I had been having strange intermittent electrical problems for a couple of months. They ran the gamut from the tack skipping back and forth to the engine shutting down when I hit the trim/tilt. The best one was the fuel gauge going to full when I turned on the cockpit lights. Strange indeed. I dedicated last Tuesday to finding what was going on. Armed with my high dollar multi meter and several schematics of the boat and the engine, I went to work. After about two hours of work I had just about checked every circuit on the boat. The last thing I checked was from the negative side of the start battery to the engine block— dammened. There were 56 ohms of resistance between the battery and the block. I checked the cable and the block luge and all looked good. I removed the cable from the block and it looked fine, just a little dark. I hit it with some sand paper and nooxid and bolted it back on. Son of a gun! everything works fine, even the charging relay works perfect for the house battery again
The engine is a standard rotation Yamaha F150. About as basic as you can get. So learn from something I learned a long time ago, if you get strange intermittent electrical problems, look for a ground problem.