posted 03-12-2011 07:59 PM ET (US)        

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On a 1994 Mercury 75-HP classic two-cycle I recently replaced the rectifier-regulator, and everything was good except the tachometer did not work. I assumed [the tachometer] had corrosion or a bad connection, and waited [until] I had some time to troubleshoot it. I replaced all the connectors and checked continuity on all the wires. This model has a black ground wire, a [violet] ignition wire and a [gray] sensor wire. The [violet] wire shows 12.6-Volts with out the engine running and 13.9 volts with the engine running. The gray wire shows 0-Volts with out the engine running and 1.7-Volts with the engine running.

Just to be sure I took the motor cover off and checked the voltage for the gray wire at the engine and I got the same reading, 1.7-Volts when running.

My understanding is I should see a higher voltage reading on the gray wire. I took all the connections off the regulator then reseated them and still have the same results. I assume that I cannot return electronics--especially after running it for a month--and hate to drop another $150 on another voltage regulator or even $70 on a new tach. [Suggest] any ideas [how to investigate the cause of the tachometer not working].

posted 03-13-2011 10:26 AM ET (US)            

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The tachometer signal is a pulse train whose frequency varies with engine speed. The positive limit of the pulses is near battery voltage. The negative limit is near 0-Volts or slightly negative. When you measure this signal with a DC Voltmeter you get some sort of random reading. I cannot offer any diagnostic advice based on a DC voltage meter reading of the tachometer signal.

Look at the tachometer signal on an oscilloscope to see what is happening. You should see a rough square wave whose frequency varies with engine speed.