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Author Topic:   Tachometer: Interchangeable Among Various Brands
Command57 posted 04-25-2006 04:02 PM ET (US)   Profile for Command57   Send Email to Command57  
[Are outboard motor tachometers interchangeable among brands? For example,] can a Yamaha-brand analogue-dial pointer tachometer [be connected to and properly measure the engine speed of a] Johnson-brand outboard? I tried to hook it up but got nothing. It worked with my old motor (Yamaha). Thanks
jimh posted 04-25-2006 07:27 PM ET (US)     Profile for jimh  Send Email to jimh     
Most outboard motor tachometers are adaptable to other brands, as long as they are calibrated for the same number of alternator poles and the engine pulse voltages are compatible, although your experience seems to point in the contrary direction.
Command57 posted 04-25-2006 07:48 PM ET (US)     Profile for Command57  Send Email to Command57     
There are two poles that are connected with a bridge. I was thinking of trying it connected to just one pole. Does that make any sense?

jimh posted 04-26-2006 09:37 AM ET (US)     Profile for jimh  Send Email to jimh     
Most tachometers are designed to accept a pulse signal from the outboard. The pulse signal voltage is clamped to ground and to battery positive by the action of the battery charging rectifier. The pulse rate is proportional to the engine speed, the number of poles in the alternator winding, and the number of magnets in the flywheel. If all of these are the same, you can use your existing tachometer.

Many alternators have calibration controls to adjust to different pulse rates.

The common color codes of the wiring for Mercury and OMC are given in the REFERENCE section. This may help:

Normally all that is necessary is to conform the wiring to the proper terminals and to adjust the calibration as necessary.

I would consider purchasing a new tachometer before I would undertake extensive modifications to the electrical system of my outboard motor in order to accommodate an tachometer designed for a different motor. I arrive at this decision based on the comparative costs of a new tachometer and a new motor. A new tachometer can be had for $25 on eBay. A new motor will be about one hundred times more expensive, at a minimum.

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