A/C Performance Testing
A/C PERFORMANCEThe 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, and the combination coil on vehicles equipped with the optional rear heater-A/C unit, is/are cooled to temperatures near the freezing point. As warm damp air passes through the cooled evaporator or combination coil, the air transfers its heat to the refrigerant in their tubes and the moisture in the air condenses on their fins. During periods of high heat and humidity, an air conditioning system will be more effective in the recirculation mode (Max-A/C). With the system in the recirculation mode, only air from the passenger compartment passes through the evaporator. The optional rear heater-A/C unit is a recirculating unit, which can only condition air already within the passenger compartment. 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 and the combination coil have to perform a double duty. They must lower the air temperature, and they must lower the temperature of the moisture in the air that condenses on their fins. Condensing the moisture in the air transfers heat energy into the fins and tubing of the evaporator and the combination coil. This reduces the amount of heat the evaporator and the combination coil can absorb from the air. High humidity greatly reduces the ability of the evaporator and the combination coil to lower the temperature of the air.
However, cooling 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 in the General Information 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.
Testing
1. Connect a tachometer and a manifold gauge set.
2. Set the heater-A/C mode control switch knob in the recirculation mode (Max-A/C) position, the temperature control knob in the full cool position, and the blower motor switch in the highest speed position. For vehicles equipped with the optional rear heater-A/C unit, the front and rear units must be tested separately
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 closed, and the hood must be mostly closed.
5. To test the front unit, insert a thermometer in the center A/C (panel) outlet. To test the rear unit, insert a thermometer in the left front overhead duct outlet in the headliner. Operate the engine for five minutes.
6. The compressor clutch may cycle, depending upon the ambient temperature and humidity If the clutch cycles, take all readings before the clutch disengages.
7. With the compressor clutch engaged, record the panel outlet discharge air temperature, the discharge pressure (high side), and the suction pressure (low side).
Performance Temperature (Front Unit):
Performance Temperature (Rear Unit):
8. Compare the outlet discharge air temperature reading to the Performance Temperature charts. If the front unit temperature reading is high, open the hood and unplug the vacuum harness connector at the front water valve while observing whether the water valve lever moves. If it does not move, see Vacuum System in the Diagnosis and Testing. If it does move, plug the vacuum line connector and clamp off both heater hoses (inlet and outlet), wait five minutes and record the temperature again. If the rear unit temperature reading is high, remove the rear heater-A/C cover to inspect the rear water valve for proper operation. If it does not operate, see Rear Mode Control in the Diagnosis and Testing. If it does operate, clamp off both heater hoses to the rear unit, wait five minutes and record the temperature again. For both the front and rear unit, compare the second reading to the Performance Temperature charts. If the temperature reading is now OK, see Blend-Air Door Control (front unit) or Rear Mode Control (rear unit) in the Diagnosis and Testing. If the temperature reading is still too high, see Refrigerant System Leaks in the Diagnosis and Testing, and Refrigerant System Charge in the Service Procedures.
Pressure Diagnosis:
Performance Diagnosis:
9. If the discharge air temperature fails to meet the specifications in the Performance Temperature charts, refer to the Pressure Diagnosis chart and Performance Diagnosis chart for more test information.