I have a 1978 Cobalt 19' Bow Rider. I'm the kinda guy that likes everything on the boat to work. Even if I don't need it. Like my dash & interior lights even though I don't boat at night.
I decided to get my trim guage working. I'm kinda new to boating but fairly handy around a car and my house. I pulled the sender apart and cleaned the contacts an checked for power. Have power so I put it backed together. Guage doesn't move. Pulled the wires off from the guage, which you have to get to from underneath, like working under the dash of a car, cleaned the post, wire ends and nuts, put it all back, guage doesn't move.
Go to Google to try to get some help. They all say basically the same thing. Bad wires at the sender or water has gotten into the sender, replace the sender. Everything in the sender looks new, the wires look good. This boat is a 78 with just a little over 400 hours on it. Can't be the sender.
Pull the sender apart again. Ground it to a known good ground, jumper cable hooked to the battery, and the needle pegs. OK so I know the guage is good. Break for lunch.
I didn't at that time fully understand just how the sender worked. I knew that the contacts worked against a plate that moved with the motor, but that's about it.
Pulled the guage out of the dash and used jumper wire directly off the sender. Guage didn't move. Back to Google. Finally found a forum where a guy had said that one of the wires is 12V and the other is ground. Pulled the sender apart again put the ground wire side to a known good ground, jumper cable again. Guage moved. OK so I have a bad ground.
Back to Google again to see where it gets it's ground at. I Know that it's not ground to the chassis like a car because it's a fiberglass boat. Google no help.
Pull the back part of the interior out so I could get down around the motor to see if I could see the sending wires coming through the transon. No luck. Then I look up high above the motor and see a comon ground block. Checked the screws and sure enough one loose screw. Pulled the wire off, cleaned the contacts and now the guage works.
A lot of work to see a little needle move. But I did learn a few things and it didn't cost me any money. Next time I think I'll check for a loose ground first.