A/C Performance Testing

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 in the engine compartment, 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 Max (A/C) mode. With the mode control switch set in the Max (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 evaporator's ability 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 Warnings and Precautions before performing this procedure. The air temperature in the test room and in the vehicle must be a minimum of 21°C (7O°F) for this test.
1. Connect a tachometer and a manifold gauge set.
2. Set the heater-A/C mode control switch to Max (A/C), the temperature control lever in the full Cool position, and the blower motor switch in the High speed position. For vehicles with rear air conditioning, the front and rear units must be tested separately The front unit is disabled by unplugging the wire harness connector at the front air conditioning unit blower motor.
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, windows, and hood closed.
5. Insert a thermometer in the left center A/C (panel) outlet for the front air conditioning unit test. For the rear air conditioning unit test, use the center side panel outlet. Operate the engine for five minutes.
6. The compressor clutch may cycle, depending upon the ambient temperature and humidity. After five minutes, note the discharge air temperature. If the clutch cycles, take the reading before the clutch disengages.
7. Open the hood and disconnect the vacuum line going to the heater core water control valve(s). Observe the valve arm for movement as the line is disconnected. If it does not move, see the diagnosis for the Vacuum System. Plug the disconnected vacuum line to prevent vacuum leakage.

Fig 7 Performance Temperature (Front Unit):

Fig 8 Performance Temperature (Rear Unit):

8. Operate the air conditioning system for two more minutes and take the discharge air temperature reading again. If the temperature increased by more than 3°C (5°F), check the temperature control (blend air) door cable for correct adjustment and operation. When the increase is more than 3°C (5°F), compare the discharge air temperature, suction pressure, and discharge pressure with the Performance Temperature charts. Connect the vacuum line for the heater core water valve(s).
9. If the discharge air temperature fails to meet the specifications in the Performance Temperature charts, refer to the Pressure and Performance Diagnosis charts for more test information.

The following diagnostic charts have been developed for quick reference.

Pressure Diagnosis:

Performance Diagnosis: