tachometer

Tach for 2005 johnson 115 operates erratically from 0-7000 rpm while engine is idling in neutral. Should I just replace the tach or is there another problem?

(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.

Joe..as I have found out with the older air cooled rectifiers, they can be blown and the tach will still work. I have found the same on the 9 amp water cooled units too. My 150 V-6 was run all season, but it wasn't until near the end that I found it wasn't charging the battery. However, the tach worked all year. I swapped out the rectifier/regulator once I had it out of the water, test ran it, now it's charging, and the tach still works.

And I can vouch for the water cooled units catching fire....the first 115 I bought had one that had literally gone up in flames!!!

Bobkill..does the tach work normally above idle??

No, I've had the boat in the water at full throttle and the tach continued to operate erratically showing various rpm's without moving the throttle.

bill_w said:

Joe..as I have found out with the older air cooled rectifiers, they can be blown and the tach will still work.

Click to expand...

Bill... I'm aware of that. The test I replied with pertains to the units whereas the gray tachometer sensor lead functions from the water cooled voltage regulator/rectifier only.

What year is that 150hp V6?

My 150 is a 1989. I replaced the reg/rect that was on it with one from a 1985 115 that I own, and I think it was you who looked at the mess I had on the 115 last year, and you told me to rip off all the old stuff the PO had on it, and replace the water cooled reg that had gone up in flames before I bought it.