Automotive Air Conditioning Information Forum

Hi,
AC clutch does not engage despite gets juice and good ground. Also clutch turns fine by hand. The following resistance readings are with compressor in vehicle and the coil disconnected (from bat. & to gnd.) at connector on top of compressor. A= anode fr. bat.+, C= cathode to bat.- & H= housing of mounted compressor;
A to C = 2.7
C to H = 2.7
A to H = 0.7
(When I touch the tester probes directly to each other I get 0.7)
Additionally (with both coil wires still disconnected) when I run a 14 gauge jumper wire from bat.+ to A it gets very hot very quick before I quickly pull it.
Interject here that previously a jumped fan belt had partially stripped both coil wires but not too bad. AC was not on at the time and the wires were easy to re-insulate after which the AC ran fine for a month. Current problem started intermittently but now seem permanent.
My thought (and hope) is that compressor, clutch and coil are OK but there is a short to the compressor housing in the inch or so of anode wire that turns to the coil between the pulley and compressor (not able to view it). It could have started when the fan belt "hit" the wire but anyway something is wrong now. I don't know if the diode at the coil plays any part here.
Thanks for reading and your comments will be greatly appreciated as I was unfamiliar with many of the terms used here a week ago.

arty4444 wrote:Hi,
AC clutch does not engage despite gets juice and good ground.

Simple: problem with AC clutch. With positive power and good ground, should behave like a very strong electromagnet and engage. So bad AC clutch.

Could be bad diode

Thanks- yes I know there is a fault- a short. It may be something that can be fixed without replacing any major component. I was hoping with the resistance readings given someone might know something specific about the nature of the short like "it must be the wire", "the coil is OK",or "cut out and replace the "backfire diode". Anything like that so when the repair shop says "you gotta replace the whole compressor" (for well over a grand) I can go to the next shop knowing at worst I might have to replace only the coil and maybe even only a diode.

The clutch coil should be about 2.7 ohms to ground.

What sort of vehicle is this?

It would appear that your overly complicated testing is the fault. If "A" is effectively zero ohms to ground, and "C" is 2.7 ohms to ground - which one do you think the ground wire is?

Obviously shorting the battery to ground through a zero ohm path will make the jumper wire hot.

Most likely there is no electrical fault. An excessive clutch gap will give intermittent operation, and eventually no operation. You can adjust the clutch gap on the vehicle in many cases.

I think I understand that your saying I've got what I'm calling the anode and cathode mixed up. When I attach the end clip of a "test light" to the neg. battery terminal and touch the (unconnected) contact in the pigtail that would normally hold my "A" described above with the probe I get a red light. Similarly when test pos. bat. to what would normally hold my 'C" I get green light. Anyway I will reread the instructions on my test equip.
Also I read elsewhere if I'm getting 0.7 ohms when I put my digital meter probes together that I need to subtract (index) that 0.7 from all readings which takes my 2.7 thru the coil to 2.0 which apparently is the boarder line for a shorted coil.
Oh- 2001 Dodge Ram Van 3500

Checked my light probe tester instructions and have them correct. Somewhere beyond (compressor side) of the pigtail connector at the compressor incoming voltage to the coil is shorting to then thru the metal housing back to neg. bat..
Sorry for a long confusing post. I was looking for confirmation of the diagnosis but don't see how it could be wrong. I'm going to replace the coil only (compressor has only 67K miles) with a new diode.

This is a simplification of a recent question I asked about a clutch that doesn't engage. With all (2) clutch coil wires unplugged from the connector at the compressor I measure zero resistance from the battery side wire going into the coil and the grounded housing of the compressor itself. Does this indicate a short circuit? While still unplugged a fused jumper wire from battery pos. directly to the same battery side wire into the coil blows a 30A fuse instantly. I'm sure I'm dealing with the battery side wire that brings voltage into the coil.
Thanks

Sounds like the clutch coil or if there is a diode in the coil ckt shorted.

Thanks,
It was hard to find a shop that would do a clutch kit instead of the whole compressor but finally found one with a good rep. also willing to use my EBay clutch when he couldn't locate one from his sources (it was tuff to find one).
With help from this forum I diagnosed my own AC problem and will be cool again for $300 instead of $1500+.