OMC 50 amp fuse keeps blowing

[chopperdog]

Had the alternator rebuilt, and everything worked well
during short engine run. Alternator produced 13 volts.

Went to restart it today to get ready for the water. No voltage at all.
Upon inspection, found top 50 amp fuse blown. There are 2 on back of engine.

Replaced it, and it blew again immediately. This is with engine off, and only the battery power.

What happened?
This may be obvious, and dumb on my part. The alternator shop convinced me that I didn't need the small 2 wires coming out of the back of the alternator. They said I only needed the larger one, which supplies power.

Is this related?

Could the rebuilt alternator have blown something in the system (i.e. voltage regulator) thus grounding the system somewhere?

Thanks for any help.

 

[Don S]

Re: OMC 50 amp fuse keeps blowing

What year OMC? What engine, what drive.
MOST of those 50 A fuses have nothing to do with the charging system. But who knows what you have wired to what, and alternators should be putting out about 14.5 volts, not 13.

 

[chopperdog]

Re: OMC 50 amp fuse keeps blowing.

Re: OMC 50 amp fuse keeps blowing.

Hi Don,

Thanks. It is a 1988 OMC 3.0L 4 cyl with the OMC cobra outdrive.

It is in a 1988 Bayliner Ciera 2155. I'll double check the altertor output when I get it going again.

Thanks again for any help.

I am this close <<<< >>>> to getting on the water!!!!!

 

[chopperdog]

Re: OMC 50 amp fuse keeps blowing UPDATE!!!

Re: OMC 50 amp fuse keeps blowing UPDATE!!!

In viewing related threads, I found that something might be important.

I have had unreliable tach readings.

The last time the engine was running (yesterday), the tach was jumping around to 3000 rpm and back, but the engine was stable at about 600 rpm (set by mechanic during valve job).

The tach will never read below 1000 when engine is running, and will not respond to changes in idle. It reads 0 with engine off.

Also, the Tilt works fine. So the 50 amp fuse that is blowing is the 2 trim tabs fuse, I think.

There are previous post that if this fuse blows, there is no power to the ignition or instrument panel. This is my case as well.

So, I need to find what is blowing the fuse.

Any ideas are greatly appreciated.

 

[Howard Sterndrive]

Re: OMC 50 amp fuse keeps blowing

Replaced it, and it blew again immediately. This is with engine off, and only the battery power.

What happened?

You have what is called a "short to ground", meaning something Hot on the circuit fed by that fuse is touching ground. It has little or no resistance to current flow, so more than 50 amps is flowing... or trying to until the fuse blows to prevent a fire.

You could disconnect circuits... put a new fuse, reconnect stuff and when the fuse blows again, you'll know which circuit.

 

[MRS]

Re: OMC 50 amp fuse keeps blowing

I just replaced my throttle control box on a omc 4.3 cobra and all cables. When I pulled the cables and main wire harness down the way it was mounted the harness was rubbing on a screw and had worn thru three wires one was the main power wire going back to the motor. Fixed them and mounted so as not rub on anything I think this was factory mounted that way so you might check that out.

 

[chopperdog]

Re: OMC 50 amp fuse keeps blowing

I replaced the 50 amp fuses, and they didn't blow again.
I have checked and cleaned all the electrical connections.

But, now the Tach is acting like a voltmeter.
What I mean is, when I turn the key to ON, not start, the tach indicates a stable 1000 rpm.
After starting the engine, the tach runs up to 5000, even though actual rpm is like 600 or so.
Turn the key off, and the needle stays at 5000 until I turn it on again, and the cycle starts over.

So, I think that either the wires on the back of the tach are mixed up by the previous owner, possibly causing a short.
or:
The tach is bad.

What do you think?

Thank you in advance.

Also, I will thoroughly check the wires by the control box tomorrow.

 

[P 0 P E Y E]

Re: OMC 50 amp fuse keeps blowing

The fuses don't blow anymore? Did you fix it or did it heal? Wiggle the wire harness and see if it blows again. You will need to find the short.

I would take a good look at your alternator wiring to see if the positive feed wire is secure and that it can not make a connection with any ground.

Inspect all positive wires to ensure that they have sound insulation and not shorting out.

 

[Howard Sterndrive]

Re: OMC 50 amp fuse keeps blowing

So, I think that either the wires on the back of the tach are mixed up by the previous owner, possibly causing a short.
or:
The tach is bad.

What do you think?

A common tach wiring mistake is often made at the ignition coil.... the tach wire goes on the (-) side, but some folks don't understand that and put it to the (+) side.

 

[chopperdog]

Re: OMC 50 amp fuse keeps blowing

The short basically did "heal", but I actually might have inadvertently fixed it.

I disconnected the alternator wire and reattached it. I think it is possible it was touching the alternator case and grounding out. The 50 amp fuse that blew is in line with the alternator wire.

I'm still in the process of checking for shorts, and no luck yet.

Regarding the tach, I bought a new tach today. It indicates correctly at about 750 rpm, with no fluctuations.
The old tach was definately bad. I reattached the old tach after the new tach, just to be sure, and again it acted as outlined in my previous post.

 

[chopperdog]

I found the short!!

I found the short!!

I don't know if this is a standard method, but it worked.

I used an ohm meter to test continuity between the block and various
points of electrical connection. There should be no continuity, unless there is a ground wired to that specific connection.

Eventually, after a very tedious elimation process, I traced the short to the trim tab (not tilt) switch.

I disconnected the switch, and the power to the switch, and the continuity disappears.

Obviously the switch is improperly wired. I'll have to figure this out later.

thanks for all the help.