Your car air conditioner stopped cooling—how do you determine what's wrong with it? Here are some testing procedures to use and get to the problem. Hopefully this will help you understand some of the causes and cures for a non-working air conditioner.
Of course, some car AC repairs are best left to a professional technician due to the skills and tools required. If you're not comfortable doing these repairs, or you don't have the right equipment, don't hesitate to take you vehicle to a professional—they'll follow some version of the process described below.
Electrical and Mechanical AC Elements
Start the engine and turn on AC, then open your hood and observe the compressor. Is the clutch engaged and the compressor shaft rotating? If not, there are several things to check, starting with the fuse.
If the compressor looks good, move on to the low pressure switch, usually mounted on the suction hose near the evaporator or on the accumulator/receiver dryer. Unplug the wiring harness from the low pressure switch and the jump terminals of the wiring harness with a jumper wire. If the clutch now engages, you're looking at two possibilities—one is a faulty switch, the other is that the system has no refrigerant.
If the clutch doesn't engage with the low pressure switch jumped, shut off the engine and unplug the wiring harness from compressor clutch. Using a multimeter, check for continuity and resistance across the two terminals of clutch magnet. If the test shows an open circuit, the clutch magnet windings are burned out. Repair or replace as necessary. If it shows continuity and a low ohms reading then measure the clutch shoe distance from magnet.
Using a feeler gauge, determine the distance between the clutch shoe (sometimes called plate) and clutch magnet. Measurement should range between 0.020” and 0.010”. If the reading is over 0.020” then the shoe will need to be removed and a shim washer taken off the shaft to allow the shoe to be reinstalled with the proper clearance distance.
Shoe removal requires a special puller designed for the particular compressor. Attempts to remove clutch shoe without proper tools will result in damage to other compressor components that will far exceed the cost of shoe puller.
Connect a Manifold Gauge Set
If the compressor will rotate, but the air conditioner won't cool, it's time to connect a refrigerant manifold gauge set to the AC system and check system pressures with engine running and air conditioner turned on.
Connect the blue hose to suction port and red hose to the pressure port. Open the gauge valves and observe pressure readings. If the pressures do not check out to specified readings according to service manual it is possible to add some refrigerant as a test to try and get system back in operation.
If the pressure side gauge reading is high while at same time suction side gauge reading is low a clog in the system piping is suspect. The plug is most likely at the expansion valve/orifice tube as this is a restricted flow point where the expansion of refrigerant takes place to enable the cooling to happen. This will require reclaiming refrigerant and opening system to get to the expansion valve/orifice tube and clean or replace as needed. The entire system will need to be flushed before reconnecting components, vacuuming, and recharging with refrigerant.
The compressor may be rotating but not pumping, due to internal damage. If the suction side and pressure side gauges show nearly the same pressure readings while the compressor is rotating, this indicates the compressor has failed. Replacing a failed compressor and the flushing, vacuuming, and recharging of system requires equipment the average DIYer probably doesn't have.
Lack of Air Flow Over Evaporator and Condenser
The condenser (in front of radiator) must have a good flow of air to facilitate heat transfer to cool the hot refrigerant and likewise the evaporator (embedded under dash) must have good airflow to distribute the cooled air inside vehicle which is accomplished with a blower fan forcing the air movement. Be sure there is no trash or debris blocking air flow. Clean as necessary to get the maximum possible air flow.