Grounding issue?

[P3_sailor]

I just recently bought my first boat. A 1999 Capri 1952 cuddy. Overall it's in great shape with a few electrical quirks. The fuel gauge stays on full when it's in the water. When I get home and flush the engine it registers the proper fuel level. As soon as I put it back in the water though it goes right back to full. All the other gauges, with the exception of the spedo (bad pitot I think) work fine. The other quirk is the all-around light. It reads 12v at the plug. I had it working at one point then it stopped. The bulb isn't blown and I can't find a short in the post. The anodes on the outdrive look ok. A little pitted maybe. The are a few spots of galvanic corrosion on the outdrive but not bad and I'm going to fix it with some quicksilver paint. I was thinking it was an in-water grounding problem but wouldn't you have a better ground in the water? Is there a way to check the grounding system? What exactly is the ground on a fiberglass boat? Any advice would be greatly appreciated.

 

[sho3boater]

Re: Grounding issue?

Your ground is the negative 12v. The anode is only for current generated by the metal of the outdrive/LU reacting with the water. The anode sacrifices itself to corrosion rather than your drive, it does not stop it, it just corrodes easier. The engine will always be 12v ground to run the starter, some things are wired to it but on a boat you should have everything wired to the battery direct, or a fuse or ground block often under the dash for wiring purposes. A car is metal and that metal body is used for 'ground' the negative 12v, but obviously you can't do that with a fiberglass boat so the grounds are all wired just like the positive is.

Note that some fuel gauges run a positive to the sender on tank, then back to the gauge in dash. The sender cuts the current according to the level in tank and gauge reads it to ground.

This is why boats have a bunch of switches on the dash to turn on things, because you don't wire everything to the key like in a car you wire it to the battery. They often tell you not to interfere with the power plant wiring.

 

[P3_sailor]

Re: Grounding issue?

Excellent explanation. That makes a lot of sense, had been thinking the engine or the outdrive was somehow the ground. Mistakenly thinking of the circuit the same way as what's on a car. I'll look through the schematics and check the whole grounding circuit back to the battery. Thanks for your help.