A/C Compressor and Clutch


Only EPA-certified technicians can purchase R-12 refrigerant, since proper recovery and recycling techniques are required to prevent damaging the environment. Improper disposal of R-12 refrigerant can result in a fine.

Never expose A/C refrigerant to open flame. A poisonous gas, phosgene or mustard gas, will result.

A/C Sytem Test

Testing Air Conditioning Efficiency

Before temperature and pressure tests, the following list of items should be met:

  • Set the temperature control to maximum cool and normal AC. Some manufacturers call for MAX AC so that recirculating air is used. Be sure to check the specifications.
  • Set the blower position to high.
  • The temperature inside of the car must be stabilized (not getting cooler).
  • Engine speed must be at least 1500 rpm to ensure adequate refrigerant flow.
  • The compressor clutch must be engaged.

Air Discharge Temperatures

This chart below shows a chart of expected air discharge temperatures at different outside temperatures. The vertical temperature readings refer to actual duct readings. The horizontal readings refer to the day's outside temperature.

A chart showing expected air discharge temperatures at different outside temperatures. Courtesy of Ford Motor Company.

The drop in the air's temperature as it passes through the evaporator should be at least 20°F. It is very difficult to tell how well A/C works on a cool day.

Rules of thumb:

  • With 70°F air entering the evaporator, expect about a 20°F drop in air temperature (50°F).
  • With 80°F air entering the evaporator, expect about a 25°F drop in air temperature (55°F).
  • With 90°F air entering the evaporator, expect about a 30°F drop in air temperature (60°F).

It is important that the hood be closed during this test. The hardest work an air conditioner does is on "normal" because it is cooling outside air.

Pressure Testing

A pressure gauge set is used to measure pressures in the system. One of the gauges is for high side pressure and the other is for low side pressure.

A pressure gauge set is used to measure pressures in the system. Courtesy of Ford Motor Company.

The gauges and hoses are of a standard color code:

  • Red is the high side.
  • Blue is the low side.
  • Yellow is for charging and discharging equipment.

This is the same color code that plumbing follows for hot and cold water. The gauges are connected to one manifold with another hose fitting for performing service procedures. Evacuation and recharging of the system can be done through this port.

Before connecting the gauges to the service ports, close the manifold hand valves and any valves on the service hoses at a refrigerant tank or vacuum pump. Be careful that the hoses do not hang in the fan or on an exhaust manifold. Then, connect the gauges.

Connect the gauges to the service ports. Courtesy of Ford Motor Company.

The low pressure gauge is a compound gauge. It reads in either low pressure or vacuum. This is because it is in the suction side of the system. There is a hand-operated valve that opens and closes the suction line to the service hose used to charge the system. The gauge is always open to the suction line and registers that side's pressure.

The high side gauge registers pressure only. The gauge usually reads to 500 psi. Most high side gauges have a small orifice in the inlet. This keeps the needle from fluctuating rapidly as the system pulses. Many gauges have a zero calibration adjustment screw on the gauge face also. Sometimes gauges have a scale to convert pressure readings to relative temperature.

The hand control valve for the high side opens and closes the line to the high pressure service hose, used for discharging the system. The gauge always measures pressure in the high pressure side.

With manual service valves, use a special wrench to mid-seat the valve by turning the stem one or two turns. It must be back-seated before removing the hoses. Modern service hoses have a check valve in the end of the line to prevent refrigerant from escaping when the line is disconnected. A small amount can still get out. When removing a hose from a connection, put a shop rag around it to prevent oil and refrigerant from spraying. Disconnecting the low side while the system is running results in the lowest pressure at that connection.

The pressure reading in the system when it is not operating is called the static pressure. This will be at least 50 psi if there is enough refrigerant in the system. High and low side pressures will be equal when there is no movement through the system.

When you start the engine and turn on the AC system, the low side pressure will drop and the high side will increase. Pressure on the low side will change according to the temperature in the evaporator. The high side will reflect the temperature of the liquid refrigerant leaving the condenser. The pressures will continue to change until the system stabilizes (reaches its normal operating temperature). The system pressures are stabilized when maximum high side pressure when the system cycles off does not change from cycle to cycle.

The pressures will vary with humidity because high humidity puts a higher load on the system. Adequate air flow over the condenser is also required. A portable fan might be necessary to blow additional air through the grill on a car with a belt driven fan. Expected low side pressures differ between expansion valve and orifice tube systems. Troubleshooting charts in manufacturers' service manuals will help pinpoint the causes of pressure problems. The following are rules of thumb for the pressure test:

  • The higher the high side pressure, the more heat there is.
  • The lower the low side pressure is, the lower the temperature will be at the outlet.
  • High side pressure affects low side. When you squirt water on the condenser, the high and low pressures both drop at once.

Leak Detection

Leaks can be found using several means. These include soapy water, internal dyes, and several types of special leak detectors.

Soapy water is made by mixing water and soap to a thick solution that can be applied with a small brush. If the leak is large enough, soap bubbles will appear.

Soap bubbles showing a leak in a hose. Courtesy of Best Educational Trainers, Inc.

An internal charge detection method includes injecting a coloring agent into the low side of the system. Some red dyes are available in pound-size refrigerant containers. The dye is harmless and will be left in the system after the test. If dye leaks out, it will be visible at the source of the leak. A problem with the red dyes is that they can leak onto interior carpet and cause a damaging stain. If there is a leak, the dyed refrigerant will have to be recovered for reuse.

Another dye method uses fluorescent dye. A black light is used to locate the leak.

A fluorescent dye is added to the system before checking for leaks with a black light. Courtesy of Tracer Products, Westbury, NY.

This method is very effective on difficult leaks that only happen during the vibration that results from driving the vehicle. The dye will pinpoint where it leaked. The system is charged to at least 10% capacity for either test. This is so the system can develop normal operating pressure. There should be at least 50 psi static pressure (when the engine is off).

Electronic leak detectors are the most popular ones used today. Many of them work on both R-12 and R-134A refrigerants.

An electronic leak detector that works on both R-12 and R-134A refrigerants.

The tester nozzle is moved around the various joints in the system. The probe is held at a distance within 1/4" of the joint being checked. When a leak is found, the tester lights up and its slowly pulsing, high-pitched horn or clicking sound becomes more rapid. The tester has a sensitivity adjustment. You can adjust it so that it just stops ticking. When it starts to tick you have found a leak. The "gross leak" position can find leaks of 4 ounces per year. The sensitivity in this position is less so you can locate the exact source of the leak.