Yes, electrics aren't my thing either! It's a good call though.
The tacho has been intermittent. I had an issue with Tohatsu wiring last year. Someone should tell Tohatsu to use tinned cable when making outboards!
I rewired part of the outboard cable loom last year (corroded wiring) and checked continuity through all the wires to the remote. I could check the green pulse wire for continuity to the tacho again at the weekend. The tacho illuminates, but that's about it. Last year the overheat or low oil alarm sounded and strangely it did this on Sunday too, for a very short burst when starting the engine. If it's low oil the oil lamp on the tacho should illuminate.
The engine is an autolube and it's definitely using oil as I check the fill line. When it sounded last year I added a little more oil to the petrol tank rather than risk a seized engine. Call it cautionary, but better safe than sorry.
It pumps water and the tell-tale is very strong. The impeller was changed last year, and I renewed the thermostat, again a genuine Tohatsu part. I also had to renew the head gasket (stripped spark plug threads on number 1 and 2 cylinders), so a good opportunity to see water cooling passages. I cleaned it all up and reassembled.
I can't see this being a rev limiter or faulty sensor issue. As far as I'm aware the Tohatsu is a fairly agricultural bit of kit, with an overheat sensor on the block and an oil level sensor on the oil reservoir. If the rev limiter was introduced I'd be reduced to 3,500rpm. At the weekend I clocked 27mph on the GPS at wide open throttle (albeit bogged down) so I know it's not that.
I'm slowly rebuilding this engine one part at a time!