April, 2010

Air Conditioning

System diagnosis

Mark Lemay

The heat of summer is just a few short months away, so the time to begin paying attention to your customers’ air conditioning system has arrived.





The tools required to properly diagnose A/C systems typically consist of leak check equipment, charging station with pressure gauges, refrigerant identifier a temperature probe, scan tool, electrical test equipment and basic knowledge of system operation.

Basic operation

Typical compressor controls are shown in (Fig 1). The HVAC head sends a message to the BCM and the BCM informs the PCM of this request. The PCM responds by checking the A/C pressure sensor and ambient temperature before turning the compressor relay on which in turn causes the compressor clutch to engage.

Checking the electronics

If the compressor will not engage, start by examining the data with a scan tool.  Fig 2 shows typical scan data. Look to see if the PCM is getting the A/C request. If not, check the HVAC control head and the BCM for proper inputs. If the PCM is getting the request for A/C but will not turn the compressor relay on, check the data to see if the PCM is requesting that the compressor relay to be turned on. If the PCM is not commanding the compressor relay on, check the PCM pressure sensor and ambient air temperature sensor to see if any faults exist in the PCM inputs. If the PCM is commanding the compressor relay on but the compressor does not engage, use the scan tool bi directional controls to manually engage the compressor. If the compressor still will not engage check the compressor, compressor relay, compressor wiring and fuses for faults.

Checking the system charge

If the compressor engages but the system does not cool properly the system charge will have to be checked. All air conditioning systems work on the principal of heat transfer. We can use temperature readings to determine if the system has the proper refrigerant charge. Using temperature is more accurate than charging by weight alone.

Figure 3 shows a typical system with the correct evaporator and condenser temperature differences for a properly charged system.

Temperature testing

To set the car up for temperature testing, the engine should be at 1000 rpm, all doors should be open, blower on high and A/C set to max. 

Using a temperature probe, measure the inlet and outlet of the condenser as close to the condenser as you can get. A normal reading is a temperature drop of 20 dgF to 50 dgF.

If the temperature drop is less than 20 dgF, check for system overcharge, bad condenser, cooling fan not working properly, contaminated refrigerant or compressor problems.

With a temperature drop of more than 50 dgF, check for low refrigerant charge, non condensable such as air in the system and internal condenser restrictions.

And finally if you suspect that the refrigerant is contaminated the only true test is to test the refrigerant with a refrigerant identifier. These tests should help to identify problems in any air conditioning system.