I have a 2002 125 merc,Tach stoped working,I see voltage at the purple wire,good ground but no needle movemet?Have power with 12 v test light,What signal or voltage should I see at the purple wire?Also my charging system seems to have low voltage this season.My fish finder (garmin 300c) has low voltage alarm.If Im running at any speed @ any rpm range from 1100-5250 rpm range,If i hit Trim, add nav Lts or any curent draw>Voltage alarm goes off with display of 11.9 volts.Battery is 2 years old.Brand new 2 ga battery wires with fresh heat shrink and #2 ends.The voltage does come back if I turn off item that draws amps.Is it my charging system?How can I test it?Any help would be excellent.