

A power dome with two separate sides controls the shaft movement. The sealed thermal side of the power dome has a refrigerant charge. The sealed refrigerant in the thermal side responds to the temperature changes of the refrigerant flowing out of the evaporator. As the refrigerant temperature rises, the pressure of the thermal side increases.

If the thermal side pressure is greater than the system side pressure, the shaft moves down and pushes the ball off of the seat allowing more refrigerant flow through the evaporator. Likewise, when the system side pressure is greater, the shaft moves back up and re-seats the ball and refrigerant flow is reduced. There is a non-adjustable spring at the bottom of the TXV to help re-seat the ball.

Although the TXV controls the evaporator refrigerant flow, variations in the compressor piston stroke control the A/C system capacity.

Evaporator Description

The location of the evaporator core is within the HVAC module and functions as a heat exchanger between the low pressure refrigerant and the air that is entering or recirculating in the passenger compartment. High pressure liquid refrigerant turns to a low pressure liquid by flowing through the TXV just before entering the evaporator. The refrigerant then turns into a low pressure gas by absorbing heat from the air. The heat is then carried by the refrigerant to the condenser for release. Since the air passing through the evaporator core is dehumidified, condensation will collect on the core and then drain to the outside of the vehicle.

Condenser Description

The A/C condenser, located in front of the radiator, consists of the coils and the cooling fins. The coils carry the refrigerant; the cooling fins provide the rapid transfer of heat. The air passing through the condenser cools the high pressure refrigerant vapor, causing the vapor to condense into a liquid.

Receiver Dehydrator Description

The receiver/dehydrator is located on the LH of the vehicle behind the front grille.

The receiver/dehydrator functions as a moisture separator and as an additional pulsation dampener that receives high pressure refrigerant liquid. The receiver/dehydrator only allows refrigerant and refrigerant oil to continue on to the TXV. A desiccant filter, mounted on the bottom of the receiver/dehydrator, acts as a drying agent for any moisture that may have accumulated in the system. The desiccant filter is serviceable.

Compressor Description

This A/C system uses the constant run V-5 compressor. The V-5 is a variable displacement compressor that can match the automotive A/C demand under all conditions without cycling. The basic compressor mechanism is a variable angle wobble-plate with 5 axially oriented cylinders. The center of control of the compressor displacement is a bellows actuated valve located in the rear head of the compressor that senses suction pressure. The difference in crankcase suction pressure controls the wobble-plate angle and the compressor displacement.

When the A/C capacity demand is high, the suction pressure will be above the control point. The valve will maintain a bleed from the crankcase to suction side (no crankcase-suction pressure differential), and the compressor will have the maximum displacement. When the A/C demand is lower and the suction pressure reaches the control point, the valve will bleed discharge gas into the crankcase and close off the passage from the crankcase to the suction plenum. A force balance on the 5 pistons controls the wobble-plate angle. 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.