Weird electric issue in old Johnson outboard

[Jhoughton]

Hi all,

I have an old boat with a Johnson 120 on the back. It's worked faultlessly for years but recently developed a strange problem where it would sometimes start without warning when the key was in the 'on' position but I hadn't twisted it to start. I thought it might be something to do with the rusty battery terminals so took all the wiring off the battery and cleaned it. Now it does it all the time! Whenever the positive terminal touches the battery, the engine starts if the key is in the ignition. It seems that I've somehow shorted the key...

Is that right? Any ideas? I took the control box off and apart but everything looked ok inside.

Thanks,

Jim

[Brinormeg]

Hi Jim......not a sparky, but my first thoughts were faulty/damaged ignition switch. Nobody been trying to steal it by trying to bypass the ignition?
Had this on a motorbike once and when put the key in to electronic start the bike jumped forward and threw me off!
Mechanic later said wires had been tampered with when somebody tried to steal bike.

Not sure about starting /ignition systems on a outboard but a short sounds likely and if you think about the basics of hot wiring a car..... the key ignition switch sends a signal to starter solenoid which then tells battery/starter to do its thing.
So if outboards are similar I would start there with switch then solenoid, as a process of elimination if that makes sense

[jeffstevens763@g]

Short in the starter switch or in the cables I would look at the switch first IMO

[Davie]

Disconnect the harness going to the control box (the big red plug) reconnect the battery. If it still cranks over the start solenoid is stuck or somehow your getting 12 volt on the yellow wire with the red strip. If it’s not cranking with the red plug out it’s got to be either the ignition switch or the red from the battery on the back of the switch touching the yellow/ red that powers the start solenoid somewhere.

[Jhoughton]

Thanks. Really helpful