Hauling the 5er home from Aztec, NM and 75 miles to the house, Check Engine light comes on and the voltage on the gauge drops to under 12 volts. Its always been right around 13.5... Made it home, overdrive works fine. Pulled the ground cable off and put the charger on the battery. Serpentine belt is fine as are all wiring and connections to the alternator. Before I pull the alternator, any other suspects? I forget, is there a Battery idiot light? If the alternator's bad, shouldn't that be the light that's lit? Blort the Blue Moose has never let me down and it would not surprise me if this is the factory alternator. Up to a measly 107k now. THANKS for your help. H

 

Try swapping the ASD relay with the A/C relay and see if it charges. If not it's probably the alternator.

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Check your obd codes, probably a combo of these:
41-field not switching
42-no asd voltage
47-voltage too low
If 47 only, you hav either a bad alt or bad PCM regulator
If 42, then do as wanna suggested
If 41, ground the upper wire/nut behind the alt and measure voltage at the bat post, just for a few secs while engine is running. If voltage goes up, alt is good, pcm VR is bad; if not alt is bad.

To read obd codes
Key on, key off
Key on, key of
Key on
Count the flashes, e.g. One flash followed by two flashes = 12 and thrn a pause before the next set of flashes

 

Like he ^ says. The quickest and least expensive path to a repair is proper diagnostics.

The CEL comes on when something is wrong.
The PCM sets a code that is a real good clue to what it is.
Go get it.

 

I'm on it. THANKS for the speedy (and great) advice. H

 

Check Engine light flashes: Looks like one followed by two and five followed by five. 12 & 55?

 

Oh no, the codes got erased when you disconnected the bat cables :-) lol

Back to square 1.

And your check engine light is still on? Or off?

Voltage above 12V?, alt charging, when all acc are turned on? Is your battery on its last leg? Any shorts? Sometimes you could get a 47 under heavy electrical load but it goes away later on its own.

I would proceed as if you got a 47, a 42, a 41 in that order.

 

Code 12 = battery's been disconnected
Code 55 = no more codes to display

 

LOL, I figured the codes would nuke when I unhooked batt cable.... No CEL presently. Volts still at 11.5. Will drive 'er around tomorrow until I get codes again and give an update. THANK YOU all. H

 

I think, according to both the diagrams and my experience, that the upper field nut on the alternator goes to the ASD relay. If you ground that, you'll blow a fusible link. It's the bottom one from the PCM you can ground for a test.

It's both easier and safer to use a test light. Top one should be bright, and bottom one should be flickering.

If the top one is dark, there's no power from the ASD relay.
If the bottom one is dark, the field (alternator) is bad.
If the bottomone is bright, the PCM or wiring is bad.

 

On mine the blue wire goes to the lower nut, and the green wire to upper. Anyway, the nut where the green wire goes is the one to ground. But because the wires are encased you can't tell which is which until you've put a continuity meter to them which I have on mine, just recently in fact.

Edit: J_Martin, I understand now why you were adamant about grounding. If you look at the diagram in the FSM plate AD-6 the alternator wiring shows the illustration of the connector and the alternator. Problem is they are upside down. For example, terminal A (ground) of the connector block is at the top of the diagram, but we all know that the ground terminal tip points down to ground, ie lowest part of the alt. The terminals in question are B (blue, ASD voltage) next to ground, and C (green, PCM field driver). So physically, from top to bottom goes C, B, A. The two nuts are C and B. The ground A is separate way to the side.

And to the OP whom we're trying to help, sorry for the confusion :-)

 

My main point is an accidental ground on the supply side of the field will let the smoke out of something, fuse link or ASD relay. That's why I'd probe with a test light or meter instead.

Also, the field will run with either polarity, so if a harness had those 2 connections reversed, it would not make any difference running. On mine the top was hot and the bottom switched, just like the diagram says.

 

Got up early this morning to investigate my charging issue. NipponDenso alternator??? Looks like killer good quality. Made in Tennessee??

Upon inspection, found the large power lead terminal loose and it had been arc-ing and finally carboned up enough to prevent passage of power altogether.


Tried to tighten the nut and the post gave it up obviously from a severe reduction in diameter due to the arcing. From the looks of it, its been doing just that for a while. Glad it wasn't worse.....



Managed to get a new panduit terminal on the power lead and there were just enough threads left on the post to land one nut, yes, without the lower locknut. Bought me some time. I'm going to need the alternator harness and a new alternator I guess, unless I can get in there and replace the post. Started up the truck, 13.5 volts just like it oughta be.

Once again, not a component failure at all but a lack of maintenance that led to the failure. Just gotta keep the terminals clean and snug....

Now, everybody go out and check the terminals on your alternator. Might want to pull the ground cable off the batt first....

Anybody know where I can get the alternator harness? New or used? THANKS to all for the support and input. H

 

Awesome, you found the culprit.

Yeah, i love those Denso alternators, good stuff.

I doubt you cand find a harness at the wreckers' yards. Maybe try Dodge ***********? For the alternator, I don't know where you are in Az, I recently replaced my good stock alt with a Delco 28SI, so it's in Craigslist cheap.

Also you don't have to replace the harness. There is an interconnect about 2 feet from the end. Cut that off and splice a #4 cable to it and an eye terminal for the end to your alternator. Just disable the stock cable at the terminal block.

 

What your alternator is upside down? LOL . . .