How to Tell If the A/C Compressor Is Going Out on a Vehicle?by Don Bowman
Diagnosing air conditioning compressor problems requires gauges to check the existing pressures in the system. One of the biggest reasons for compressor failure is a lack of oil due to leaks, which are common with air conditioning systems. The refrigerant oil is suspended in the Freon; when a leak occurs, the oil is the first thing forced out through the leak. More often than not, the system will be recharged with no regard to the oil lost. Each system is meant to have a certain amount of refrigerant oil, which can be determined by reading the service manual. Without sufficient oil, the vanes in the compressor will accelerate wear, resulting in a loss of compression and vacuum. The oil must also be distributed throughout the system.
Inspect the air conditioning belt for tension or slipping. Start the engine and turn on the air conditioning. Snap the throttle a few times. A loud squeal could mean the belt is slipping or the compressor is locking up. To determine which is the case, have a helper snap the gas pedal while you watch the compressor in action. When the engine is suddenly accelerated, a bad belt or tensioner will allow the belt to flap, indicating the lack of tension. If the belt does not indicate looseness, watch to see if the belt simply slips on the compressor pulley, even if the pulley does not attempt to stop turning. This indicates the need for a new belt. If the compressor pulley attempts to lock up when you accelerate the engine, the compressor is either locking up or is overcharged.
Listen to the compressor in operation with the engine running. If any knocking or tapping sounds are emanating from the compressor, it is failing. Your best move is to shut the air conditioning off immediately until the compressor can be replaced, even if it is cooling. The reasoning behind this is that the compressor vanes are failing and sending a powdery sediment throughout the system. An air conditioning system is a conglomeration of many parts, and taken as a whole can be quite expensive. This powder can plug up much of the system if left operating. This is the easiest diagnosis.
Watch the compressor clutch operation. If the compressor itself is quiet, but the compressor makes a small grinding noise when cycling, the clutch is bad. This should be viewed as good news, because it is an easy, inexpensive fix. There is no need to remove the compressor or discharge the system. Just replace the clutch mechanism.
Shut the engine off. Check the compressor with the leak detector. A compressor can be malfunctioning even if it cools and is relatively quiet if it leaks. Hold the sniffer under the compressor pulley, between the compressor and pulley. Freon is heavier than air, so it will always drop. Next check the compressor case. Most of the late model rotary vane compressors have two sections. Check where the compressor is pieced together -- if a leak is found in either of these two areas, the compressor needs to be replaced.
Hook up the air conditioning gauges. Make sure the two gauge valves are closed. Connect the red hose to the high-side air conditioning line -- this will be the smaller of the two lines and have a red cap on the Schrader valve. Hook up the remaining blue hose to the largest line with a blue cap.
Start the engine and turn on the air conditioning. A good compressor will display between 20 to 45 pounds per square inch on the blue, low-side gauge and between 200 to 280 psi on the high-side, red gauge. If the red, high-side gauge shows little pressure, feel the small, high-side line. If this line is cool to the touch, the compressor is malfunctioning.