A/C Performance Test

The air conditioning system is designed to remove heat and humidity from the air entering the passenger compartment. The evaporator, located in the HVAC housing, is cooled to temperatures near the freezing point. As warm damp air passes over the fins in the evaporator, moisture in the air condenses to water, dehumidifying the air. Condensation on the evaporator fins reduces the evaporators ability to absorb heat. During periods of high heat and humidity, an air conditioning system will be less effective. With the instrument control set to Recirculation mode, only air from the passenger compartment passes through the evaporator. As the passenger compartment air dehumidifies, A/C performance levels rise.

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.

PERFORMANCE TEST PROCEDURE
Review Safety Warnings and Cautions before performing this procedure. Air temperature in test room and on vehicle must be 21° C (70° F) minimum for this test.

NOTE: When connecting the service equipment coupling to the line fitting, verify that the valve of the coupling is fully closed. This will reduce the amount of effort required to make the connection.

1. Connect a tachometer and a manifold gauge set or A/C recycling/charging station.
2. Set the A/C-heater mode control in the Recirculation Mode position, the temperature control knob in the full cool position, and the blower motor switch to the highest speed position.
3. Start the engine and hold at 1,000 rpm with the A/C compressor clutch engaged.
4. The engine should be warmed up to operating temperature with the doors closed and windows open.
5. Insert a thermometer in the driver side center panel A/C-heater outlet and operate the engine for five minutes.
6. The compressor clutch may cycle, depending upon the ambient temperature and humidity.
7. With the compressor clutch engaged, record the discharge air temperature and the compressor discharge pressure.

Performance Temperature And Pressure:

8. If the discharge air temperature fails to meet the specifications in the A/C Performance Temperature chart, refer to the Pressure Diagnosis chart.

Pressure Diagnosis Part 1:

Pressure Diagnosis Part 2:

Pressure Diagnosis Part 3:

9. Compare the compressor discharge pressure to the Performance Temperature and Pressure chart. If the compressor discharge pressure is high, see the Pressure Diagnosis chart.