I hope someone here can help. I have a 2001 Durango. My daughter was driving it home from work at night, lights on, etc., when it died. Had it towed home and saw that the battery only had about 6 volts on it. Took the battery off and charged it to see if it would hold a charge. It did. It's only 6 months old, so didn't ~think~ that was the problem anyway. Did some testing and thought the alternator was bad, so replaced it and put a new belt on "just because". Alternator still not charging, so brought battery in for testing. It was good. Playing with my multimeter, we saw that if the control cable on the back of the alternator was unplugged, it would keep the battery around 12 volts. If it was plugged in, battery would just get drained. Somewhere in the midst of all this, speedometer pegs itself out at 120 mph and stays there (car sitting in driveway all this time). Based on internet research, we suspect PCM. Ordered one from an ebay seller who programmed in with vin & mileage. Arrived today. Put it on the car, and absolutely no change. Exactly same behavior.

So, battery fairly new tests good; alternator brand new; pcm new. What else could be the culprit?

 

6 volts in the battery means you have a bad cell. Replace it!!!!

Sounds like it is grounding out the system.

 

I thought the battery was the problem first. It only got to 6 volts because the alternator wasn't charging it & the headlights were on for the trip home. But after I recharged it, it's held 12 volts. On the car with the engine running, it will slowly die unless the control cable to the back of the alternator is unplugged. As long as it is unplugged, the alternator keeps it at 12 volts (doesn't ever show a charge of 14 ish when running like it should).

Since the battery tested good when we brought it in, we assumed it was the alternator and replaced that. Same thing, 12 volts across it when running, slowly dies unless control cable is unplugged.

Next we assumed it was the voltage regulator in the PCM since the alternator didn't charge at all if it was connected to PCM. Before I broke down and bought it, the speedometer pegged at 120 mph while sitting, so I thought that sort of confirmed the PCM was the problem. New PCM, and it still has the exact same symptoms:

Alternator not charging battery.
Speedometer sits at max (120 mph).

There seems to a common sensor perhaps that is telling the pcm not to let alternator charge? Would the speedo sensor do that? I can't find any schematics for the pcm that can help me troubleshoot.

 

Sounds like there is a ground somewhere that is loose or dirty. The speed sensor on the rear axle controls the speedometer but usually when that goes it also throws the ABS light.

I just had a brain surge and remembered something.... might not be your case but its worth a look.... One time my son thought it be cool to stick a screwdriver in the power outlet while i was not looking. It threw a big spark and scared the crap out of him. Once i figured out what he did i thought he had just blown the fuse for the power outlet, replaced it and moved on. Later that night i went out to the truck to go to the store and when i turned the key the gauges went nuts and the warning lights in the dash where flashing rapidly really dim.

I figured that when he put the screwdriver in there he might have fried the jacket off a wire that was causing a short. After i pulled the whole dash out and checked every inch of the harness i still had the same issue. Turns out that when he put the screwdriver in there he somehow blew one of the bigger fuses in the PDC (cant remember which one) and it was causing all sorts of weird things. Once i replaced that all was good again..

Might be worth a shot to check, if not that the only other culprit i can think of is a loose/dirty ground.

 

In the bottom of the battery box is a spring loaded disc shaped sensor that monitors the heat of the battery to keep it from over charging but at the same time will not let the battery charge if it's bad. Replace the sensor. As for the speedometer...drive it about 100 ft and it will come back to normal.

 

Thanks for the suggestions. I've checked all the fuses. Replaced the battery temperature sensor. Other than the speedometer deciding to go back to 0, no change. PCM/Voltage regulator is still telling the alternator to not charge the battery. About to just hook up an external voltage regulator so that the car is functional again, but would sure like to correctly diagnose the problem.

Open to any other suggestions.

 

I think I might have found something. On another site, I found part of a diagram for the circuitry. The two control wires that go into the back of the alternator go to PCM pins C2 #10 and C3 #25. When I tested continuity, both wires are connected to BOTH pins and to each other. That can't be right. Seems like I have a short in the harness. I'm attaching the diagram in case it helps anyone else.

 

Here is how you test your alternator on the vehicle
I suspect you have a bad computer, I've seen a external voltager regulator kit or a computer bypass kit ERCKFRM, do a search to find them
Here are the testing instructions
How to test your alternator on the vehicle:

To check to see if your alternator is working, unplug or remove the two field wires from the back of the alternator. Then ground one of the field terminals on the alternator and jumper the other field terminal to the positive battery post on the back of the alternator. It does not matter which terminal is grounded and which one get positive power. When you connect the ground and positive wires to the field terminals you will get a small spark, this is normal. This is by-passing the voltage control and will cause your alternator to run full blast, if it is working. Put a multi-meter on your battery and run your engine on high idle, do not allow your alternator to charge the battery much over 16 volts. If you find that your alternator is charging when you do this procedure, but does not charge when you connect it back to the computer, this kit will get your alternator working and charging at the proper voltage. The voltage regulator is preset at 14.5 volts. If you want to adjust the voltage regulator you use the screw on the back, but be sure the metal case of the voltage regulator is grounded while you adjust it.

Ref: QSERCK , ERCK, FRM, ERCK-FRM