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DIAGNOSING A/C PROBLEMS
As we enter into the warmer months, dormant air conditioning systems now become the focus for repair shops across the country. If it’s not working when the temperatures rise, air conditioning problems must be fixed if you’re going to keep customers happy.
Air Conditioning Basics
The air conditioning system is designed to provide low temperature and low humidity air so that the passengers can be comfortable on those hot, muggy summer days. The A/C evaporator, located in the HVAC housing, is cooled to temperatures near the freezing point as the refrigerant inside it expands. As warm, damp air passes over the fins of the evaporator, the air transfers its heat to the refrigerant through the evaporator coils. 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 operating 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 (moisture level decreases), the A/C system performance levels rise. 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. As a result, high humidity greatly reduces the ability of the A/C evaporator to lower the temperature of the air.
Troubleshooting the A/C System
The first step is verifying that the A/C is not working properly. Start by doing the standard A/C performance test. Before reviewing the test procedures, it is important to remember that the use of an A/C recycling/charging station, for the purpose of determining the charge level of the system, is not recommended. These stations do not reflect the correct refrigerant charge after a single reclaim cycle.
The following steps detail the A/C performance test:
1.Be sure the following conditions are met in the area where this test is to be performed:
✓ Max. ambient temp. 110°F
✓ Min. ambient temp. 60°F
✓ Max. relative humidity 90%
✓ Min. relative humidity 20%
Using a scan tool, check for any Diagnostic Trouble Codes (DTCs) related to the Controller Area Network (CAN) data bus, Telematics Gateway (TGW), A/C heater module, Body Control Module (BCM) and Powertrain Control Module (PCM). If no DTCs are found, continue with the test. If any DTCs are found, repair as required, then continue with the test.
✓ Engine at normal operating temperature
✓ Engine at normal idle speed
✓ No sun-load in the cabin of the vehicle
✓ Vehicle doors and windows closed
✓ Transmission in Park or Neutral with the parking brake On
✓ A/C heater controls set to Recirculation mode, full cool, panel mode, high blower and with the A/C compressor engaged (Figure 1); if the A/C compressor does not engage, see the A/C System Diagnosis table
✓ All panel outlet vanes are open and positioned straight rearward
3. Using a scan tool, operate the engine cooling fans at high speed from the PCM View>Actuators.
Note: This procedure requires you to know what the temperature and relative humidity is in your location at the time of the test
5. With the A/C clutch engaged, compare the observed panel outlet air temperature along with the ambient temperature of the work area and the relative humidity to the Maximum Panel Outlet Temperature chart (the chart is in the service manual for the vehicle being serviced).
6. If the air outlet temperature fails to meet the specifications shown in the Maximum Panel Outlet Temperature chart, the A/C performance problem is confirmed.
Now that we have determined that the system is not working, we have to determine why. More than likely, there is a refrigerant leak somewhere. Low or no refrigerant reduces the ability of the system to extract heat from the ambient air. That means warm air is likely blowing out of the vents. Before we discuss leaks, we should consider other possible triggers.
One possible problem is a corroded evaporator. The evaporator is the heat exchanger inside the car. If corrosion occurs, its ability to deliver cool air inside the cabin of the vehicle is greatly reduced. The evaporator has a drain tube to draw off the condensed water. Check this water for the presence of oil and rust.
Cooling fan failures are another source of A/C problems not directly related to a refrigerant leak. Turn the engine On, then turn the A/C on. The cooling fan should operate. If it doesn’t, test the relay circuit to ensure that the fan is good.
The fan must operate in order to draw cool air across the condenser, especially in stop-and-go city driving. If you’re not drawing enough air through the condenser, an insufficient amount of heat is drawn out of the refrigerant. A hint that the cooling fan is not working is a rise in coolant temperature, so check the temperature gauge in the car.
Leaks are the most common source of A/C performance problems. Leaks can occur in a number of locations, and finding a leak is not always easy. Using a fluorescent dye is the preferred, and most accurate, method to find a leak. The best spot to look for a leak is at any connection where two lines are joined using a compression fitting.
Another common source of leaks is the condenser. Located in front of the radiator, road debris, such as small stones, can puncture a fin, resulting in a leak. Another area to check for leaks is at the connection for rear extension tubes in those vehicles (like minivans) with rear A/C outlets.