AC Fast Facts

Modern A/C system diagnosis can be as complex as any OBD-driven electronic issue or mysterious driveability problem. Unlike many of those systems, however, a “calibrated hand” can go a long way toward eliminating unnecessary troubleshooting time and effort. Here’s how:

Is there really a problem? With the advent of computer-driven HVAC controls, a surprising number of consumers don’t understand how to properly set their systems. Try all A/C modes, including defrost, to make sure that the problem really is under the hood, and not in the dash. Use the owner’s manual if you have to.

Does the compressor clutch engage? Watch and listen as a helper cycles the dashboard controls. If it doesn’t, it’s time for the gauges.

Check for static pressure with the system off. About 40 psi is needed to allow the pressure switch to send current to the compressor clutch, so if the pressure isn’t there, go to your standard leak checking procedure. If pressure is good, jump 12 volts directly to the clutch and check for cooling.

Start and warm up the engine. Grab the discharge, liquid and suction lines as well as the compressor body and feel for temperature.

The discharge line should be just a little too hot to comfortably hold. If it’s extremely hot, look for a restriction in the line. Too hot, and the system is overcharged or there is air in the system. Also, the condenser may be not losing enough heat. If it’s only warm, the compressor is shot, or the system has a leak.

The liquid line should be warm to the touch. If it’s hot, see the instructions for a very hot discharge line above. If it’s cold, there may be a system restriction. Note that this condition is normal if the system uses an orifice tube in the condenser.

The suction line should be cold in your hand. If it’s warm, check for refrigerant leakage and the orifice tube or thermal expansion valve for blockage. Also, the compressor may have poor suction. If it’s just cool, expect a worn compressor or stuck thermal expansion valve.

Finally, touch the compressor. It should be a little less hot than a valve cover with the engine at operating temperature. If it’s extremely hot, check for a discharge line restriction. If it’s very hot, the system may be low on oil. In the less likely event that the compressor body is cool to the touch, there is excessive oil or refrigerant in the system.

It goes without saying that reaching into the engine compartment of a running vehicle should be a task performed with a lot of consideration to safety. It’s hot in there, and besides whirling belts, electrically operated fans can start at any time, without warning. Use lots of light, and remember warnings about loose clothing and jewellery. The “hand” tests above take only a minute or two, and can save lots of time when you’re tracking down a system with poor cooling performance.

Thanks to Professor Tom Brown, Motive Power Program, Centennial College, Toronto, Ontario