A/C Compressor Clutch Coil
An A/C compressor clutch coil is a simple device made of fine windings of wire wrapped into a coil. When current is passed through, it creates a strong magnetic field that pulls a splined hub into contact with the spinning pulley. The coil contains a clamping diode to prevent voltage spikes when the clutch suddenly disengages. This is important because these powerful spikes can damage relay contacts and the control device, such as the PCM, BCM or ATC module. An A/C clutch coil is an electromagnetic device used to pull the clutch hub that’s splined to the compressor shaft into firm contact with a belt driven pulley. This action engages the compressor.
Take great care when running these tests, because the coil and harness plug are very small and may be difficult to work with. Always use caution when working around hot or spinning parts. The circuit should be checked for proper voltage before testing the coil for resistance. Before testing, visually inspect the coil for overheating and cracking. Heat expands the coil and creates breaks in its windings.
Voltage Test: Remove the plug from the clutch coil and activate the A/C system by placing the selector on cold. Using the 20 volt setting on the DVOM; check for battery voltage at the harness connector. If battery or charging system voltage is present at the plug, test the coil for resistance.
Resistance Test: An A/C clutch coil resistance test is performed with an Ohmmeter. A result of 0.00 indicates that there’s a short in the windings and a result of O.L or infinity indicates an open in the windings. There's nothing special about a clutch coil, most readings are in the range of 2 to 5 Ohms, and any reading below 2 Ohms or over 5 Ohms indicates a bad coil. The diode should be tested as well. Diodes are like one-way check valves, because current flows in one direction and not the other. Many multimeters have a selection or function for testing diodes. This is because many coils contain zener diodes that allow current, but only up to a certain threshold. If unsure, check with the manufacturer's manual for specifications and special procedures for performing the test.