Engine Spontaneously Starting

[micwhite]

Hi

I have a Volvo Penta TMD31B 100hp diesel in my Contest 43 yacht. This has served me well for many years until I went to check it recently and found the engine running.

The ignition key had no effect so I couldn't stop it that way. I stopped it with a shut off lever on the injector pump.

The next question is why did it start in the first place. I had left the fuel on and the start batteries on (and fortunately, the raw water on) but this didn't explain why it turned on. I checked the wiring of the ignition switch and found nothing obviously wrong there but when I put things back together I found that the system was working again. The engine turned on and off with the key. While I was pondering the problem it suddenly started again.

This time I disconnected the start batteries completely and handed the problem to an ecletrician. He replaced the ignition switch and a circuit breaker. All was well for a week until I went to check the boat and found the engine running again. The engine start batteries were switched off!

Apart from not having a clue what is going on Im amazed that the starter motor is still in one piece.

Any ideas would be welcome

Cheers

Michael

[MerMike]

There has to be a short somewhere in the starting circuit, closing the start switch and firing the engine. However, a short like that would also continously engage the starter, with ill effect.

Usually people have the opposite problem - the thing won't start.

[NYSail]

Haunted.

[Turtle Blues]

First couple of thoughts 2c worth.
1. You state engine battery was turned off but obviously you still had current to the battery. Check battery selector switch for proper operation / isolation. Also check charger wiring, does it bypass the selector switch Battery terminal to starter motor terminal is the heavy circuit. Battery to starting solenoid (through switch) is a second circuit.
2. Switch operates relay which energizes solenoid. Something had to close the circuit. Possible shorting/defective relay, possible short in the solenoid, if solenoid circuit is powered from starter battery wire lug this bears further inspection.
3. Diesel does not require electricity to run but something is shorting power to the starting circuit. Is it possible 1 of the components in the circuit got wet?

[skipperpete]

Hi Michael, welcome to the forum, Melbourne’s my home city.
Whatever the cause , the potential damage that can result from the starter engaging while up at cruise rpm is reason enough to not use the engine until the problem has been solved, and solved properly. This happened to a boat on our marina and it damaged the battery +ve to the starter, lots of smoke but no actual fire and the starter needed a rebuild and a new cable to the battery…. The ring gear was ok though.

[Mrkva]

All was well for a week until I went to check the boat and found the engine running again. The engine start batteries were switched off!

Check the starter side of the battery switch, there should be approx. 0V to the ground (and +12.5V to the ground on the battery side of the switch).
Is the engine in gear and are you in an area with insanely strong currents? Theoretically, diesel engine without electric fuel pump and injectors can be started by manually turning it over. So if something spins your prop fast enough, it could start the engine, if it's in gear.