Ign Fuse blows when key is turned to ON, and how to tell if gauge is bad?

[liquidlew]

1989 Merc 4.3 Alpha 1, last started in April, ran on muffs fine for 10 minutes. Started it yesterday and ran for 5 seconds then abruptly shut off, blown ign. fuse. Replaced with new fuse, started and ran fine till I tapped on the trim gauge (which was replaced with new one last year), then it quit again, blew the fuse. Checked the wiring to all the gauges, no loose wires and all nuts tight, tried 2 more fuses and they blew immediately on turning the key to run. Also when it was running the tach was reading a couple of hundred degrees high from what my ears were telling me on the rpms. Where do I start, and how do you tell if a gauge is bad and is causing this problem? Thanks!!!

 

[stonyloam]

Try disconnecting the purple wire going to the trim gauge, and see if it runs.

 

[alldodge]

Disconnect the tach for now and work on that issue after the high current issue is solved. It's blowing a 20amp fuse correct? If so the 20amp fuse feeds all of the purple wires so you need to start disconnecting some of them until you find the right one. Start by disconnect the purple wire from the coil.

 

[liquidlew]

Thanks for the replies! Yes, it's the 20amp fuse. So I remove the purple wire from the coil and see if it still blows a fuse first? Next step is to disconnect purple wire from tach? Then next would be the trim gauge I suspect. I assume I connect the purple wire back on the coil before I disconnect it from gauges? Sounds too easy, I hope this leads me to the problem. Thanks again guys!!!

 

[Bondo]

Thanks for the replies! Yes, it's the 20amp fuse. So I remove the purple wire from the coil and see if it still blows a fuse first? Next step is to disconnect purple wire from tach? Then next would be the trim gauge I suspect. I assume I connect the purple wire back on the coil before I disconnect it from gauges? Sounds too easy, I hope this leads me to the problem. Thanks again guys!!!

Ayuh,.... The purple wires are the Ignition circuit, they all go Hot, when the key is turned to "On",....

Disconnect whatever ya Suspect, then Test,....
If the Short still blows the fuse, hook back up what ya unhooked, 'n move on to the next suspect,...

 

[liquidlew]

Awesome Bondo, thanks! Gonna get to it soon as I get a chance, anything electrical scares me but it sounds like maybe this is not a major bummer after all. I gotta say that if it weren't for all the good info from all you guys over the years, my bank account would be a lot smaller! Sure do appreciate this forum!!! I've pretty much fixed everything that's broke over the last 7-8 years, and of course with an old boat (that sadly spends most of it's life in the driveway), just about everything has had to be fixed and/or replaced, sometimes twice! And not to forget routine maintenance, impellers, bellows...saved myself a lot of green doing it myself :wof:! And I'm not a mechanic! Sorry, starting to ramble, thanks again to all you guys! :encouragement:

 

[stonyloam]

If you remove the purple (resistance) wire from the coil it will not run, it might start, but as soon as the key returns to run it will shut off.

 

[stonyloam]

According to the wiring diagram that I have, the purple wire goes first to the tach and from there run voltage is distributed to the other gauges and the ignition, so if you remove the purple wire(s) from the tach it may not run at all, again will start but shut off. The diagram also shows the purple wires tied together downstream from the tach so if you remove those wires i don't think anything will work. You might better remove the wires from the other instruments one at a time. I would start by removing the trim gauge (purple) wire.

 

[Bt Doctur]

from manual 7 ,

 

[stonyloam]

Yeah that's it, thanks Dr!

 

[liquidlew]

Update. Unhooked the trim gauge, put in a new fuse and turned the key to ON, fuse does NOT blow. Hooked trim gauge back up and turned key to ON, fuse does NOT blow. Started engine, tapped somewhat hard on the trim gauge, fuse does NOT blow like it did before. Ran it for a good 10-15 minutes tapping on all the gauges, fuse does NOT blow, so I guess I'm good to go? The trim gauge however is now pegged way below the full down position marker and it doesn't move at all when raising the drive up and down, this gauge is still fairly new, bought a year ago to replace the old gauge that did the same thing, peg below the full down position. What would cause the gauge to go bad other than the gauge itself giving up the ghost? Trim wire from the drive shorting out somewhere?

 

[nola mike]

Did you adjust the trim sensors on the outrive when you replaced the gauge? Do your trim sensors even work (probably not)? I don't think a short in the sensor would cause your problem, but I don't have the wiring diagram in front of me ATM...

 

[liquidlew]

Trim gauge has always worked correctly (old and new gauge) when they worked so I don't suspect the sensor or wire to be bad, and yes, I adjusted the trim sensor when I installed the new gauge, so I am going to check out the sensor and wiring today or tomorrow. I know the limit sensor/wire is not working correctly, sometimes the drive stops, sometimes it goes all the way up if I hold the trim up button, so I will deal with that also, plan to put new ones on soon when I remove the drive to change the bellows. I'm not 100% sure my new gauge is toast either, it worked fine and the needle was on the lowest marker when I first had the trouble. When I put in the first new fuse and it blew when I turned the key to ON, the trim gauge needle jumped and pegged to the far left lower than the last marker. Since I got it running without blowing the fuse, the needle stays pegged when I move the drive all the way up and all the way down. The old gauge was stuck in the pegged position too when I replaced it, so would a surge of something make that gauge go into seizure?