In the middle of my 4bt swap into my half ton, using a Dodge Cummins PCM and Engine/Manual transmission wiring harness so everything works like it had the 12 valve in it. Anyways, I hooked everything up and started it up and the Alternator was charging at 18 volts. So I went to the junk yard and robbed an older Chrysler car of its external voltage regulator and hooked It up and bypassed the one in the PCM. Plug it in and same thing, charging at 18 volts. So I throw on a new alternator thinking that that is the issue, and why not because it's original. Plug it in with the PCM again and charging at 18, and then switch back to the External Regulator, and again at 18. So I unplug all connections to the alternator and as expected, it didn't charge at all. So I'm trying to figure this situation out here and am stumped as to why its overcharging the pickup. Any help would be appreciated.

Thanks, Christian.

 

Are you going by the instrument panel or did you measure with a voltmeter ? If the latter, did you check at the alternator and the battery ?

 

I was going off of both my dash gauge and a volt meter on the battery. I did not test at the alternator, but I don't see why I would have to test at the alternator if its overcharging at 17 to 18 volts at the battery.

 

Did you ground the body of the regulator? And where did you tap into the ignition switch?

 

yes I grounded the body of the regulator to the alternator, and i'm getting power through a relay direct from the battery. Using an add a circuit out of the fuse block in the dash to activate the controls side of the relay. Also verified I have power all the way to the regulator too.

 

Is the regulator not switching the field on the alternator? Got a short in the wiring somewhere?

 

No reason to if you checked at the battery. Might be a stupid question: What alternator are you using ? Don't know if it's purely physical dimensions or internal differences too but at least for the later models the 136A alternators have different part numbers between gas and diesel.

 

Yeah, but, they still work the same way......... I am thinkin' wiring problem. Given that it exhibits the same behavior under two different system, I would suspect the field positive wire is shorted to power somewhere.

 

Actually the field coil gets permanent 12V from either the ASD or the PCM in later models, and the PCM switches ground on and off in a pulse width modulation fashion. The PCM likely has some minimum/maximum values for PWM which could be different depending on alternator. Hence my question. But along your suggestion, the field negative could be shorted to ground which would make any attempt to regulate the output fruitless.

 

That seems perfectly reasonable.