I need M939 Generator HELP!

[nate_o23]

Well I have a problem with our M939A1. It has gone through 3 generators in the past month.The first generator caught fire and even melted the wires. We fixed the wires and put on another gen. That Gen just went out, so I changed it again. When I fired it up and put a load on it (A/C and lights on) I found that I am getting 27.5V at the gen but only 25.5V at the battery. The gen also is getting very hot after only a few minutes of running. I didn't think this was right so I fired up a different 5Ton to double check.I turned the A/C and lights on, and after several minutes the gen was still cool to the touch. Something is definitly not right with our 5 ton so I did some more checking.

OK so here is what I have checked so far.

Main wire from gen to PCB= .1 ohms, wire from PCB to starter lug= .1ohms, starter lug to batteries= .1ohms

Neg wire from gen to ground= .1ohms

changed PCB

wire #568(I think) = battery voltage. I think this is the wire that powers the windings.

wire #566(I think) this is the one wire Im not sure of. According to the schematic it goes to the PCB but Im not sure what it does.

I have checked all the connections between the gen. and the batteries and everythng is tight and corrosion free. I just dont get why I am losing voltage between the gen and batteries.

So does any one have any ideas?? Something is causing the generators to heat up while running which Im sure is what is making them go bad. The question is what is causing this?? Any help would be GREATLY appreciated!

 

[acetomatoco]

Engine to Frame ground?

 

[nate_o23]

the gen ground goes from the gen to the frame down by the rad. I already took it off and cleaned it up. Didnt help though

 

[Recovry4x4]

I would also look towards a body to chassis ground Sir. That could also be the source of the problem.

 

[Rattlehead]

If you are dropping 2 volts between the gen and the bat, there has to be some resistance somewhere in that circuit. I am not sure how that circuit is routed to know where to inspect. If you are tracing the circuit with a meter, doing a voltage drop while the circuit is under load is the best way to find a problem area. For instance, if you hook one end of the meter to the alternator and the other lead to the battery, while the truck is running, in a perfect world the meter would read 0 volts. But you could drop maybe .5v and it would be acceptable. Then you move one of the leads up or down the circuit to see where the drop is occurring.

Checking voltage drop across the ground side is also important. Even though you checked ohms, the circuit was not under load and so problems may not be apparent.

Did you inspect the entire length of that wire from the gen to the batteries? I was just thinking that maybe the insulation is chafed somewhere, and now corrosion has set in.

Could it also be possible that you have one or more batteries that have an internal short, overloading the alternator? Maybe disconnect the parallel jumper battery cables and check the open circuit voltage of each battery.


Hopefully this board can help you out! By the way, you said "A/C", do you mean air conditioning?

 

[nate_o23]

Recovery 4x4 - Do u mean a ground from the cab to frame? I will look for one.

Rattlehead- thats a good idea about the voltage drop test. I will try that. I will also look over the wires a little more to see if maybe something is shorting out.
Yes these have A/C in them. Its an after market red dot system that was added when they put on the armor cabs.

 

[steelsoldiers]

I will take a look at my truck tonight and study the schematic to see if I can find any more potential problems. Sounds like something is back-feeding into the charging system. My truck had some terrible grounding issues when I got it. It's got to be something big though to keep cooking the alt.