Refrigeration System Operation

V-5 Compressor:

TXV Cross Section:

V-5/Thermal Expansion Valve System:

TXV SYSTEM

This A/C system uses both a thermal expansion valve (TXV) and a variable displacement 5 cylinder compressor (V-5). The TXV's only purpose is to regulate the amount of refrigerant flow through the evaporator to achieve optimum performance. Too much flow through the evaporator "floods" (the refrigerant stays liquid all of the way through the evaporator and boils in the compressor inlet hose), not enough flow and the evaporator "starves" (the refrigerant boils too early in the evaporator). Both conditions will cause A/C performance to be less than optimal. The goal is to get the refrigerant to boil around the middle of the evaporator. The TXV achieves this by sensing the evaporator outlet pressure and outlet temperature to control refrigerant flow to a predetermined factory set point. It should be noted that the TXV is a no bleed design (to prevent liquid slugging upon start up). This means that when the compressor is disengaged the TXV closes off completely and high & low-side pressures take a long time to equalize (they will eventually equalize due to a very small amount of bleed back through the compressor reeds). It should also be noted that a good TXV on the bench (removed from the system) will be wide open due to atmospheric pressure and temperature acting upon it.

The V-5 compressor does not need to cycle on and off. Instead it varies the length of its piston stroke and hence changes its displacement to prevent evaporator core freeze-up. The goal is to run as much displacement as possible for good cooling and yet not too much as to freeze. The V-5 tries to sense evaporator temperature via its control valve. which knows only one thing: suction pressure. Think of pressure as being roughly equivalent to temperature, and the suction pressure at the compressor being about the same as the pressure inside the evaporator, and hence the temperature at the evaporator. When the low-side pressure increases (due to higher evaporator load and/or reduced compressor rpm), the control valve tells the compressor to upstroke to pull the low-side pressure back down. When the V-5 is all the way to full stroke and can do no more, any additional increase in evaporator load and/or reduction in compressor speed will result in a corresponding increase in low-side pressure and temperature. When low-side pressure decreases (due to lower evaporator load and/or increased compressor rpm), the control valve tells the compressor to destroke to keep the low-side pressure and temperature from going too low (going too low could result in evaporator freeze-up).
The V-5 used in this application has a high rpm destroking mechanism built in. When the trigger speed is reached, the compressor goes to a partially destroked condition until the rpm is reduced. The action is subtle and probably would not be detected during normal driving conditions.

REFRIGERANT-134a

The air conditioning system contains Refrigerant-134a, which requires special handling to avoid personal injury. Refer to "Handling Refrigerant".

COMPRESSOR

The V-5 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, a clutch plate and hub assembly is drawn rearward toward the pulley. The magnetic force locks the clutch plate and pulley together as one unit to drive the compressor shaft.

As the compressor shaft turns, the compressor performs two main functions. It compresses the low-pressure refrigerant vapor from the evaporator into a high-pressure, high-temperature vapor. It also pumps refrigerant (and refrigerant oil) through the A/C system.

PRESSURE RELIEF VALVE

The compressor is equipped with a pressure relief valve which is placed in the system as a safety factor. Under certain conditions, the refrigerant on the discharge side may exceed the designed operating pressure. To prevent system damage, the valve is designed to open automatically when high side pressure exceeds a specified amount. Conditions that might cause this valve to open (defective A/C pressure sensor, inoperative electric cooling fan, etc.) should be corrected, and the refrigerant oil and refrigerant should he replaced as necessary.

CONDENSER

The condenser in front of the radiator is made up of coils which carry the refrigerant. Cooling fins provide rapid transfer of heat. Air passing through the condenser cools the high-pressure, high-temperature refrigerant vapor, causing it to condense into a liquid.

COOLANT FAN

The coolant fan operation is crucial to the proper operation of the A/C system. The coolant fan ensures the proper amount of air flow across the condenser throughout the vehicle's operating range.

The fan operation should be checked during any A/C diagnosis procedure. Special attention should be given to the fan whenever excessive high-side pressures are encountered, the high pressure relief valve discharges, or if the A/C pressure sensor opens. The coolant fan is mounted to the rear of the radiator.

Coolant fan control circuits vary with carline and engine application.

THERMAL EXPANSION VALVE

The Thermal Expansion Valve (TXV), with its monitoring bulb and attaching capillary, mesh screen, and variable orifice is located in the evaporator inlet pipe. While sensing the temperature of the evaporator outlet pipe, the monitoring bulb's internal pressure raises or lowers in direct proportion. (By sensing a higher temperature, the TXV opens further to allow more R-134a into the evaporator, and the opposite when it senses cooler temperatures). This provides a variable restriction for the high-pressure liquid refrigerant in the liquid line, which meters the flow of refrigerant to the evaporator as a low-pressure liquid. The thermal expansion valve is protected from contamination by a filter screen.

Do not attempt to change flow rate of the valve. It is preset for maximum performance. Any attempt to alter will reduce cooling capabilities and could possibly cause system damage.

The thermal expansion valve is located near the evaporator at the front of dash. The thermal expansion valve is serviced only as a replacement assembly.

EVAPORATOR

The evaporator is a device which cools and dehumidifies the air before it enters the passenger compartment. High-pressure liquid refrigerant flows through the TXV; becomes a low-pressure liquid, and then enters the evaporator. The heat in the air passing through the evaporator core is transferred to the cooler surface of the core, thereby cooling the air. As the process of heat transfer from the air to the evaporator core surface is taking place, any moisture (humidity) in the air condenses on the outside surface of the evaporator core and is drained off as water. The evaporator is located inside the HVAC module at the front of dash.

RECEIVER DEHYDRATOR

Connected to the condenser outlet pipe, the sealed receiver dehydrator assembly functions as a liquid storage container. At the bottom of the receiver dehydrator is the desiccant bag. It acts as a drying agent for moisture that may have entered the system. If the system is open to air for an extended period of time (due to a front-end collision, removed parts, etc) replace the receiver dehydrator. The desiccant bag will be saturated with moisture.