I had some problems, a few years ago, with starting the Yanmar
3GM30F on my boat. I discovered the problem was with the panel switch, which had become internally corroded enough to inhibit a dependable start. I have since rebuilt my entire engine
instrument panel in the cockpit, and replaced the switch with a "waterproof" one (which eventually corroded inside) and replaced it with another, this time packed in the stem with grease. This switch was German and had a simple red plastic, replaceable key. Sadly, West Marine
no longer carries it in their catalog.
The red ignition wire in the harness, with its 30 Amp fuse, connects to the input side of the switch. The output side of the switch connects to the input side of the start button, and the output side, the white wire of the harness, goes to the starter solenoid.
The red ignition switch wire output, in my boat, goes to a buss, from which the power to the engine
instruments, etc., are connected for their electrical
If anyone knows of a current supplier of these switches, I would be very happy to know. Since the key stem can be packed with grease, they can be made into very serviceable outdoor switches. Having a couple extra plastic keys is very handy. To secure the engine against unwanted use by boarders, you simply shut off the battery
switch from the engine, removing power to the key switch. Any bad guy who can't figure out how to hot wire an ignition switch isn't going anywhere soon. The one who does then has to face the consequence of having entered a secured environment
without permission. No details revealed here due to security