MerCruiser 496 keeps blowing / melting 90 AMP fuse on the starter

[OC_RiverRat]

This has happened several time after long 1 hour plus runs at various speeds the 90 AMP fuse on the starter is blown / partially melted we can get home by removing the fuse and direct connecting but I want to try to determine what is causing this situation to happen obviously something drawing a lot of current has anyone ever experienced this ? any ideas on how to troubleshoot. The boat is a 2004 22 ft Laser any help would be appreciated

 

[Bt Doctur]

if its wired to the red wire, that's supplies system power to the boat, if its wired to the orange wire, that's the feed from the alt to the battery.
might have a alt overcharging or running wild

 

[alldodge]

+1^ and for clarity, only the orange wire should be connected to the fuse. All other wires are connected to the starter post (Red (battery) and red/purple. The fuse is used to protect the battery if the ALT would short out

 

[Fun Times]

AllDodge, Just for further clarification, you are correct in saying the main job for the 90 AMP fuse is to protect the alternator to battery system but every Mercruiser that has come from the factory with the 90 amp fuse that I recall seeing, both the orange wire to the alternator and red wire to the 50 amp circuit breaker are connected together at the 90 amp fuse cube. So in essence the red wire between the 90 amp fuse and the 50 amp circuit breaker is protected as well and depending on failure point any one could go. Here's an example from Merc quoted below.

Marine Power International Pty Ltd has become aware that some MerCruiser 3.0L MPI engines may experience the following issues:
The starter engages and cranks the engine, but the engine does not start
The starter does not operate or crank the engine
There is a potential for a portion of the engine wiring harness to be pinched or its insulation chafed due to an incorrect routing of the harness, possibly resulting in a direct electrical short circuit.

The system is protected from electrical short circuits with a 90-amp fuse located on the starter solenoid and a 50-amp circuit breaker located on the wire harness bracket. Depending on the circumstances, either the 90-amp fuse may blow or the 50-amp circuit breaker may trip. http://www.mercurymarine.com.au/hom...ruiser-30l-mpi-engine-harness-inspection.aspx

http://www.justanswer.com/boat/2m9hd-1994-mercury-magnum-inboard-turning-key-not-click.html
Random online photo.

 

[alldodge]

AllDodge, Just for further clarification, you are correct in saying the main job for the 90 AMP fuse is to protect the alternator to battery system but every Mercruiser that has come from the factory with the 90 amp fuse that I recall seeing, both the orange wire to the alternator and red wire to the 50 amp circuit breaker are connected together at the 90 amp fuse cube. So in essence the red wire between the 90 amp fuse and the 50 amp circuit breaker is protected as well and depending on failure point any one could go. Here's an example from Merc quoted below.

FunTimes I'm sure your right but I'll have to recheck.

 

[OC_RiverRat]

So I have replaced the Alternator, Mechanic thought this was the likely cause the volt meter shows charging at 14 volts when the engine is running I suppose it could still be overcharging on long runs since this seems to happen after running for an hour or more at medium to high speeds. It has never happened on short runs or multiple short runs. Before replacing it again it appears we should check the orange cable for mis-route insulation break or short ? Anyone have a good source for these fuses at a reasonable rate

 

[alldodge]

So I have replaced the Alternator, Mechanic thought this was the likely cause the volt meter shows charging at 14 volts when the engine is running I suppose it could still be overcharging on long runs since this seems to happen after running for an hour or more at medium to high speeds. It has never happened on short runs or multiple short runs. Before replacing it again it appears we should check the orange cable for mis-route insulation break or short ? Anyone have a good source for these fuses at a reasonable rate

The ALT should be putting out 14 to 14.5 volts when running, but it could still be the ALT. It only takes a second for something to short and pull more then 90amps and blow the fuse. Hope the ALT was the problem. The orange wire should run from the fuse directly to the ALT output terminal. Don't know of any "cheap" places for the fuse

http://www.iboats.com/Fuse-Kit-90-A...9067781--session_id.166950524--view_id.222151

 

[Bt Doctur]

also consider the battery