So my 91w250 quit charging on me, First I took apart a few plugs and proceeded to pull the harness out of the engine bay and went through the whole damned thing, wired everything into a nice little aftermarket fuse/relay box. Finally got the harness back in, truck runs and everything works as it should. Other than it charging, so I went about and tested the Alternator and its only throwing out 12 volts (plus or minus a few tenths). Next I went online looking around for a new one and I see they all come up to have an external Regulator. And well I have no idea where in the hell it would be. I have a few empty plugs here and there but theyve been that way since I bought the truck. Now Let me tell you a little about my truck, the title says its a 91 gas, But infact it is a cummins, from what I got from the P/O they took a 89 cummins and swapped the drive train out. It is intercooled now, no idea if it was originally. but all the piping looks factory to me, so at least that parts done well. Back to the charging, there is a computer or something of the like bolted to the inside of the drivers fender right behind the lights, Does that control the charging? Ive been driving this thing for a little over a year now like this with no issues until a month or so ago. Also the check engine light flashes code 12 and then code 41. First pic is of the computer thing, second is of the truck.

 

i am assuming your truck has a 91.5 or newer engine. the fact that it is intercooled, and has that controller, is what makes me believe that. Yes, the voltage regulator is in the controller. most charging failures with this set up are caused by the crankshaft sensor.
it isnt that difficult to change to an external regulator if you have failed components.

 

The regulator could very well be in the middle of the cowl, near the windshield wiper motor. '91.5 and earlier used an external ubiquitous mopar regulator mounted there. They can be used/retrofitted with any year ecm.

 

Got it, Took the Alternator to be tested, said that works perfect. So I was looking at the crank sensor, there was some grease stuck to it, I guess it wouldnt let it read, cleaned it off, regapped it and it goes perfect now.

 

I had a similar issue, it turned out to be a burned charging wire that run from the battery to the firewall. New alternator would work then go to full charge and burn up. Took three alternators to figure out.