Motor / electrical advice needed 😳

[Richo1]

Hi all,
Here is one for the Electrical experts out there.
I have a 40hp EFI mercury 4stroke with just over 20hrs on the clock. Electric trim tilt & start.
Did a couple of beach launches over Easter that went well with no dramas except a little water over the stern on retrieval.
Problem was after around 15hrs of sitting without a freshwater wash down I turned the battery on to play with the GPS and the motor leg started smoking! Immediately isolated the battery. It turns out that the earth wire comming out of the leg got so hot it burnt through, the heat of the wire burnt some of the plastic covering causing the smoke. Due to it all being under warranty I haven't pulled anything apart untill the merc mechanic see it.
Anyone had this happen to them?
Any ideas what could have caused this? Salt build up?
Motor still runs well and did another two trips with no problems.
Regards
Richo

[Fed]

Was that the wire from the motor to the transom bracket?
Take a pic or give a better description Richo.
Sort of sounding like a bad earth combined with a secondary earth an an alloy hull.

[Richo1]

Thanks fed, will see if I can track down a photo, away for work for the next 5 weeks.
Not to the transom bracket that one is okay.

[Spaniard_King]

I have seen this before.. something has powered up the hull and is transfering the current to the engine to track back to the battery via the earth battery lead to make the circuit. It normally needs a decent connection. I have seen this when the battery swtich has no isulation between the battery leads and the hull

[johncar]

Yes would happen if as S_K said something powered up positive to the hull assuming Ally boat and then with no other return to the negative on the battery except the engine leg bonding would cause that kind of current flow and cooking of the bonding wire. So if it wasn't an accidental short to the hull, a thorough check of all the main battery power cables and isolator switch and engine connections need to be made. Any sharp edges where cables insulation could be cut through.
It has happened to my engine also when the previous owner accidentally shorted one of the battery leads to the hull even though he didn't realise at the time that he melted the engine bond wire in the process, I just found it after I bought the boat and put two two together, no major harm done.
That's my best guess without any other details anyway. Good luck getting it sorted.

[dnej]

One for Garry I recon