Purpose of the 90A starter fuse?

[nola mike]

Doing some rewiring. Noticed that my 4.3L didn't have a starter fuse, so I decided to grab one. Looking over the wiring diagrams however, I'm not really sure of its function. It's unclear to me which wires are connected to the protected side. It appears only the red wire to the slave. If that's true, isn't it already protected by the 50a breaker?
How about orange to alternator?

 

[Bt Doctur]

You are correct, the 50A CB protects the main wiring and the 90A should only protect the alt output lead. On newer boats both are connected together, So if the 90A should blow ,your left dead in the water where as protecting the alt only you only lose your charging system

 

[achris]

The only wire on the 90A fuse is the orange alternator wire. It's to protect the system if an alternator failure dumps full current to ground (like internal diodes going short)... It also may be quick enough to blow (to protect the alternator) if you hook the battery up the wrong way, but I wouldn't rely on it for that...

Chris.........

 

[nola mike]

OK, thanks. Saw a few conflicting wiring diagrams, and the manual doesn't show which wire goes where. Orange wire was the only one that looked like it could benefit from protection. Glad I didn't waste $25.

 

[JustJason]

Before Merc started putting that fuse there, if a battery was ever hooked up backwards, the alternator will burn up, and possible catch fire, and the alternators main output wire would turn into 1 long fusible link, and that will definitely burn up and catch fire. That fuse is there to prevent electrical fires should the battery ever be hooked up incorrectly. The fuse should be in place.

 

[Bt Doctur]

Could have used a Maxi-Fuse to make life easier but that would be too easy.