Hit the battery switch with the engine on

[Mossman]

This is the situation, a friend of mine has a PCM 351 he was working on the motor on the trailer. He had to start and stop it, one time when he cranked it the starter got stuck on and was grinding against the flywheel. At this point he turned the key off, no good, so he turned the perko switch off and a big spark shot out of the perko. Then he called me? Now it won't start, we obviuosly need a new starter, but what else? Did he blow the alternator or perko or both? All suggestions welcome.

Thanks
Drew

[skipperbrown]

It appears that the solenoid got stuck in the on position probably due to a short probably due to moisture probably due to the starter being in the bilge. That's speculation on my part.

When you first engage a starter capable of rotating a v8 at 200 rpm or so, it draws a lot of amps at first, roughly 300. Then once the engine is rotating, the requirement drops to about 150-200 give or take. Your buddy was probably drawing about this amount when he shut down the starter via the battery switch if the engine was rotating. If the starter was locked up in the flywheel teeth, it was probably drawing close to 300a. Things were heating up fast in that starter.

You can weld w/ about 40a. When your buddy flipped the battery switch, he opened the circuit and the big spark was 150-200 amps of angry electrons arcing over the switch contacts and creating a lot of heat as their path to home was shut off. Enough heat to melt the contacts. These switches were never designed to be switched under load. However, they are designed to carry a lot of amperage so you really need to dissect the switch to access the damage.

Take the switch apart. It may be ok. On the other hand, if 25% or more of the contact plate has melted away, I'd toss it.

The starter itself may be ok too. Your buddy may only need a relatively inexpensive solenoid to be back in bizness. It depends on how hot the starter got and if any of the wiring melted.

I hope the engine wasn't running. I've seen flywheel teeth get really messed up when the engine was spinning at 700 rpm and the starter was spinning much slower.

Mark