350 L engine electrical puzzle

[griff4]

Hi all. My starboard engine would not start. This is a 350 small block Mercruiser with 2 barrel carb and tks choke system. I found the 20 amp engine harness fuse blown and the diode inoperative. The fuse and diode are replaced. Now the fuel pump is running even with the ignition key off. The engine now will start up, but won't shut off with the ignition key! The diode has to be removed th shut the engine off! I have checked over the engine wiring harness and tried to locate where the unwanted 12 volts are coming from, but am stumped. Has anyone had this in the past? Any tech suggestions are appreciated. Griff

 

[scoflaw]

What diode did you replace ? Diode wired backwards ? Back feeding from alternator ?

 

[Espos4]

+1

Make sure the diode is oriented correctly.

It’s like a one way valve for current flow.

 

[griff4]

What diode did you replace ? Diode wired backwards ? Back feeding from alternator ?

Thanks, skoflaw. I ordered a new diode, and it hasen't arrived yet. I used the diode from the port engine, which is still okay.

 

[griff4]

Thanks all. Especially Scoflaw and Espos4. The diode and the 20 AMP fuse were the problem. Found a 2006 Merc service bulletin #2006-04, that detailed the problem exactly. Griff