Tach & Charging issue 1988 225hp Johnson

1988 225 Johnson outboard TACH & CHARGING ISSUE....
For 8-10 years my tach would suddenly stop working (zero RPMs) and at the same time the motor would stop charging (voltage would show 12-12.5v). I continue to run it and the voltage would continue to drop (I have a second battery that is disconnected, so I was never really worried). When the voltage dropped to 11 1/2v, the tach would come back on and the voltage would show 15-16v (charging again).
Since this boat only gets used 2-3times a year, and sometimes it would not fail, I never did anything about it.
This past weekend the symptoms changed. Now the tach works but the RPMs were approx 1.5k slower the entire time. And....the voltage never gets above 12v (drops to 11.5v & then back to 12v).
Ideas?

Re: Tach & Charging issue 1988 225hp Johnson

You're extremely lucky your engine didn't catch on fire!

(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.

Remove that 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.

Re: Tach & Charging issue 1988 225hp Johnson

I had it in the shop a couple of times...but since it is so intermittant, they found nothing wrong. Now...it is in a failing state so I should be able to try what you suggest.
Unfortunately, I won't be down there for two weeks, but I WILL let you know what I found out.
Thanks for the help....
MEANWHILE...if anyone else has anything additional to add or suggest, ...please do. Open to all suggestions.
Thanks..

Re: Tach & Charging issue 1988 225hp Johnson

Not very familiar with outboards.....heading down to the shore this Sunday to try what was suggested. Looking for any other tips/ideas since I will only have the one day to play with it.
* going to check all grounds
* going to get rid of "wingnuts" on batteries and replace with nuts/lock washers
* then I am going to try what Joe suggests

Any other ideas (in case I still haven't figured it out)?

Also...just curious...Can a faulty Tach cause the voltage to stay below 12.5v? (99% sure it can't....but figured I'd ask anyway).

Thanks...andy