new tachometer not working

I have a 1977 Evinrude, 115hp, model#115793S. Just bought a new tach and installed, but not working. Tach seems to jump a little when trying to start but nothing when engine is running. Light works and seperate feul gauge works. First boat I have ever owned and no manual or experience; any help would be great. Old tachometer never worked, maybe something in the wiring or did I buy the wrong tachometer? Set to 6 on back and wired correctly,... I think.

Check the rectifier, as follows.

(Small Rectifier Description & Location)
(J. Reeves)

On most 2,3,4,6 cylinder engines, the small rectifier is located on the starboard (right) side of the engine just in front of the engines electrical wiring strip. There are a few older V4 engines that have the wiring strip on the rear portion of the engine and the rectifier would be located just under that terminal strip. The smaller horsepower engines usually have the rectifier located on the starboard side of the powerhead close to the carburetor area.

The rectifier appears to be a round object approximately one inch (1") in diameter and also about one inch (1") high. The base of it is sort of triangular in appearance and is attached to the engine with two (2) screws/bolts..... usually one screw/bolt is larger than the other. The rectifier, depending on which one your engine uses, will have either:

One Red wire, one Yellow wire, and one Yellow/Gray wire, or One Red wire, and two Yellow wires.


Note that either of the above rectifiers could have a fourth wire which would be Yellow/Blue


Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay store at:

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(Small Rectifier Test)
(J. Reeves)

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier.

Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.


Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay store at:

http://stores.ebay.com/Evinrude-Johnson-Outboard-Parts-etc?refid=store

Okay Joe, I tested the rectifier and had readings both ways to ground and no readings either way on the red wire. I also noticed that the red wire is broken and burned on both ends, so I am guessing that I need a new rectifier. Is there a specific one, or are they mostly universal? I know that it has to be round with 3 leads but I have found a couple of different ones, is there a difference and what should I look for? Thank again for all the help.

Your engine requires either the OMC/Bombardier rectifier #583408 or the Sierra rectifier #18-5708, both of which interchange and are no doubt available on this site.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay store at:

http://stores.ebay.com/Evinrude-Johnson-Outboard-Parts-etc?refid=store

Finally got that new rectifier; put in on today and tachometer is working beautifully. Thank you very much for all the help Mr. Joe Reeves, couldn't have done it without you.

Nah..... you would have got it eventually.

I'm still trying to figure out why it works at all, if he has it set at "6" on the back. Isn't a 115 a 4 cylinder engine?

Martino..... The OMC tachometers operate off of the engines charging system. The OMC charging systems emit a six (6) pulse signal. That signal is fed to the tachometer's sensor terminal via a GRAY wire that leads from the charging system's rectifier. The number six (6) on the dial on the back of the tachometer pertains to the pulse setup, not the number of cylinders.

The tachometer above as it now stands could be transfered to a 2, 3, 6, or 8 cylinder two stroke OMC outboard and function perfectly.

Thanks! Learn something every day.