2.3 liter OMC Cobra tach issue

[bstig60]

I have a 1990 Reinell 17M with a 2.3 liter 4 cylinder Ford engine and an OMC Cobra outdrive. The engine has Mallory ignition and runs well. Sometimes we will be running along at about 3500 or so RPM and the tach will read 5000 and above. Other times is reads correctly. Wiring on the tach is tight. I am not sure it the source for the Sensor lead is on the coil or comes off another location. This is a factory installed system. Any thoughts would be welcome.

 

[o2batsea]

Jumping tach is a sure sign of a failing alternator

 

[bstig60]

Thanks for the reply.... This tach is powered off the negative terminal of the ignition coil, not the alternator. Please explain why the tach acting up is a sign of a failing alternator....