Headed to Mark Twain Lake today but the 2002 50hp Mercury won't start. First time it let me down in 15 years. It blow the 20 amp fuse under the bonnet as soon as you turn key to the run position and the power trim quits also. I was able t

 

Something is shorted out but haven't found it yet. The fuse has 12 volts to one side as it should but as soon as you turn key to run position there is 12 volts on the other side with the fuse out so of course the fuse will blow. Anyone have wiring diagrams or suggestions? 

 

Thanks in advance!

Take the fuse out and connect a 12v tail lamp across it. Unplug the connectors you can and turn the switch on. If the bulb doesn't light or is dim, you probably unplugged the short. To verify this, put a fuse in, it will not blow if you did. Now, put the bulb back in, turn the switch back on and start plugging things back together. When the bulb goes bright, that might be your short.

If the bulb burns bright when you first turn the switch on,, then you probably didn't disconnect the short, but you can try a fuse to see.

Since you are dealing with a 20 amp circuit, some good components can make a small bulb burn bright so something like a headlamp might be needed, but I would start with something like a single element tail lamp.

I use a $200 circuit checker because I can trace the signal to the problem, but before that, the bulb trick has saved me a lot trouble many times.

Take the fuse out and connect a 12v tail lamp across it. Unplug the connectors you can and turn the switch on. If the bulb doesn't light or is dim, you probably unplugged the short. To verify this, put a fuse in, it will not blow if you did. Now, put the bulb back in, turn the switch back on and start plugging things back together. When the bulb goes bright, that might be your short.

If the bulb burns bright when you first turn the switch on,, then you probably didn't disconnect the short, but you can try a fuse to see.

Since you are dealing with a 20 amp circuit, some good components can make a small bulb burn bright so something like a headlamp might be needed, but I would start with something like a single element tail lamp.

I use a $200 circuit checker because I can trace the signal to the problem, but before that, the bulb trick has saved me a lot trouble many times.

I have read post after post from you sir. You always have great advice. You should be charging for your services on here. Hope you are still around if I have issues with my older merc.

I think there are some that will disagree with you but thanks for the comment. Most of it's just called being and old fart that pretty much fits the old saying, "Jack of all trades, Master of none" and have been fairly heavily involved with cars, boats, guns, fishing, electronics, wood working, metal working, radio controlled planes and helicopters, and I could keep the list going on and on. There is just not a whole lot that I don't do or haven't done. As for charging, that's not in my nature, I don't even charge people to help them face to face, but it's rare for me to work on something for someone else. I've had that bite me in the butt more than a few times, while I'm trying to do them afavor.

However, as for your Mercury, I'm afraid I would be very little help, other than my basic knowledge. I haven't done much with them since I quit racing them back in the early 70's and forgotten most of what I knew then. The JohnnyRudes, I know pretty good. The old two stroke stuff anyway, I've never had my hands on or even seen a four stroke with the cover off.

Worked on the boat today and seem to have the problem solved. Didn't find any shorts ended up disconnecting the yellow/ red wire that goes from the ignition key and the same wire at the  solenoid and back in business.  Hopefully I can get out on the water and test it out next week.