A/C Performance Test

The A/C system is designed to provide the passenger compartment with low temperature and low humidity air. The A/C evaporator, located in the HVAC housing is cooled to temperatures near the freezing point. As warm damp air passes over the fins of the A/C evaporator, the air transfers its heat to the refrigerant in the evaporator coils and the moisture in the air condenses on the evaporator fins. During periods of high heat and humidity, an A/C 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 A/C evaporator. As the passenger compartment air dehumidifies, the A/C system 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 A/C system. When humidity is high, the A/C 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 coils. This reduces the amount of heat the A/C evaporator can absorb from the air. High humidity greatly reduces the ability of the A/C 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 A/C system on humid days. A performance test is the best way to determine whether the system is performing up to design standards. This test also provides valuable clues as to the possible cause of trouble with the A/C system. The ambient air temperature of the vehicle and the location where the vehicle will be tested must be a minimum of 21° C (70° F) for this test. Also the evaporator temperature sensor probe (located between the evaporator fins) must be a minimum of 18° C 6.5° F) for this test as well.

WARNING: REFER TO THE APPLICABLE WARNINGS AND CAUTIONS FOR THIS SYSTEM BEFORE PERFORMING THE FOLLOWING OPERATION. FAILURE TO FOLLOW THE WARNINGS AND CAUTIONS COULD RESULT IN POSSIBLE PERSONAL INJURY OR DEATH.

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. Perform the Heating and A/C System Self Diagnostics. If no diagnostic trouble codes (DTCs) are found in the body control module (BCM) or the powertrain control module (PCM), go to Step 2. If any DTCs are found, repair as required, then proceed to Step 2.
2. Connect a tachometer and a manifold gauge set and adjust the heating-A/C controls as outlined below.

NOTE: The ambient air temperature of the vehicle and the location where the vehicle will be tested must be a minimum of 21° C (70° F) before performing this test. Also the evaporator temperature sensor must be a minimum of 18° C 6.5 F) for this test as well. If the vehicle has been sitting in the sun prior to testing, place the vehicle in the testing area until the temperature within the vehicle reaches a minimum of 18° C 6.5° F).

Manual Temperature Control (MTC) System
- Blower motor control to the highest speed position.
- Temperature control to the full cool position.
- Mode control to the Recirculation (max-A/C) position.
- A/C control to the ON position.

Automatic Temperature Control (ATC) System
- Blower motor control to the highest speed position.
- Temperature control to the LO position.
- Recirculation control ON (A/C and RECIRC symbols should be lit).
- Mode control to PANEL position.

NOTE: The word MANUAL should appear in the ATC display, confirming that the system is set manually.

A/C Performance Temperature And Pressure:

3. Start the engine and hold the idle at 1,000 rpm with the compressor clutch engaged. If the A/C compressor does not engage, see the A/C Pressure Diagnosis chart.
4. The engine should be warmed to operating temperature with the doors closed and windows open.
5. Insert a thermometer in the driver side center panel outlet and operate the A/C system until it stabilizes.
6. Start the engine and hold the idle at 1,000 rpm with the compressor clutch engaged. If the A/C compressor does not engage, see the A/C Diagnosis chart.
7. With the A/C compressor clutch engaged, compare the air temperature at the center panel outlet and the compressor discharge pressure (high side) to the A/C Performance and Pressure chart. The compressor clutch may cycle, depending upon the ambient temperature and humidity. If the clutch cycles, use the readings obtained before the clutch disengaged.

A/C Pressure Diagnosis (Part 1):

A/C Pressure Diagnosis (Part 2):

8. If the air outlet temperature fails to meet the specifications in the A/C Performance Temperature and Pressure chart, or if the compressor discharge pressure is high, refer to the A/C Pressure Diagnosis chart.