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Author Topic:   Mercury Engine Tachometer
mercuryman1113 posted 10-04-2010 11:54 PM ET (US)   Profile for mercuryman1113   Send Email to mercuryman1113  
[I] have a [c.1980] [Mercury] 50-HP with [Thunderbolt] ignition. [T]he boat [on which this motor is installed] sat up for 20 years. [I] finally got [the Mercury 50-HP outboard] running. The [tachometer connected to the the c.1980 Mercury motor] stayed on 3,000-RPM while [the motor speed was] [idle]. [I] want to change the [tachometer] to an after-market one. [T]he original [tachometer] had two wires going to it, a GRAY and a BLACK wire. [M]ost new [tachometers] have three or four wires going to them. [W]ill a new [tachometer of unknown type or brand] still work [if connected to the c.1989 Mercury 50-HP motor]?
jimh posted 10-05-2010 08:09 PM ET (US)     Profile for jimh  Send Email to jimh     
I am certain there is an after-market tachometer that will work with a c.1980 Mercury outboard.
L H G posted 10-05-2010 11:05 PM ET (US)     Profile for L H G    
I came across one of these older tachometers recently, actually furnished by Boston Whaler from the factory in the late 70's. It also had no purple ignition (+) terminal, much to my surprise, but did have a light terminal to which a blue wire was attached. Maybe that is how the + was fed into the gauge?

Or evidently a tach only needs the sender and neg to function? But as mentioned, all newer tachs also have the "+" terminal (maybe it's only for the light), or simply for jumping the purple (ignition) to other gauges.

A newer tach will work on the older Mercs, which have the purple, black and gray leads already there in the ignition harness. Just hook up all three, and set the selector pole setting correctly for your engine application.

jimh posted 10-06-2010 08:49 AM ET (US)     Profile for jimh  Send Email to jimh     
The tachometer signal (usually on a GRAY wire) is a pulsating signal which has 12-volts. A meter could easily power itself from this signal. The power required to move the dial pointer of a D'Arsonval or Weston type movement is extremely small. It is likely that the meter movement found on most tachometers is full scale at 1/1000-ampere. The notion that a tachometer must be supplied with 12-volt power via a separate conductor has come from the normal practice of manufacturers in designing these devices, but it would be completely possible for a tachometer to derive all of its operating power from the tachometer signal itself. In most cases the tachometer signal is connected directly to the alternator of the engine which can supply currents of up to 50-amperes. Supplying 1/1000-ampere to the tachometer is completely inconsequential.
seahorse posted 10-06-2010 10:00 AM ET (US)     Profile for seahorse  Send Email to seahorse     
The old Airguide tachometers and OMC tachs from the '60's and early '70's used only the pulse wire and aground to operate. Back then they ran off the magneto and breaker points. It wasn't until the introduction of CD ignition that they used the alternator for the pulses though some early battery powered CD units had a separate tach wire built into them.
L H G posted 10-07-2010 02:30 AM ET (US)     Profile for L H G    
So would a 2-wire tach like described, work for a conventional 2-stroke, any brand, say up through 1998 models? Or do the 80's and 90's engines need the later 3-wire versions? What about working on a newer 4-stroke, assuming there is a pole selector sqwitch?

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