How to Check the Condition of a Car Compressor?
3 Answers
There are three main aspects to check the condition of a car air conditioning compressor: 1. Checking the compressor's sealing: Use a manifold gauge to test by connecting the high and low-pressure hoses of the manifold gauge to the compressor's high and low-pressure service valves, respectively. Start the engine and let it run at an idle speed of 2000r/min. 2. Measuring the resistance value of the compressor's electromagnetic clutch: First, remove the electromagnetic clutch coil from the compressor, then measure its resistance with a multimeter. Since the standard resistance of the electromagnetic clutch coil is (3.7±0.2)Ω, use the ohmmeter function of the multimeter to avoid measurement errors. If the multimeter needle does not move during measurement, it indicates that the electromagnetic clutch coil is open. If the measured value is less than 3.5Ω, it means the coil is short-circuited, open, or the electromagnetic coil should be replaced. 3. Checking the operating condition of the compressor: Whether the drive belt is broken or loose—if the drive belt is too loose, it will slip, accelerate wear, and fail to transmit power; whether there is noise inside the compressor; whether the compressor clutch is slipping.
Start the car and turn on the air conditioning, setting it to the coldest temperature with the highest fan speed. Observe the changes in engine RPM. When the compressor engages, the RPM should slightly increase by around 100 RPM. If there is no change, it may indicate the compressor is not working. Next, feel the high and low-pressure lines of the AC system—the high-pressure line should be hot to the touch, while the low-pressure line should be cold. If both lines have similar temperatures, there may be a fault. Listening for sounds is also important; normal operation produces a steady hum, but squeaking or metal grinding noises suggest internal compressor damage. Finally, have a professional connect an AC pressure gauge to measure the high and low pressures. Standard readings should be approximately 2-3 bar on the low-pressure side and 13-17 bar on the high-pressure side. If the readings deviate significantly, consider replacing the system. Remember to run the AC for a short period every quarter to prevent seal aging.
My two-decade driving experience has taught me some practical tricks: Start the engine and turn on the AC, wait three minutes, then feel the vent temperature. In normal July/August weather, it should blow around 10°C cold air. If noticeably warm, don't rush for repairs—first check if the fan spins, then inspect coolant. Belt inspection is simplest: after turning off the engine, try rotating the compressor pulley by hand—if it spins easily, the clutch coil is likely faulty. Old cars most commonly suffer leaks—use a flashlight at night to check pipe connections for oil stains (indicating refrigerant leaks). Here's a tip: During traffic jams, turning off AC and opening windows saves fuel. Regularly driving at high speeds to make the compressor run at full load actually extends its lifespan. For repairs, always replace the receiver-drier before recharging refrigerant to completely solve the issue.
