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  Mercury 90-HP FOURSTROKE Blows Main 20-ampere Fuse

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Author Topic:   Mercury 90-HP FOURSTROKE Blows Main 20-ampere Fuse
nats posted 04-17-2010 12:34 PM ET (US)   Profile for nats   Send Email to nats  
I have a 90 Mercury EFI [FOURSTROKE] on my 170 Montauk. [The] power tilt stopped working. [I] opened the fuse panel on the engine and noticed that the MAIN 20-ampere fuse was blown. I replaced it, and it keeps burning out. Any solutions maybe to my problem? Thanks
jimh posted 04-17-2010 03:56 PM ET (US)     Profile for jimh  Send Email to jimh     
Do you have the schematic diagram for the electrical system of your engine?
nats posted 04-17-2010 07:35 PM ET (US)     Profile for nats  Send Email to nats     
No, I do not have a wiring schematic. Since I just recently opened the boat up, I tried to start it also today and nothing. It[']s not getting any juice. But the horn lights do work.
jimh posted 04-18-2010 01:15 AM ET (US)     Profile for jimh  Send Email to jimh     
I am afraid that we are not going to be able to gain much insight from learning that the horn and some lights on the boat still work, inasmuch as it would be very unusual that the electrical distribution branch circuit that feeds them would be from the fuse panel on the outboard motor. Generally the fuse panel on the outboard motor only feeds circuits which are directly associated with the outboard motor.

To make trouble shooting this problem easier, you really need to get a wiring diagram that shows all the electrical branch circuits which are fed from the 20-ampere fuse on the fuse panel of the outboard motor. Or, if you can't get a schematic, you can try to draw one yourself by tracing all the wires. You need to find out what branch circuits are being fed by the 20-ampere fuse.

Once you identify the branch circuits, you should be able to see where they connect to the distribution from the 20-ampere fuse. Then I'd suggest disconnecting them all temporarily. Replace the fuse. The fuse shouldn't blow, unless there is something wrong with the fuse circuit itself. Assuming the fuse does not blow, then you can connect the branch circuits back to the distribution one at a time, looking for which one is blowing the fuse.

Once you identify which circuit is blowing the fuse, look for the problem in that circuit. To blow a 20-ampere fuse you will need quite a strong load, probably a dead short.

nats posted 04-18-2010 08:17 AM ET (US)     Profile for nats  Send Email to nats     
jim thanks for the advice. I will try that today.
jimh posted 04-22-2010 08:56 AM ET (US)     Profile for jimh  Send Email to jimh     
The under-cowling main engine fuse is typically fed from a connection to the battery lead feeding to the engine starting solenoid. The branch circuit being fed from the fuse is typically the remote throttle and shift controls, which are fed typically via a wire with red insulation with a violet stripe in the electrical harness routed between engine and remote controls. There is no guarantee this is precisely how it is done on your Mercury 90-HP FOURSTROKE engine, but the wiring as I described above is very typical among outboard engines.

If the under-cowling main engine fuse repeatedly blows, the problem is likely in some accessory wiring at the helm or a result of a loose connection creating a short circuit somewhere along the way.

When working on electrical circuits and trying to diagnose problems, I find it much easier to work through the problem if there is a schematic diagram showing the circuit. The schematic diagram allows you to visualize the circuit, to look at it in a logical way, and to make a series of progressive deductions and tests. Without a schematic diagram you just have a bundle of wires, and without a logical series of tests you just poke around among those wires hoping the problem hits you in the face and you are smart enough to recognize it when it does.

jimh posted 04-29-2010 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Any news on the remedy?
nats posted 04-29-2010 10:40 AM ET (US)     Profile for nats  Send Email to nats     
The problem was a wire under the console that was not connected correctly--my fault. [The wire under the console that was not connected correctly] was causing [unclear] that shorted out the main fuse on the engine. Something so simple can cause so much fustration. Knock on wood, so far so good.
jimh posted 04-30-2010 02:02 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the follow-up reply with the actual diagnosed problem and remedy.

Quoting myself from previously:

quote:
"If the under-cowling main engine fuse repeatedly blows, the problem is likely in some accessory wiring at the helm or a result of a loose connection creating a short circuit somewhere along the way."

I should have put some money on that prediction!

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