M939A2 starter system dead short. Any help where to start?

[jefjoc]

When I turn switch, sparks and smoke from batteries.

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[Mullaney]

When I turn switch, sparks and smoke from batteries.

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.
Any chance you might look at the ground strap and ground wires?
Loose grounding will cause what you describe.

 

[Jbulach]

First WAG would be batteries are wired incorrectly. Is this a new to you unknown truck, or this condition started after…?

 

[ericp351]

Agree with above, and I'd triple check batts AND pull starter hot, hit switch and confirm 24V pos. Lots of power in 2 or 4 batts to risk an acid bath or fire! Good luck.

 

[cbrTodd]

If it's a truck that is new to you or if there is any question about its engine health, try barring the engine over by hand to make sure it isn't seized or something. A starter trying to turn a locked up engine can do funny things.