Help! Mercury Tachometer not working.

[FlyingV]

Does anybody here know how the tach works? Mine quit working. Its a '94 115hp 2 cycle. I'm thinking its an analog voltage level from somewhere on the engine. Is there a way to test the tach to be sure it is not the problem?

Thanks.

[eurolarva]

Most outboards tap off of the voltage regulator for the tach. Hook a voltage meter up to your battery and see if it is still charging your motor. When you hook it up with motor off it should read about 12.75 volts for a fully charged battery. When motor is at cruising speed it should be over 13 volts. Make sure meter is set to DC volts and not AC volts.

[Valv]

There should be a gray color wire coming from remote control or motor, usually there's a connector plug on motor with 3 wires (purple, black, and gray) that is to connect a tachometer.

Gray wire should come from rectifier on motor, but it's bundled with main harness up to the remote control.

I don't think you can test it, but if you want to stop by I can lend you one to try out.

There's only 4 wires to a tach:

Purple ( + with key on)

Black (- neg.)

Gray (sender from motor)

Blue (backlighting)

[FlyingV]

Guys, it was the Voltage Regulator/Rectifier that was the problem. Thanks for your help!

[Whoaru99]

To answer your original question, many tachs work off a tap on one of the windings in the alternator. That provides a variable frequency signal - higher RPM, higher freqency; lower RPM, lower frequency.

The newer "electronic" engine are probably more sophisticated than this and likely use a signal from the ECM.