A/C Performance Testing

Performance Temperature And Pressure Table:

The air conditioning system is designed to provide the passenger compartment with low temperature and low humidity air. The evaporator, located in the heater-A/C housing under the instrument panel, is cooled to temperatures near the freezing point. As warm damp air passes over the fins in the evaporator, the air is cooled and the moisture is removed as it condenses on the fins. During periods of high heat and humidity, an air conditioning system will be more effective in the Recirculation (A/C) mode. With the mode control switch knob set in the Recirculation (A/C) position, only air from the passenger compartment passes through the evaporator. As the passenger compartment air dehumidifies, the air conditioning system performance levels improve.

Humidity has an important bearing on the temperature of the air delivered to the interior of the vehicle. It is important to understand the effect that humidity has on the performance of the air conditioning system. When humidity is high, the evaporator has to perform a double duty. It must lower the air temperature. and it must lower the temperature of the moisture in the air that condenses on the evaporator fins. Condensing the moisture in the air transfers heat energy into the evaporator fins and tubing. This reduces the amount of heat the evaporator can absorb from the air. High humidity greatly reduces the ability of the evaporator to lower the temperature of the air.

However evaporator capacity used to reduce the amount of moisture in the air is not wasted. Wringing some of the moisture out of the air entering the vehicle adds to the comfort of the passengers. Although, an owner may expect too much from their air conditioning system on humid days. A performance test is the best way to determine whether the system is performing up to standard. This test also provides valuable clues as to the possible cause of trouble with the air conditioning system.

Review the Service Precautions before performing this procedure. The air temperature in the test room and in the vehicle must be a minimum of 21~C (70~F) for this test.

1. Connect a tachometer and a manifold gauge set.
2. Set the heater-A/C mode control switch in the Panel and Recirculation positions, the temperature control knob in the full cool position, and the blower motor switch in the High (A/C) speed position.
3. Start the engine and hold the idle speed at 1,000 rpm with the compressor clutch engaged.
4. The engine should be at operating temperature. The doors and windows must be open.
5. Insert a thermometer in the left center A/C (panel) outlet. Operate the engine for five minutes.

Fig 7 Low Pressure Cycling Clutch Switch -- Typical:

6. The compressor clutch may cycle, depending upon the ambient temperature and humidity. If the clutch cycles, unplug the low pressure cycling clutch switch wire harness connector from the switch located on the accumulator. Place a jumper wire between the two cavities of the low pressure cycling clutch switch wire harness connector.
7. With the compressor clutch engaged, record the discharge air temperature, the compressor discharge pressure, and the evaporator inlet pressure.
8. Compare the discharge air temperature reading to the Performance Temperature and Pressure chart. If the discharge air temperature is high, see Refrigerant System Leaks and Refrigerant System Charge.

Pressure Diagnosis Chart:

9. Compare the compressor discharge and suction (evaporator inlet) pressure readings to the Performance Temperature and Pressure chart. If the compressor discharge pressure or suction pressure is not normal, see the Pressure Diagnosis chart.