Main fuse keeps blowing.

[Gonefishing760]

The main fuse on my 1995 Mercury 40hp outboard keeps blowing. I've searched the boat and motor for any frayed wire that would cause a short. I even rewired the ignition switch that looked suspect. We would replace the fuse and run the engine one of us would watch the fuse and the other would start wiggling wires searching for a bad connection and nothing. We found that the fuse would only blow when the boat would be running at 12+mph. Which leads me to believe the voltage regulator is suspect it has a direct connection to the fuse so I can only guess the voltage builds as the Rpms do and the fuse pops because of a faulty regulator. Am I crazy or does this make sense? Any suggestions would be great.

 

[Texasmark]

The manual states that voltage measurements for proper operation are made at 1000 rpm or higher. At 12 mph you are probably up around 1500. The rectifier/regulator module is designed to keep a fully charged, good battery, fully charged within the limits of the module, per the manual. It's not made to charge a dead battery. What is the voltage on your battery just before the fuse blows?

 

[Silvertip]

What value is the fuse you are replacing. Should be around 20 amp. If the value is correct then there is an electrical issue and it may be inside the shift/throttle control box. Wires are easily pinched and frayed in there.

 

[Gonefishing760]

Not sure of the voltage of the battery right before the fuse blows. That's good info what should the voltage be and what voltage would indicate a problem? If the voltage surges right before the fuse blows this means bad regulator?

 

[Gonefishing760]

It's a 20 amp fuse. I will dig a little deeper into the throttle control that could be a pinch point.

 

[Gonefishing760]

The main fuse stopped blowing! I went and got new 20amp fuses from the auto parts store and I also took apart the throttle control unit and found that some wires were pinched One had a very very small cut in the wire sheath I taped that up and out the unit back together. I also noticed that the throttle control unit had wires for a trim and tilt motor one the my boat does not have the trim and tilt must be operated manually. I traced these wires back to a connection plug and I unplugged them. I took the boat out and ran the same test of increasing speed until the fuse blew but no blown fuse! Great!!! I'm not sure if it was pinched wires new fuses or unplugging the tilt trim wires, of which have no purpose, that fixed the problem but one of those three things worked.

Thank you silvertip and Texasmark for your input I found it helpful and I hope this form can help others.

 

[Texasmark]

Glad you got-r-fixed.