I have a 2003 Dakota 3.9 v6, I can't figure out what is going on with the charging system. On a cold start the battery indicator is perfect right in the middle. After the truck warms up ~15 miles or so the battery indicator falls all the way to the negative side. At that point if I continue to drive it, it will drain the battery until the truck shuts off.
I have tested the battery, alternator, cabling as well as the battery temperature sensor and all check out good.
Any idea of what could be going on? I erased all P-Codes yesterday because I was testing out the battery temp sensor and had a code for that while it was disconnected. I have driven ~30 miles and the check engine light has yet to come back on.
Thanks in advance...

 

When you're testing the system, is it warmed up or cold? Put a real voltmeter on it, not just the gauge, when the output drops. Measure across the alternator output to a clean ground.

Heat increases resistance. It could also be a circuit that "opens" when it gets hot, possibly in the alternator itself. Was the only code for the battery temp sensor? How did you test it?

 

I tested it both warmed up and cold with a mutl-meter. I am waiting for the gauge to drop and will test the alternator output across to a clean ground. The only code I received when testing was for the battery temp sensor when it was disconnected. I used an OBD II scanner I have.
My suspicion is that the alternator is just not kicking on, it seems it will kick on charge for a while and then kick off and never kick back on. I am afraid that if I just replace the alternator that the issue is in the PCM and I just wasted money. I will report back my findings on the readings once the car gets into the bad state.
Thanks for your quick response.

 

check your tensioner mine would not put the proper tension on the alternator

 

Tensioner is good to go. It charges when it wants to so the tension on the serpentine belt is ok, it is something with the alternator and being told when to kick on and off...

 

00DakDan,
I tested it in the bad condition and it is what one would expect. So when the engine is cold and everything is working properly I get 14 volts at the battery and about 14.2 across the alternator. In the bad condition the battery drops to about 12.2 (I would expect this because the truck is running off of all battery) and I get an erratic reading across the alternator anywhere from 3-5 volts, it seems to jump around a bit.

 

rebuilt alternators always suck low dependability but I know a new one is what $300 or more I would be more inclined to find a junker replacement and hope for the best.

Never had any luck with rebuilt alternators, starters and so on they last two or three years and Die.

Remember those parts are on other autos like Ram trucks Jeeps and many other autos just ask a junkyard what you want they should be able to tell what other autos have the same part.

 

By across the alternator you mean the field terminals? Yeah, 3-5 volts is low. My only concern is that the PCM switches the field very rapidly - up to 1000 hertz. It may not be really that low just maybe your meter can't read it. Still, if the voltage is low it should be "on."

The alternator and battery voltages should be about the same. Put the meter from the alternator output to the battery positive, measure the voltage drop across the charging leads.

Keep your code reader on and look for low charging voltage codes, P1682. It likely won't trigger the CEL but your code reader may detect the code.

Here's another, how many miles do you have on it? I had an alternator get erratic at about 135k on another vehicle, the brushes wore out.

PS: I agree with 98. Rebuilts are junk. I'd prefer the boneyard too.

 

Hey 00DakDan I wasn't blowing you off I was just working on my truck At any rate the alternator has never been replaced and the truck has 160 on it. It is time... But before I go and do that I found a open connector just hanging in the truck. I looked at the wires and they are Violet/Lt Green and Black/Lt Blue. So I went searching through all of the wiring diagrams in the Haynes Manual and I found that this is related to the Ambient Temp Sensor Signal. My question is I don't have a clue where it plugs into inside the Engine compartment. It is a really short run of wire so it can't be far from where it is.
It is located off of a main cable run that supplies the headlamps. Off of that is another run of 3. 2 of which go to the Washer fluid controllers and this one I speak of... Any clue where the heck this gets plugged into?
Thanks...

 

Post a few pic's that would help