Boat Repair Questions? Ask a Marine Mechanic for Answers ASAP
No worries, this happens more often than you might think.
If you look at your starter, you will likely see a square 90 amp fuse mounted to the starter solenoid (some 3.0L engines used a square 55 amp fuse). Check for voltage on both sides of this 90 amp fuse, if you do not have battery voltage on both sides of the fuse, then the fuse is blown and will need to be replaced (it is usually the first thing to blow if a battery is hooked up backwards).
Here is a reference picture of the 90 amp fuse at the starter (item #8)....
If the 50 amp breaker will not reset after replacing the 90 amp fuse (assuming the 90 amp fuse is blown), it may be damaged and you'll likely have to replace the breaker as well.
I will also attach a general wiring diagram of a 3.0L engine for reference should you need it....
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