Faulty tachometer?

[unclebob]

I have a 1994 Mercury, 60 hp, 2 stroke outboard that has developed a problem with erroneous tachometer readings. This problem has surfaced before recently and then went back to normal readings without any adjustments. Last couple of times out the tach reads about 300 rpms too high at idle, (normally 750 rpms) and all the way through max throttle. Max rpm used to be 5500 rpm with present prop on cool, dry day. I have traced wiring from tachometer back to engine wiring at control module and cannot find anything that represents a poor or intermittent connection in wiring using ohm meter. Engine performs fine without any issues and trim angle does not have any effect. It seems that it must be either control module or tachometer. Any suggestions to verify root cause.

 

[galamb]

The "signal" for the Tach comes from the rectifier that charges the battery. So good idea to check the feed to the battery (check the voltage and see if it's jumping around when the tach is acting wacky - ideally you want about 13.6 volts when it's charging) - you don't want to fry a battery as well.

If all is ok it could simply be the tach coming to the "end of it's days" - they do die...

 

[unclebob]

GALamb, The battery voltage is 14.4 volts when engine is running, (the tach is reading high, 1100 rpm @ idle in gear). This engine has a voltage regulator/rectifier. The factory manual specifies an output of 14.5 volts. The manual shows two versions, the regulated model and the rectifier model. Also, what I originally called the control module is the switch box, (this seems to be where tach signal is generated).