Atlernator Field Switch?

My son keeps getting this error code (41) on his 91, 4.0L, AW4

We've already cleaned all the grounds and replaced the alternator.

Completely stumped.

Whats the fix?

That code is for open or short in alternator field driver. PCM pin 20, dark green. That circuit goes nowhere but alt directly to computer so I'd probably disconnect the pcm connector and the alt connector, check continuity, make sure there's no short to voltage or ground.

Is the ground from the alt to the engine block connected? Had that same error on my 93 a while back and found that the ground was just sitting on the post without being connected. When you say you cleaned all the grounds did you actually do all of them or just the ones for the alt?

JJacobs said:

That code is for open or short in alternator field driver. PCM pin 20, dark green. That circuit goes nowhere but alt directly to computer so I'd probably disconnect the pcm connector and the alt connector, check continuity, make sure there's no short to voltage or ground.

Click to expand...

I'll check that one...again. Arggggggggggghh. We've tracced every wire all the way from the Alt to the PCM and back again.

dave92cherokee said:

Is the ground from the alt to the engine block connected? Had that same error on my 93 a while back and found that the ground was just sitting on the post without being connected. When you say you cleaned all the grounds did you actually do all of them or just the ones for the alt?

Click to expand...

Yes it's connected, and yes we went thru every ground in the engine bay.

Have you checked the alternator itself? Might have a shorted or open field coil, could be bad luck on a new/reman unit.

Other than that and the wiring harness, probably your PCM has a bad field coil driver in it. If you are suitably motivated and skilled with a soldering iron you have a chance of repairing it, else hit the salvage yard.

Yes we had the alternator tested.

We swapped in a spare PCM, same problem.

It's got to be in the harness since Daniel is getting sporadic voltage drops and spikes.

The frustrating thing is that we've gone over the harness numerous times. Back probed connectors, checked for continuity, wiggled the wires around, pushed and pulled on connectors....

Just when we think we've narrowed it down the damn thing will run perfectly and the voltage stays steady....CEL goes off.

Wait a minute or two and just look at the motor cross-eyed and the voltage changes and the CEL comes on.


Very frustrating.

wow that's pretty screwy.

Not overly familiar with the way the field coil wiring is done, is the field coil ground common to the rest of the harness or run separately all the way back to the PCM? I would put the voltmeter on the connector at the PCM and wait for it to flake out, see if it shows a problem... if it does, move it one step downstream, continue till you find the bad spot.

Yeah, intermittent wiring faults are about the most frustrating problem there is to debug. Intermittent software/firmware issues are the only worse thing, as they can completely blow away any trace of info that you could use to find them... at least wiring and hardware stay in the same place usually.

I'm thinkin'....BONFIRE!!

I'll post up more results/developments as they occur.

Sounds like an intermitent bad alternator to me

Frank Z said:

Yes we had the alternator tested.

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Just tested or replaced? I doubt the PCM would be an intermittent problem.

I believe there is a +12-volts to the field coil on the alternator (from the ASD relay?), and the PCM controls the ground side of the field coil. The problem might be on the supply side of the circuit and not between the alt and pcm.

Both. When the code initially popped up I put a new alternator in. A couple days later the code came up again. Had the new one tested and checked out fine.