Quote Originally Posted by barn dog View Post
IBNFSHN I have a 94 120 johnson that the tach dosen't work right. It bounces all diffrent ranges but some times it works properly till I change speed then it starts bouncing all over the place from no reading to correct reading. Any suggestions?
Your tach works off an AC pulse from the stator. You can take the rectifier/regulator out and throw it away and as long as the tach is connected to one of the yellow wires coming from the stator, it will work (if the stator is good). The connection for the tach is made at the rectifier/regulator simply because it is a convenient place to make it. The 2 yellow wires coming from the stator feed a full wave bridge rectifier consisting of 4 (sometimes 6) diodes. It depends on which diode gets damaged whether it will affect the tach or not. Bottom line is the charging system can be damaged but the tach still work.

With all that said you have either got a bad stator, a bad rectifier and or regulator, a bad connection, or a bad tach. Check all your connections first and look for something loose or corroded. Have you got a way of monitoring the voltage while running? Maybe a voltage gauge installed or your depth finder capable of showing voltage? (most nowadays have that capability). Watch your tach and when it starts acting up watch your voltage to see if its fluctuating too. Also note your voltage before you start the motor and again after you start it. If your charging circuit if working properly you will see an increase in voltage.