Will you just stop...
QFor 12 years we have been able to stop our boat’s engine via a 12v stop solenoid on the injector pump by turning the ignition switch ‘off’. Recently, the engine would cut out but then not start again. A replacement solenoid appeared to solve the problem, but several days later, the engine then began running on for a few seconds after ‘switching off’. Eventually I became unable to stop it from the switch. On switching off, the charge warning light comes on and the rev counter continues registering but the engine keeps going. I now stop use the manual stop lever.
If I start the engine and turn it off before increasing the revs while the 12v ignition light remains on, the engine will stop as it should, but after revving up to perhaps 1,000, the engine will not turn off. Does the ignition switch incorporate diodes, and would failure be a possibility?
AThere should be no diodes in the ignition switch but there will almost certainly be some associated with the warning buzzer; however, unless the buzzer has shorted I cannot see how one failing could put 12v on the stop solenoid circuit.
Your solenoid is an ‘energise to run’ type so if there is 12v at the solenoid, the engine will keep running. The question is: how does the 12 volts get to the solenoid with the ignition off? I have known a few ignition switches develop short circuits, but usually accompanied by a fishy electrical burning smell. Check the main battery earth (negative) circuit. If it’s loose or dirty, the charging negative might be back-feeding through the solenoid. Make sure no multi-plugs in the main engine wiring harness are sitting in water, wet, dirty or have any loose connections. .
Check under the instrument panel for things like piggyback connectors touching other terminals and causing a short.
Try disconnecting the solenoid, the engine should not start. Then connect it with a length of wire from the battery and it should start. If it is a two-wire solenoid, do this once using the positive wire and battery positive and again using the negative wire and battery negative. If you don’t get the same result both ways, you know whether to look for positive – or negative – side faults.