starter only bumps engine after alternator replacement

[brazengoat06]

So had an alternator fail on a m934a2 and replaced it with a used unit. first time i fired it up was fine but realized i had left the small gauge wire unplugged so of course it didn't charge. shut off truck and plugged it in and when i went to crank it the starter would bump the engine and then stop, then bump the engine and then stop, and so forth. Eventually the engine started and the alternator charged as expected. Shut off truck and unplugged small wire and truck starts fine, plug it back in and the starter only bumps the engine.

Does the PCB receive an rpm signal from the alternator? And yes it was a used alternator.

 

[Tinstar]

Did you look at the wiring diagram?
I would start there.

When I replaced my alternator, the new one (Delco 33si) is a true one wire design.
The other wires are simply folded back and taped since their not needed.

I rebuilt my PCB and it was very easy.
But.....I've also seen PCBs with circuit cards. Mine did not have one.
Maybe yours does and it needs to "talk" to the alternator to function properly.
The TM will say for sure.

 

[brazengoat06]

Yes ive looked at the diagram and it appears to have multiple circuits running to the pcb from the alternator. My pcb is a circuit board type as i have also replaced the solenoid inside. the problem is that im not finding in the tm a description of each circuit.

 

[Tinstar]

I'm not sure why some PCBs have that circuit board and some don't, on identical trucks.
Both of mine do not have it.

Sounds like your starting truck fine.
Does the alternator output change any when you reconnect that wire, while truck is running?

 

[74M35A2]

OK, here is how it works.

The "smart" PCB's have a circuit board, part of that has a sense portion which is wired to the alternator. This sense wire is specific, and is an AC signal, picked up from inside the alternator before the diodes, where the alternator makes AC power anyway. When the PCB senses AC frequency on this line, it knows the engine is running, and it locks out the starter from being re-engaged.

Sounds like the PCB board is going flaky. Leaving this small alternator wire undone is no problem.

 

[Tinstar]

Clearly I don't know the 5 ton electrical system like I want.

Helicopters I fly, I know the electrical (and all other systems) in exact detail forward and backward from memory.
Theory, how it works, troubleshooting and how everything works together.

I was trying to help and all I did was show my ignorance in the system.


Its a work in progress
At least the truck won't fall out of the sky.........

 

[Jbulach]

Uh oh, I smell a