Problem with testing polarity on A/C clutch coil

[JoJoMix]

Hello everyone,

I have a problem with my Dodge challanger 2015 with AC problem as it was not cooling. Took it to a mechanic, a friend of mine, to fix it and he doubted the clutch coil is the problem. He disassemble the A/C and took the compressor out to test it. Not sure why he wanted to test the compressor clutch by connecting 12v directly through the clutch terminal (a plug with 2 pins) and he somehow burnt the flywheel diode that protects against the surge. I knew after reading though forum that clutch coil doesn't have a polarity and in case a diode exists, it would be installed in the harness. Seems like newer clutch have built-in diode or thermal fuse and that got burnt just because my friend didn't bother check polarity first.
Now the old clutch with burnt diode is shorted so coil would heat up where usually from what I searched if diode is damaged it would be open and compressor clutch would work normally but only the problem of voltage spike would occur. Mine just complete short circuit at diode. 
I went and bought a used clutch but got promised that it works. The one I got has green/black wire to harness while old one was white/black. The clutch is the same but the harness is different and not sure how to first test it and how to connect it. Is the green +pos and black -neg
I put picture comparing both old burnt coil and the supposedly working one. I did try diode test using a multi-meter but I get a reading of 0.05v on both sides of terminal. Also when trying ohm test the reading fluctuates alot (maybe because i'm testing a magnetic coil) till it settles to 3.4 ohm and the same reading if I switch polarities.

So I got nothing from the multi-meter to indicate the diode position as I do not want it to get shorted while testing.

[Dougflas]

The black band on the diode should face the ground.

[bohica2xo]

Always a tricky test.

Because the coil & diode are in parallel, the resistance through the coil is the same both directions. But the diode has a 0.5v drop one direction, and is effectively an open circuit the other direction. A DMM will not display the very small change when switching polarity.

A test could be done with a current limited power supply and a second resistor.

You would probably be money ahead to just get the right part for your car with the correct connector already installed, rather than the wrong part from a junkyard. A mistake that damages the wiring or electronics in a post 2000 model year vehicle can be very costly.

[JoJoMix]

The black band on the diode should face the ground.

Hi Dougflas,

you are right about black band (cathode) should be connected to ground but that would only help me in case i want to replace the burnt diode which was completely toasted and crumbled. These diodes are hard to access and I have to cut open the sealant on the wire.
I just wanted to test the correct polarity without touching or harming the diode

[JoJoMix]

Always a tricky test.

Because the coil & diode are in parallel, the resistance through the coil is the same both directions. But the diode has a 0.5v drop one direction, and is effectively an open circuit the other direction. A DMM will not display the very small change when switching polarity.

A test could be done with a current limited power supply and a second resistor.

You would probably be money ahead to just get the right part for your car with the correct connector already installed, rather than the wrong part from a junkyard. A mistake that damages the wiring or electronics in a post 2000 model year vehicle can be very costly.

hey bohica2xo,

Thanks for the info. Testing ohm and diode forward voltage with multimeter is truly tricky as you mentioned coil and diode are in parallel. I do however have a bench power supply with adjustable voltage/ampere but still if I connect it correctly given i set current to 0.25A, current will flow through coil and the opposite polarity not sure but current will still flow either trough diode only like short or split between both and would get the max limited current flow. What is the resistor you mentioned for?

[bohica2xo]

Because a diode tested by itself will give a resistance higher than the coil in either direction on your DMM, you need to apply a small load to cause current to flow through the diode.

The resistor is used to build a voltage divider.

With the coil & diode assembly connected as one of the two resistors in a divider, you will get different output voltages when you reverse polarity of the circuit.

You could of course add three resistors & build a Wheatstone Bridge, but the simple divider should be good enough for your needs.


Remember the 0.5v drop through the diode when calculating resistors. Choose an arrangement that will give a substantial voltage drop with the 3.4 ohm coil, otherwise the drop through the diode may make it hard to measure.


Here is a handy calculator

http://hyperphysics.phy-astr.gsu.edu/hb ... oldiv.html

[CariKahn]

Hi... all cars have a high pressure sensor and a low pressure sensor in the AC line somewhere that are there to protect the compressor from being burned out and that would have been a couple of easy checks to do before checking the clutch on the compressor. I would want to know what checks he did before going to the compressor clutch. I don't remember how to check the clutch coil without doing some research first, but I would have ruled out a lot of other things before I went with the clutch and coil.

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