Tach is not working on a 1989 evinrude 150xp ranger 363v. Engine running smooth and has full power. Tach reading was jumping, die, work good, now it is dead. I need a procedure to troubleshoot. I bought a new marine starting battery, out of the store measured 13.7 volts, installed in boat running battery measured approx 17 volts. I had the tach problem before installation of new battery.