Mercury Trim Switch Blowing Fuse (unusual circumstance)

[Texas41]

I have a 2020 Mercury outboard, 115HP Pro XS. The trim switch works in all scenarios, up and down, unless I am in reverse. Then, it blows the 15 amp fuse under the cowling.

Any thoughts would be helpful

 

[alldodge]

I am in reverse.

NO expert, just a idea
When in reverse the pressure is increased inside the trim cylinder so the valve that keeps the drive from coming up out of the water holds tight. The trim motor turns ON and the motor stalls because the pressure is to great

Why ya trimming up in reverse?

 

[Texas41]

NO expert, just a idea
When in reverse the pressure is increased inside the trim cylinder so the valve that keeps the drive from coming up out of the water holds tight. The trim motor turns ON and the motor stalls because the pressure is to great

Why ya trimming up in reverse?

Nice thought, but I don't think that's it. I agree there is pressure, but the fuse blows, and the engine dies, quite literally, the moment the instant the engine slips into reverse.

I may be at a complete stop. Picture me lowering the engine while unloading from the trailer at the ramp. The engine is dropping. If I slip it into reverse, pop goes the fuse.

If I wait until the engine is lowered and get off the switch, then slip into reverse, all is good. The problem appears to be of I do both at the same time.

 

[alldodge]

Ok got it, that shouldn't happen
If you have a manual should the fuse be 15 amp?

Otherwise I don't know

 

[Texas41]

Ok got it, that shouldn't happen
If you have a manual should the fuse be 25 amp?

Otherwise I don't know

My manual, and sticker on the engine, says 15 amp. I was thinking about bumping it up to see what happens though.

 

[alldodge]

Did notice it posted as 25 but I meant 20
It says 15 so I would keep it at that

Look in your shift handle to see if there is any chaffing or the wires going on

 

[Texas41]

Did notice it posted as 25 but I meant 20
It says 15 so I would keep it at that

Look in your shift handle to see if there is any chaffing or the wires going on

Yeah, I'll give that a try.

 

[Faztbullet]

If it has the tilt limit switch check for pinched wire at cowl

 

[Texasmark]

2 stroke engines of that HP (like mine) use a 20 amp fuse in the wiring harness. If you are running a 15 that extra 5 amperes could make a difference in high stress conditions...course one would think the design goal would be to size the fuse for 50% of the normal load.

Increasing fuse size, over the specified value can cause problems, maybe a fire under some conditions as wiring AWG and insulation rating are designed for a certain amperage load under certain operating and environmental conditions.

 

[QBhoy]

Not to give you a hard time or anything…but in 40 years of owning and working professional on boats and using them at least every weekend in summer, I don’t think I can think of a single occasion that I’ve used the trim in reverse.

But all that aside…there was a recall on the merc 115 engines like that to do with the trim shuttle valves. Was more an issue with them passing though. I got a new set under warranty for my 2016 engine.

 

[alldodge]

I use trim down every time the boat goes in the water. Its up some while on the trailer because if it was full down it would drag on the ramp. Put in REV and when it floats off and gets a bit away from the trailer, it goes the rest of the way down

So I have to retract what I said at first

 

[QBhoy]

I use trim down every time the boat goes in the water. Its up some while on the trailer because if it was full down it would drag on the ramp. Put in REV and when it floats off and gets a bit away from the trailer, it goes the rest of the way down

So I have to retract what I said at first

Yup. Me too. I do this when reversing off the trailer at a certain slipway I use about once a year. Take it back