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What brand might this tachometer be?

Do you have any other gauges? Do they all work?

Do you have a Voltmeter? This is where I would start.
Either a Voltmeter or Fish Finder that shows the charging system is working or not.

Here is a partial list of OMC style gauges.
http://www.whalercentral.com/articles...icle_id=43

If your charging system is not working, then you most likely need a Voltage Rectifier for your model outboard.

This in only an example.
Make sure whatever you purchase is the proper one for your motor.
http://www.caliboatstore.com/Johnson-...-72320.htm