Vetus M4.45 Failure to Crank

[workmaster2n]

I've got a cat with 2 Vetus M4.45's in it.

I've always had issues with the starboard side. Late last year I replaced the starboard engine battery and starter. A year before that I had replaced the ignition switch. With the new starter, I got about 2 months of trouble free starting, but now the engine seems dead in the water.

Today I pulled things apart and had the starter battery tested. Tested better than new. I cleaned the battery terminals and connectors. I cleaned the terminal that connects to the engine block and the terminal that connects to the engine block. Still no dice.

I took the starter out and bench tested it. Starts up seemingly without issue.

When I attempt to crank the engine, I hear the fuel pump going, but I don't hear the starter even attempt to crank over.

The only 2 things left I can think of are corrosion somewhere in the battery line (it runs from the starboard side to a battery isolator switch on the port side and back) or a bad wire in the loom/harness. I'm leaning towards the loom/harness because the engine oil light, which should always come on as a precheck, is intermittent at best.

I'll give the team at Vetus a call tomorrow, but curious if anybody had any insight of other things to check. Or curious if anybody has any ways to diagnose which wire it is in the loom if it is in fact that.

I'm terrible at reading wiring schematics, but best I can figure there is a relay between the ignition switch and the starter motor. How do I go about seeing on which side of the relay (or the relay itself) is the problem?

Much thanks

[Lepke]

On the starter, if you run a wire between the post the battery cable is attached and the small terminal where the ignition wire attaches, the starter should run. See the red line on the image. This will bypass the key. If the starter works then it's a matter to trace back the start wire or start with the key.
Some start systems use a relay near the starter so the power doesn't have to flow to the key and then back to the engine, but has a much shorter run.
You need to find where power stops in the path from the battery to the key and from there to the starter. There's no easy way.