Will you just stop...

Canal Boat - - Back Cabin: Experts - TONY REPLIES… P DEA­CON, via email

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.

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