Saturday, January 21, 2017 Detailed Auto Topics
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Two things most people know about automotive air conditioning are, it is hard to live without, and repairing it is expensive. The next few articles give increasingly more complex ways of checking and servicing air-conditioning systems in vehicles.

Understanding the basic idea of air conditioning is imperative to diagnosing problems. How Automotive Air Conditioners Work gives a good overview.

Two sides of the automotive air conditioner

Automotive air conditioner problems fit two broad categories. The refrigeration side, removes heat and produces cold air. An electrical-side of the system controls the refrigeration function. For instance, the compressor is part of the refrigeration side, but is controlled by the electrical side. When the system does not cool, knowing which side is causing the problem saves untold money and additional problems.

Electrical air conditioner issues

Simple tests can be used to help identify the source of the problem, when the air conditioner stops cooling. The first step is to inspect the compressor with the engine running, and the system turned on. A good electrical side is shown by a compressor clutch that is turning. If the compressor is not engaged, the reason must be found before preceding.

a voltmeter is used to test the air conditioner clutch

A digital voltmeter is used to check for power and ground at the compressor clutch connector. With 12 volts or more, between the leads, and the clutch not engaged, the compressors clutch or connector is bad. To test further, remove the connector and test for power and ground at the connector terminals. If no current flow is present, the connector is bad otherwise the compressor clutch coil has failed.

On high-mileage compressors, replacing the compressor and clutch as an assembly may be more reasonable. The costs of many clutch assemblies are almost as much as a new compressor with a clutch.

a jumper wire used to test the AC cycle swtich

If there is no current or ground at the compressor, several things can be the cause. A cycle switch is designed to shut the compressor off when internal pressure is too low. For instance, a leak in the system allows the refrigerant to escape. The cycle switch will cut the power to the air conditioner clutch. Air conditioner cycle switches can also fail. When this occurs, the compressor clutch will not engage, and the system will not be cool. A simple test is to use a short wire, temporarily connecting the terminals. The compressor may start to turn. Touching the outlet line from the evaporator with the hand will determine the temperature. If the line is now cold, suspect the cycle switch.

Refrigerant problems with air conditioners

The evaporator outlet tube should be cold, with the system working 

The outlet and inlet lines near the same temperature may mean the system is low on charge. Checking the level of refrigerant charge will be covered in the next article in this series.

Jumping the cycle switch may not engage the compressor clutch. When this happens, the compressor clutch relay and fuses should be located and checked, as shown in the article How to Check Fuses and Relays. Electrical problems beyond these may best be referred to a qualified technician.

Reasons why an air conditioner does not cool enough

Vent temperature is a good test of the AC system

A refrigeration thermometer can be used to see if an air conditioner is working properly. Place the thermometer in a center vent. The system should be on re-circulate, fan on high and temperature set to the lowest setting.

Actual vent temperature depends on several factors. With the engine at an idle, a vent temperature in the mid-forties Fahrenheit is okay. Lower is better and temperature in the fifties or above usually indicates a problem.

 Low refrigerant charge is ONE possible cause of insufficient cooling. Adding additional refrigerant, without first learning the cause, may severely damage the system and not fix the problem. DIAGNOSIS OF THE ACTUAL PROBLEM IS IMPERATIVE.

The speed of the condenser fan is important

A very common mistake is simply to add refrigerant to a system that does not cool. Often improper cooling is the result of a non-functioning condenser fan. When the condenser fan fails to move the proper amount of air, the pressure in the system rises drastically. This causes a lack of cooling and will damage the compressor and evaporator coil. Repairing the fan corrects the problem, and if it has not gone on too long the system may be fine. Adding additional refrigerant to such a system, results in an overcharge. Over charging will raise the pressure even further and destroy the system.  It is important to check not only that the fan turns, but that it operates at the proper speed and moves air.  A bad condenser fan motor can run too slowly and cause problems.  Condenser fins, blocked by debris, creates much the same situation.  If you are not certain, have a trained technician measure the air flow.

a new cabin filter on the left and a restricted one on the right

Another common problem is a restricted cabin air filter or evaporator core. Many vehicles use these filters to clean the air flowing through the evaporator. If the filter becomes restricted, air flow through the coil will drop and cooling will fall off. Lack of air flow may allow the refrigerant not fully to flash to vapor in the evaporator.

Liquid refrigerant that leaves the evaporator is caught in the accumulator. An overcharged system may allow the accumulator to overflow. If the liquid refrigerant is sucked into the compressor, it will be immediately damaged.  Vehicles without cabin filters may suffer from a similar problem. Debris can clog the fins of the evaporator and restrict air flow. If not corrected, or additional refrigerant is added, the compressor will be damaged.

Cost's saving with air conditioner repair is the result of proper diagnosis. Improper service and diagnosis will result in frustration and continued expensive problems.

Please also see:

Part two, understanding refrigerant gauges

Part three, finding and repairing air conditioner leaks

Part four, preventing and repairing compressor failure





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