AC Diagnoses

By Jim Taddei - June 17th 2010

A/C not working? What is necessary to get to the bottom of what is wrong.

The first step in any diagnoses is to confirm the fault. Possibilities inculcate: A/C not cold, abnormal noise, incorrect air flow, or poor air flow. Once the problem has be determined then diagnoses can begin. Is is quite possible to have more that one of these conditions present at the same tine. Here is a brief overview of each problem and some diagnostic procedures.

A/C not cold:

The first step is to confirm operation of the a/c compressor. If compressor is running and the evaporator is cold then the fault is with the temperature control system inside the vehicle. Some newer vehicles have “dual” front temperature controls, if only one side is getting cold chances are a temperature door actuator has failed. If the compressor does not run the system most likely has a leak and will need to be at least partially recharged for leak diagnoses. It can be difficult to detect a very small leak. In those cases dye is added to the refrigerant and the system is used normally until it shows signs of not getting cold again. The system can then be re-inspected for traces of the leak detection dye previously installed. In most cases the dye will show the source of the leak, if not the system may need to be recharged and the leak detection process started anew.

Abnormal noise:

Abnormal noise may come from the A/C compressor, drive (serpentine) belt, or HVAC blower motor. Compressors are generally replaced when they fail. In the case of a major compressor failure, metal debris can be pumped through the A/C lines. If this debris is ingested by the new compressor failure is almost guaranteed. To help try and prevent this problem the accumulator and any filters are replaced along with the damaged compressor. Papers. leaves, or other debris can enter the HVAC blower motor causing a noise or vibration issue. The blower motor must be removed for inspection if it is noisy in order to confirm the cause of the fault.

Incorrect air flow:

There are three systems in use these days to adjust the in car air distribution, mechanical linkage, vacuum operated, and electronic operation. These systems can be mixed and matched within the vehicle. Manual systems generally use mechanical or vacuum operation. Automatic systems generally use electronic operation and require the use of a scan tool for proper diagnose. Problems with vacuum operated system are generally related to vacuum supply or distribution lines. Problems with electronic system are generally related to the electric motors (actuators) which control the air temperature and distribution.

Poor air flow:

The HVAC blower may operate at some speeds and not others or not at all. Proper diagnoses of the blower motor speed control system must be performed before replacing the blower motor. Due to the high current flow associated with high speed blower motor operation it is not uncommon to melt blower motor electrical connectors if the bower motor is operated at the highest speed for an extended period of time.


2 User Comments

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How about if your trying to remove a A/C compressor clutch and the pulley assembly will not slide off the shaft due to what looks like a sleeve with a lip on the end.
I have a plug under the dash not connected and was wondering where is plugs in. I believe it is to the air conditioning, my ac is not blowing cold air, any ideas?