Main fuse on 1987 Mercury Classic 50 45hp keeps blowing

[DDizz]

Hey All, Brand new to this forum and trying to get some help on this problem. The fuse under the cowling keeps blowing but randomly. I have cleaned all the wires underneath the Cowling and have looked at all the wires in the control module and cleaned up all those wires as well. I thought something might be shorting out somewhere but I have double checked all the wiring and cannot figure out why this fuse keeps blowing. Any suggestions on anything that I might have missed? I have a two batterie system and all the accessories run off of one battery and the motor runs off the other one. Those are hooked up to a battery selector switch which has one, two, both or off.

 

[Texasmark]

What is the blow rate on the fuse? Does it only occur while engine is running? Will it occur with just the ignition switch in the ON position without having started the engine? Can you disconnect the wire from the engine battery to the battery selector switch at the engine battery?

 

[DDizz]

It will blow without the key in the ignition. If I had to say there was commonality, it seems to happen more often when I’m messing with the trim. But not every time.

 

[jimmbo]

Well, you have an intermittent short. Those wires in the control box going up to the Trim Switch in the Throttle Lever, do move around, and eventually the Insulation cracks, or the wire gets pinched and exposes the conductor. That Cable harness that goes from the control box to the engine can hide many damaged wires, and if moisture is trapped in there...(I used to work for a Phone Company, and we had lots of Waterlogged Cables and resulting shorts).
Do you have a Wiring Diagram for your engine?

 

[DDizz]

Yes I do. Where do you suggest I start? I have worked on wiring in cars before so this isn’t that unfamiliar, but a boat is wired differently than from what I have seen before.

 

[Texasmark]

The trim switch has 3 contacts:

Center switch terminal is 12v power input, usually red and comes directly from the engine battery supplied power....may be (apparently yours is) down wind of the 20 amp service fuse found in mid range engines. Point here is that it does not go through the ignition switch (key doesn't have to be in the ON position to get power).

The spring loaded Single Pole Double Throw (SPDT) switch then has 2 outer pins for Up and Down power to the respective PTT relay solenoids. Blue is for the UP (sky) and Green is for down (grass). Unless the switch is bad AND you have a short, you are after the red wire supplying power to the switch. The blue-green wires only get in the picture when you press the button!

Guessing the short is either in the control box where the shift-throttle lever movement and things "jimmbo" said are occurring, or in the cable harness at the engine or somewhere that there is a lot of movement of the engine wiring harness.

 

[DDizz]

Thanks! That gives me a place to start. Also, I looked at a few of the connections on the accessories that the previous owner did... needless to say I ended running all new wiring for all the accessories as most the connections that I found were questionable. So now that that’s done, I am going to dive into those two connections you mentioned and see what I find!

 

[Texasmark]

Post back so we know how it went.

 

[jcark79]

I just went through the same exact thing with the same exact motor. When I bought my rig the previous owner had an issue with the trim, what he failed to mention was he had cut and spliced almost every accessory wire on the boat apparently using the willy nilly method. Needless to say I decided to run all new wiring and while removing the ground wire going to the trim switch on the bow, a mouse had chewed through a very small portion of the insulation which had caused all of the original issues. Hope this helps

 

[DDizz]

Long time since I posted on this. But the connections in the control box were a nightmare. I carefully rewired everything in there as well. I also decided to add in a fuse block and rerun all the power and ground wires to that location and added in a battery selector switch. The boat ran all last summer without any fuses popping. Now the only issue I have is getting the Tach to work, at all. I've double checked the wiring on that and still can't get it to go. Anyone know if I can bypass the control module and run new wiring for that? I suspect there is an issue with the mess of wires I found when fixing the control module.

 

[BWR1953]

Long time since I posted on this. But the connections in the control box were a nightmare. I carefully rewired everything in there as well. I also decided to add in a fuse block and rerun all the power and ground wires to that location and added in a battery selector switch. The boat ran all last summer without any fuses popping. Now the only issue I have is getting the Tach to work, at all. I've double checked the wiring on that and still can't get it to go. Anyone know if I can bypass the control module and run new wiring for that? I suspect there is an issue with the mess of wires I found when fixing the control module.

When I was installing the tach for my 1985 Merc 50, it didn't work. I found two problems.

1 - the yellow tach wires from the stator to the rectifier were shaved off at the flywheel. I had to replace the stator. Pricey!

2 - after I replaced the stator, I hooked everything up and it still didn't work. So, I swapped the leads from the rectifier to the tach and it came to life and works perfectly.

I hope this helps.