A/C Refrigerant System Operation

The A/C refrigerant system is a A/C evaporator core orifice (19D990)— cycling A/C clutch (2884) type. The system components are:

• A/C compressor (19703)
• A/C clutch
• A/C condenser core (19712)
• A/C evaporator core (19860)
• Suction accumulator/drier (19C836)
• and the necessary connecting refrigerant lines.
• System operation is controlled by the A/C evaporator core orifice and the A/C cyclic switch (19E561) .

NOTE: When diagnosing or servicing the A/C refrigerant system, time can be saved if the proper procedures are carefully followed.

It may seem difficult to understand how heat can be transferred from a comparatively cooler vehicle passenger compartment to the hot outside air. The answer lies in the difference between the refrigerant pressure that exists in the A/C evaporator core and the pressure that exists in the A/C condenser core . In the A/C evaporator core , the A/C compressor suction reduces the pressure and the boiling point below the temperature of the air passing over the A/C evaporator core . Heat transfers from the air to the boiling refrigerant.

• In the A/C condenser core , the A/C compressor raises the condensation point above the temperature of the outside air. The heat transfers from the condensing refrigerant to the outside air. The A/C evaporator core orifice and the A/C compressor simply create pressure conditions that permit the laws of nature to function.
• The refrigerant system incorporates a A/C compressor controlled by an A/C cyclic switch connected in series with the A/C clutch . The A/C cyclic switch senses A/C evaporator core pressure to control A/C compressor operation. Using this method of A/C compressor control, the A/C cyclic switch will stop A/C compressor operation during ambient temperatures below approximately 10°C (50°F) and also prevent A/C evaporator core icing during normal system operation. An A/C compressor pressure relief valve (19D644) is installed in the high-pressure (discharge) A/C manifold and tube (19D734) to protect the refrigerant system against excessively high refrigerant pressures.
• Operation of the A/C clutch is dependent on signals from the powertrain control module (PCM) (12A650) . The system is programmed to interrupt A/C compressor operation when certain conditions exist. The A/C clutch can be shut off (or kept off) for several seconds at engine start-up, at high engine speeds, during acceleration, when the engine coolant temperature exceeds a predetermined temperature and during low engine idle conditions (approximately 200 rpm below low idle specifications).
• An A/C evaporator core orifice is used to meter the liquid refrigerant into the A/C evaporator core for A/C evaporator core cooling.
• A/C evaporator core temperature is controlled by sensing the pressure within the A/C evaporator core with a pressure operated electric switch. The A/C cyclic switch controls A/C compressor operation as necessary to maintain the A/C evaporator core pressure within specified limits.

Refrigerant Flow

During stabilized (A/C system shutdown) conditions, refrigerant system pressures are equalized on both the high and low sides of the refrigerant system.

When the function control knob (18519) is moved to an A/C, mix or defrost position the system operates as follows:

• The magnetic A/C compressor clutch field coil (2987) is energized and the A/C clutch is pulled into contact with the belt-driven A/C compressor clutch pulley (2E884) .
• The A/C clutch assembly then rotates the compressor shaft.
• When the A/C compressor shaft is rotated, the pistons are alternately pulled out and forced into their respective cylinder bores.
• As each piston is pulled from its cylinder bore, the pressure in the cylinder suddenly reduces to a pressure (or vacuum) considerably lower than the refrigerant vapor pressure on the suction side of the refrigerant system.
• The higher refrigerant system vapor pressure overcomes the suction reed valve spring pressure, forcing gas through the reed valve and into the lower-pressure (or vacuum) area inside the A/C compressor cylinder.
• The spring pressure on the reed valve closes the valve when the refrigerant system suction vapor pressure and the compressor cylinder vapor pressure are equalized.
• As each piston is forced into its respective cylinder bore, the refrigerant vapors from the suction side of the refrigerant system are compressed into a decreasingly smaller area, increasing the refrigerant vapor pressure and also raising the refrigerant vapor temperature.
• The higher refrigerant vapor pressure now assists in sealing the suction reed valve closed and also opens the discharge (high-pressure) reed valve as the cylinder pressure exceeds the higher-pressure side of the refrigerant system.
• When the A/C compressed higher-pressure and temperature refrigerant vapor is discharged into the high-pressure side of the refrigerant system, the discharge reed valve spring pressure and the high side refrigerant pressure closes and seals the reed valve.
• Closing the reed valve prevents the discharge pressure from re-entering the compressor cylinder.
• The A/C compressor refrigerant vapor compression cycle begins as the pistons are again pulled from their respective compressor cylinder bores by rotating the A/C compressor shaft.
• The high-pressure and high-temperature A/C compressor discharge refrigerant vapor is released into the top of the A/C condenser core assembly, through the A/C compressor to condenser discharge line (19972) .
• The A/C condenser core , being close to ambient temperature, causes refrigerant vapor to condense into a liquid when heat is removed from the refrigerant vapor by ambient air passing over A/C condenser core fins and tubing.
• Liquid refrigerant from the bottom of the A/C condenser core enters the high-pressure condenser to evaporator tube (19835) and then the inlet side of the A/C evaporator core orifice located in the A/C condenser core inlet tube.
• The inlet filter screen of the A/C evaporator core orifice assembly removes coarse contaminant particles, which may be present in the liquid refrigerant, before the liquid refrigerant enters the calibrated opening of the A/C evaporator core orifice.
• The outlet end of the A/C evaporator core orifice assembly has a fine mesh filter with four open side slots, in the body of the A/C evaporator core orifice assembly, upstream from the filter.
• The side slots and filter act as a refrigerant flow noise suppressor.
• Refrigerant pressure is reduced in the A/C evaporator core as a result of the A/C evaporator core orifice and the A/C compressor suction.
• As the A/C evaporator core pressure is lowered and the condenser to evaporator tube pressure increases, the liquid refrigerant passes through the A/C evaporator core orifice and enters the A/C evaporator core at a low pressure and as a cold liquid.
• As airflow passes over the plate/fin sections of the A/C evaporator core , the refrigerant inside absorbs the heat and changes into a vapor.
• A/C compressor suction draws the vaporized refrigerant and oil mixture into the suction accumulator/drier where the heavier oil-laden vapors fall to the bottom and the lighter vapors and oil mixture continue their path to the A/C compressor through the top of the vapor return tube.
• Two desiccant bags, located inside the suction accumulator/drier , absorb and retain moisture which may be circulating in the refrigerant system.
• The heavier oil-laden refrigerant also returns to the A/C compressor through a small liquid bleed hole near the bottom of the aspirator tube.
• The liquid bleed hole provides a controlled second opportunity for the accumulated refrigerant and oil mixture to vaporize as it passes through the opening to re-enter into the main vapor flow path to the suction side of the A/C compressor .

Refrigerant Systems

NOTE: It is necessary to determine whether or not the refrigerant system contains R-134 refrigerant BEFORE any refrigerant system service is performed.

 CAUTION: DO NOT use R-12 special tools and equipment when servicing an R-134a system. Doing so may cause damage to the R-134a A/C system.

Mustang vehicles offer the R-134a A/C systems.

• The main difference between these systems and R-12 A/C systems involves the mandatory requirement of the use of different refrigerants.
• The R-134 system uses a non-chlorofluorocarbon (non-CFC) based refrigerant R-134.
• R-134 is a hydrofluorocarbon (HFC) based refrigerant.