Engine auto start

While checking on my 1979 34 ft. Mainship after getting home from out of town business I found my Main Engine Running. Yanmar 4LH-DTE. After investigating I found that the starter ingaged to start until all wires burned up. Little damage to flywheel but starter and wires are Tost. The cooling seawater piping passes within inches of the starter and I belive if it had been rubber instead of fiberglass it would have burned up and caused water to flood the boat.
Anybody have ideals on what could have caused this. All ignition switches are new. No wires touched before this happened.
Weird? ?

Once in the 70s I was in the factory where I worked and a backhoe that was there the day before digging a hole for a machine base started on it's own. I saw the headlights flash then it fired up. The starter relay shorted out.

Once in my driveway the 1970 Dodge Dart I had tried to start on it's own. It was backing up out of the driveway on the starter motor when I ran outside and pulled the battery cable. The starter relay which was mounted below the battery tray and corroded and shorted out.

Just to elaborate on jleonard's comments... Something, somewhere HAD to have short-circuited. Switch, wires, solenoid, something!


Good luck finding it!

The problem has to be in the circuit that activates the starter solenoid. Probably a relay. Relay will be on or near the engine.

Seen a few boats do this. Common cause was an ignition or start switch on a flybridge or uncovered console. Water works its way into the switch and corrodes the contacts. Eventually they bridge and energize starter.

Had a similar thing on an Olds Ciera. Going down the highway it made a loud grinding noise for about 30 seconds. As I was slowing to pull off the road the noise stopped so I kept going. When I got home I shut down and tried to restart, no click, no nothing. The starter had energized itself and it spun itself to death with the car going 70 at the time. Had the starter replaced and the shop could not explain how it happened. Said they'd never seen it before. No shorts or wiring problems found, it had to be internal to the starter solenoid. Drove that car for a couple more years and it didn't happen again.