Forbitron

I have a new compressor in my 98 cherokee 4.0 but the clutch will not engage without directly applying power. I brought it to someone to test and he tells me it's the ECM, what are the chances that is right?
What was done: (to the best of my ability to explain)
-Pressure of refridgerant
-Leaks
-Low pressure switch
-High pressure switch
-Relay and fuses, engine box and passenger side box. All do receive current.
-Connected power right to compressor and it engages.

Any assistance would be appreciated before I spend money on the ECM.

CCKen

When you say Low pressure switch and High pressure switch, are you saying you replaced these switches? Same with the relay and fuses?

El_Guapo

Before getting too worried about the ECM, check your clearance between the pulley face and clutch plate with a feeler gauge. It should be somewhere between .015" and .025". If it is bigger, the residual voltage drop of going through the system can cause it not to engage.

Next, get your VOM out and make sure the coil polarity isn't reversed relative to the harness. I chased my tail for two hours after changing the compressor on my TJ before my neighbor the electrician figured that one out.

Forbitron

The relay was replaced and was tested as good.
Fuses were swapped and the panel was checked for current.
Low pressure switch was tested by pulling the plug and putting in the paper clip, unsure of how he tested the high pressure. I was not present for all of it.

CCKen

Reversed polarity? Do you mean the Diode was installed backwards?

El_Guapo

Apparently 1 leg off the coil is grounded to the case - he swapped the wires and it worked like a champ. He told me they see it a lot on aftermarket coils on the hoists he normally works on.

CCKen

I'm really interested in how this electrician swapped the wires.


Your TJ, like the XJ, has only one wire going to the AC Compressor clutch. The 2002 Liberty KJ has two wires going to the clutch so I can see where that could be reversed.


See if you can ask the electrician how he did the swap.


Here's a pic of the XJ clutch wiring (unwrapped)


Click the image to make it larger, then click it again to make it larger yet.


https://www.cherokeeforum.com/g/album/3583260

El_Guapo

I was there when he did it. He cut and reversed the 2 leads on the compressor side of the plug. I don't particularly care WHY it worked, I care THAT it worked.

The dude is a master sparky with 30+ years in the game. That's why I knock on his door when I'm stumped.