How To Test AC Compressor Clutch Coil

AC Compressor clutch coil

A friend of mine asked me the other day about the A/C compressor, and I promptly said that the A/C compressor is simply a pump, and sans a good compressor, the unit will never work. I further told him that the compressor clutch engages the compressor. This compressor clutch consists of a powerful electromagnet as well as a drive plate. 

As the drive plate is pulled into its proper place, you will hear a particular distinct click. The clutch does rotate, but the electromagnet doesn’t. It is bolted or fixed into place on the compressor’s front.

As you look closer at the A/C compressor, you will see a device consisting of fine wire windings. As current passes through the coil, it produces a strong magnetic field that engenders the splined hub to contact the spinning pulley, which, in turn, engages the compressor.

The coil has a clamping diode that checks voltage spikes as you disengage the clutch. It protects the control devices (PCM, BCM, or ATC module) and relay contacts. My friend simply scratched his head afterward. 

Steps on Testing the A/C Compressor Clutch Coil

You can quickly diagnose a defective clutch. But when running tests on your clutch, you should be very careful considering that the coil’s harness plug is small and is quite challenging to test. It will be helpful to take caution when you work around spinning and hot parts. Below are simple steps on how to do the tests:

Step 1: Examine the A/C Clutch

With the use of a flashlight, you can examine the A/C clutch. You will see a burnt or discolored clutch if your clutch fails. The clutch has flexible rubber isolators that let its disc move in and out. These rubber isolators will melt due to intense heat. 

Moreover, you will notice that the metal plates would appear rusty. If you see something like this, you should replace its compressor assembly.

Step 2: Check if the Magnet is Working Well!

After inspecting the clutch, you can unplug, then the compressor connector. Afterward, install the jumper connectors into the compressor’s electromagnet. You can utilize a wire crimp tool for installing small connectors onto the jumper wires that you would plug onto the compressor. 

Remember that the pins of the compressor connector are small. Thus, it will help if you chose connectors that fit onto the compressor plug and should not short out by inordinately touching each other. 

You should directly route the wires to the battery of the car. Then, connect a wire to the battery’s negative side. Afterward, let the other wire touch the positive side. The compressor clutch will indeed click onto the place if the magnet is working. 

After that, release the clutch as you remove the positive wire. You should repeat this process several times and listen to the click indicating that the clutch is energized. Nevertheless, if the magnet is not working, you should replace the compressor.

Additional Tests that You Can Perform

Aside from the tests mentioned above, you can also perform the following additional tests to ensure that you will have the right course of action:

Voltage Test

You can do a voltage test by removing the clutch coil’s plug and activating the A/C system by putting the selection on cold. You can use the 20-volt setting of the DVOM and check at the harness connector for the battery voltage. If you figure out that the plug has battery voltage, you can subject the coil for a resistance test. 

Take extra precautions not to hit the moving parts and hot parts like the pulley, fan, belts, exhaust, and radiator. Keep your hands, clothing, and meter wires off these parts. With the use of DVOM, you can back-probe the clutch coil connector’s positive side using the positive probe. Then, back-probe the negative side using the negative probe. The alternator output voltage should be around 1-volt. 

If there’s an excessive drop in the voltage available, chances are, the coil may produce extreme heat. In turn, this excessive heat can lead to early clutch failure. You can use the voltage drop testing and wiring diagram to check for excessive resistance cause if your reading gives you the wrong operational voltage.

Voltage Drop Testing Feed (Negative and Positive) Circuit

You can start the engine and position the air conditioning system in the maximum position. Utilizing a DVOM, position the positive lead at the positive post of the battery and then back-probe the clutch coil connector’s positive side using the negative probe. You should get 1.0 volts or less using the DVOM. If you get higher readings than 1 volt, it shows that the circuit has an excessive resistance. You should figure out then the reason behind this higher resistance.

You should also perform a voltage drop testing feed for the negative circuit. You can follow the same process as mentioned above, and using DVOM, you can back-probe the clutch coil connector’s negative side using the positive probe and position at the battery’s negative post. 

The DVOM should indicate 0.2 volts or lower. If you get readings higher than the one specified, it is indicative of excessive resistance. You need to figure out the source of this excessive resistance and correct it likewise.

Resistance Test

You can also do an A/C clutch coil resistance test using an Ohmmeter. The clutch coil should have a range between 2 to 5 Ohms. If you get a reading lower than 2 Ohms or beyond the 5 Ohms, this is indicative of a bad coil. If your reading is 0.00, then, most probably, there is a short circuit in the windings. A 0.L reading shows that there is an open circuit in the windings.

You should also test the diode. The diodes function like a one-way valve because the current only flows in a single direction. You can use your multimeter for testing the diodes. Your coils may be using Zener diodes that let the current flow. It will be useful, therefore, to check the specification manual before performing this diode test.


Working and testing the A/C compressor requires great caution because the harness plug has a small feature. Moreover, there is always the danger posed by spinning parts and hot parts. If you feel that you are incapable of testing the A/C compressor clutch coil, you can still avail of the service of experts in this field. In this way, you will have informed knowledge about what to do and how to perform the tests on your AC compressor clutch coil.

The usual problem with the clutch is that the flexible rubber isolators have already melted. Another problem is that the magnet may not be working well. So, if you want to test the A/C compressor clutch coil, you can always follow the steps mentioned above to test the A/C compressor clutch coil.