Tachometer goes backwards

[dhowebuf2]

I recently tried to install a Teleflex tachometer on 1991 Mercury 60HP. Per Tach instructions I connected Gray wire from engine harness to send input, and connected ground and power. Tach seems to read correctly up until around 4000 RPM after which it DECREASES as the engine further speeds up. Per the tach instructions the alternator setting is set to "6P".

Thinking it may be a bad tachometer, I exchanged it, but the new one did the exact same thing.

Any ideas?

 

[Barnacle_Bill]

Re: Tachometer goes backwards

You could have a diode breaking down in your rectifier/regulator. Have you any way of monitoring the voltage? If this is the case the voltage will go down when the tach starts to act up.

 

[dhowebuf2]

Re: Tachometer goes backwards

Would I need an oscilloscope to measure this? Or would a multimeter work? I'm guessing this is a periodic square pulse coming from the motor. Is it easy to just replace the diode? If it were bad, could this be having an effect on actual motor performance?


Thanks!

(Sorry for the delayed response - I kind of gave up on things in the fall, and just got the boat for spring).

 

[Barnacle_Bill]

Re: Tachometer goes backwards

A multimeter will do it or if you have a depth finder connected to your starting battery, a lot of them can display the voltage.