Electrical problem - I give up

Posted this at Ohio Gamefishing but was referred over here...

This happened last year and I thought I had it fixed until it happened again Friday. It's a 19 foot Sylvan with a 115 Merc Fuel Injected 4 stroke. The main/tilt fuse blows for no reason I can find. Everything works with the switch off. Turn on the switch and it's a crap shoot. Sometimes it will trip the fuse within 2 seconds. Other times you can't make it do it. Yesterday in the driveway it blew every time for a while. I cleaned up every single wire that I could reach in that rat's nest of wires and then I couldn't get it to do it again. It frequently blows without the switch on and nothing happening. This morning it was fine when I left the house. I drove 7 miles and stopped to put some gas in it. Fuse was blown so I turned around and brought it home. Obviously there's a ground or short circuit problem somewhere but I just have no more tools to troubleshoot it further. Unless someone here has some genius advice it's off to the shop. I don't even care as much about the cost as the fishing time I'm likely to miss.

Thanks for any ideas.

MC

Re: Electrical problem - I give up

Do you have a wiring diagram? If so post it and I will take a look. I have found that the best way is to unhook the wires one at a time on both ends and ohm them out. Depending on what is all on that circuit you can unhook each load and add it back into the circuit one at at a time then which ever one trips the fuse stop there, then see how many amps that load pulls.

Re: Electrical problem - I give up

Here is the diagram for the engine. I don't have one for the boat. I tried a version of your method but the trouble is nearly all the wires are bundled in a harness. I disconnected all from the ground side of the battery, turned on the switch and touched the main cable to the engine - pow, it blew. Took it off replaced the fuse and touched the harness without the main - pow. Everything on the boat works with the switch off - power trim, electronics, pumps, lights. Turn on the switch and pow. I suppose that points to a bad switch but I'm not great at using the volt/ohm meter on that.

Thanks,

MC

Re: Electrical problem - I give up

Unplug the rectifier and retest...