M931A2 Alternator is overcharging???

[Mike929]

Have an issue with the alternator overcharging. In the Red at idle and any throttle makes it go even higher.

Originally this fried my first set of batteries and blew out at least one of the gauges in the dash. I replaced the Alternator, batteries, etc., and upon hooking up the new alternator, I'm getting the same issue.

New alternator is a bulldozer alternator with only a ground and positive connections. The small plug in wire (568 ) is not connected in the new alternator.

Did I just get two bad alternators in a row, or is there some obvious issue that I'm missing. I've gone through the TMs searching on overcharge, voltage, and Alternator. Beyond the test steps in TM 9-2320-272-23-1, I haven't found anything.

Truck is 2 hours away, and I have verified the cables are connected correctly, but will check again tomorrow, and run through some of the multimeter checks inside the cab.

Brother is suggesting we rewire everything based on standard Semi Truck to replace the ECU(? black box) thing.

Any advice before I spend my Labor Day inventing new curse words?

 

[Mullaney]

Have an issue with the alternator overcharging. In the Red at idle and any throttle makes it go even higher.

Originally this fried my first set of batteries and blew out at least one of the gauges in the dash. I replaced the Alternator, batteries, etc., and upon hooking up the new alternator, I'm getting the same issue.

New alternator is a bulldozer alternator with only a ground and positive connections. The small plug in wire (568 ) is not connected in the new alternator.

Did I just get two bad alternators in a row, or is there some obvious issue that I'm missing. I've gone through the TMs searching on overcharge, voltage, and Alternator. Beyond the test steps in TM 9-2320-272-23-1, I haven't found anything.

Truck is 2 hours away, and I have verified the cables are connected correctly, but will check again tomorrow, and run through some of the multimeter checks inside the cab.

Brother is suggesting we rewire everything based on standard Semi Truck to replace the ECU(? black box) thing.

Any advice before I spend my Labor Day inventing new curse words?

.
Question for you: The bulldozier alternator - does it have a built in regulator? Obviously something somewhere is passing more voltage that it should be so that is where I would look first if I were you. Maybe a meter rather than relying on the truck gauges would be good.

it won't fix it by Labor Day but a couple of the guys on this board sell alternators that work well on the 939 Series trucks.

 

[Mike929]

Re output from alternator
I believe my brother did that last weekend. I think it is a problem with the alternator, just wanted to see if not hooking up that one 568 wire was causing some issues. Hard to believe I've had two bad alternators in a row on top of killing the original one, was hoping for a "don't do this" or "are you sure you did that" type solution. Really don't want to buy a third alternator and see the same problem.

Main goal tomorrow is to install the hard top I bought from another member. I was just trying to see if I was doing something incorrectly, or if I needed to look for something that was killing the new alternators that I was missing.

Thanks,
Mike

 

[162tcat]

Simple solution is to install a new one wire alternator grounded properly and positive wire ran to starter or directly to battery. No need to rewire everything if nothing else is wrong.


Sent from my iPhone using Tapatalk

 

[Tow4]

Instead of throwing parts at it, take the alternator and have it tested. That way you know if it's the problem.

 

[Mike929]

Agree about testing alternator off truck, just confused about having two overcharging alternators in a row.

What do you do with the small wire when hooking up a civilian alternater. Positive and ground cables are obvious, looks like the small wire is needed for anti lock, etc.

Concern with black box is that it didn't stop my first replacement military alternator from blowing out all my lights, gauges, and frying 4 batteries. Thus the thought of abandoning the black box and setting up new breakers to save replacement lights, batteries, etc.

Ended up helping family over the weekend, and didn't work on the truck.

 

[FullSpecial]

Does it have a good ground? Try running an extra ground wire to the frame and see if that helps. Have seen some goofy things with bad ground at alt.

 

[Mullaney]

Does it have a good ground? Try running an extra ground wire to the frame and see if that helps. Have seen some goofy things with bad ground at alt.

.
Go cheap to test that ground theory...

Get your jumper cables out of the back of your truck - and scratch them into the frame on one end and to the battery on the other.

If that doesn't work, try the same onto the bracket for the alternator and to the battery. Just trying to give your ground "another path". If it works, you need to hunt down that flat braided ground wire already attached to the frame - and scrape and clean and apply dielectric, then bolt it back up.

 

[Mike929]

Thanks, i will give that a try