90 HP Johnson 2 stroke outboard blows 20 amp fuse intermittenly

My 2005 90 HP Johnson engine blows the 20 amp fuse intermittently. Is there a good way to troubleshoot this problem?
Any known problems with this engine?

Usually that problem is caused by a failing electric starter BUT the fuse would blow when the starter is engaged.

If the fuse is blowing "out of the blue" as the saying goes.... just exactly when is it that you notice the fuse is blown?

Is the starter directly connected to this fuse? I thought the fuse goes to the solenoid, power trim and gauges. I can't tell if it blows when trying to start since I don't know if it has already blown before I turn the key.

The starter is not in the fuse circuit, most likely a rect/reg problem as if failing it will blow fuse when key is turned on .

bronx68 said:

Is the starter directly connected to this fuse? I thought the fuse goes to the solenoid, power trim and gauges. I can't tell if it blows when trying to start since I don't know if it has already blown before I turn the key.

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The 20 ampere power fuse leads FROM the battery cable that is connected to the starter solenoid, picking up the voltage FROM that battery positive cable... NOT... applying power to the solenoid!

The voltage/power/12v carries the battery voltage through that 20 ampere fuse to the "B" (battery) terminal of the ignition switch which supplies voltage/power to the "S" (starter) terminal of the ignition switch to power and engage the starter solenoid.

And yes, a faulty water cooled voltage regulator/rectifier can also cause the fuse to blow... BUT... normally if this component is acting up, the tachometer will also be acting erratic and you've said nothing about that as of yet.

Waiting to hear your detailed exact explanation as to when and how that fuse blows to pin it down. Just exactly what is taking place when that fuse blows?

Is there anything I can do to test the rectifier/regulator assembly?

The problem is very intermittent. The last time it happened I had the boat out I driven it about 1/2 hour and shut down to do some fishing. Tried starting it up and the fuse was blown. Replaced the fuse and it start right up. Took boat back home tried to get problem again while running on garden hose but could not produce problem again. The rectifier assembly is expensive so don't want to just shotgun the problem.

Going by a OMC specific service manual when those water cooled voltage regulator/rectifiers first came out, there was a test procedure given but it was so complex... carbon load test etc (ugh) etc that I closed the book and devised my own test (as follows)... HOWEVER, you have stated that your tachometer has been functioning properly which in my opinion indicates that the regulator/rectifier is also functioning properly.

However, should your tachometer suddenly start to act up...................

(Testing Tachometer With Water Cooled Regulator/Rectifier)
(J. Reeves)

A quick check is to simply plug in a another new tachometer as a piece of test equipment. If the new tach works properly and the old tach didn't, obviously the old tach is faulty.... but usually boaters don't carry around a spare tach (see below).

A faulty rectifier wouldn't damage the tachometer, the tachometer simply wouldn't work. This is due to the fact that the tachometer operates off of the charging system and the rectifier converts AC voltage to DC voltage, enabling the charging system. A faulty rectifier disables the charging system, and the tachometer simply doesn't register.

However.... those water cooled regulator/rectifiers that are used on the 35 ampere charging systems (and some others) bring into play a different type problem, and as you've probably found out, they are really a pain to troubleshoot via the proper procedure. There's an easier way.

The tachometer sending/receiving setup operates off of the gray wire at the tachometer. That same gray wire exists at the engine wiring harness which is connected to the engine electrical terminal strip. You'll see that there is a gray wire leading from the regulator/rectifier to that terminal strip, and that there is another gray wire attached to it. That other gray wire is the wire leading to the tachometer which is the one you're looking for.

NOTE: For the later models that DO NOT incorporate a wiring terminal strip, splicing into the "Yellow Wire" mentioned will be necessary.


Normally the Gray wire leading from the tachometer is attached at the terminal strip to another Gray wire which leads from the water cooled voltage regulator/rectifier...... remove the gray wire that leads to the tachometer. Now, find the two (2) yellow wires leading from the stator to that terminal strip. Hopefully one of them is either yellow/gray or is connected to a yellow/gray wire at the terminal strip. If so, connect the gray wire you removed previously to that yellow/gray terminal. Start the engine and check the tachometers operation, and if the tachometer operates as it should, then the regulator/rectifier is faulty and will require replacing. If the tachometer is still faulty, replace the tachometer.

If neither of the yellow wires from the stator is yellow/gray, and neither is attached to a yellow/gray wire, then attach that gray tachometer wire to either yellow stator wire, then the other yellow wire, checking the tachometer operation on both connections.

I've found this method to be a quick and efficient way of finding out which component is faulty.... the tachometer or the regulator/rectifier. It sounds drawn out but really only takes a very short time to run through. If the water cooled regulator/rectifier proves to be faulty, don't put off replacing it as they have been known to catch on fire with disastrous consequences.

Do you have accessories connected to the ignition circuit? Intermittent faults are normally wiring related it could be a bare wire in the engine rubbing at a certain throttle position? If accessories are connected to the ignition switch you need to add a fuse block and pull two wires from the battery for the house wiring. Keep ignition separate from the house wiring! Be sure and fuse any wires you add to battery posative at the battery.

kimcrwbr1 said:

Do you have accessories connected to the ignition circuit? Intermittent faults are normally wiring related it could be a bare wire in the engine rubbing at a certain throttle position? If accessories are connected to the ignition switch you need to add a fuse block and pull two wires from the battery for the house wiring. Keep ignition separate from the house wiring! Be sure and fuse any wires you add to battery positive at the battery.

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Amen... In complete agreement with Kimcrwbr on his statements.

No there are no accessories connected to the ignition circuit. I suspect the starter solenoid might have an intermittent short. Is there any way to test the solenoid?
You mentioned if it were the rectifier/regulator you said the tach would be erratic. Exactly what do you mean? Would it also change the engine RPMs

Erratic...... needle jumping back and forth to different readings, falling to zero, NOT functioning properly. It wouldn't actually cause the engine to change rpms.

joereeves said:

Erratic...... needle jumping back and forth to different readings, falling to zero, NOT functioning properly. It wouldn't actually cause the engine to change rpms.

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The tach is not erratic. I guess that brings me back to the starter solenoid any way to test if this is intermittent. I would just change it but poor design makes it very difficult to remove.

The way I would go about debugging this is like so:

Rig up a 12v automotive bulb in such a way that it is connected to the fuse holder, one wire of the bulb to one side of the fuse holder, the other wire of the bulb to the other wire of the fuse holder. You will have no fuse in the circuit, but a light bulb in it's place, and the term for this is wiring a light bulb in series where the fuse normally would be.

Now, if all is correct, the light will be off. When you turn the key to ON, the light should still be off. When you turn the key to START, the light will come on bright.
That's the way it should work if all is well.

Turn key to ON, and start wiggling the harness, the control handle, all connection points, all the while watching that light bulb. When it flashes ON very bright, you have found the place where your short is.

I think you have a rubbed wire inside the control box, shorting to ground, most likely the red/purple striped one.

And you don't keep blowing fuses in the process.

You could splice in a fuse holder on the purple wire to rect/reg and use a 7.5 amp fuse as no charging amp goes thru purple. If it blow its the rectifier ,if not look at harness..They can fail without showing erratic tach.

I went through cleaning all the connections on the starter and starter solenoid and have not experienced the problem again. I don't understand how this might fix the problem since even if one of the connections was bad it should not of increased the current to blow the 20 amp fuse. I would think a bad connection would increase the circuit resistance and therefore reduce the current. Anyone make any sense of this?

What actually happens is electrical components are designed to pull a certain current when there is resistance in the circuit that component is still trying to draw the same power so it increases the amps through the fuse.

kimcrwbr1 said:

What actually happens is electrical components are designed to pull a certain current when there is resistance in the circuit that component is still trying to draw the same power so it increases the amps through the fuse.

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Cannot let that pass. So wrong on so many levels.

ANY added resistance in a series circuit reduces the current in the circuit. Period. That is assuming the voltage remains constant.
Resistance goes up, current goes down.

Corrosion is the most common form of added resistance in outboard electrical circuits.

The fuse should NOT BLOW if there is corrosion resistance.

Whatever happened, you cleaned things up, and probably fixed whatever was shorting.

Dasel, I liked the light bulb trick in the fuse holder. I did something like that years ago with a carbon pile and a volt meter. Easy way to find a short without spending money on fuses.

I have this issue on an 1990 40hp Johnson and have isolated/correlated the fuse blowing to whenever the key is pushed in activating the electric primer circuit.