Mercruiser 4.2l D-Tronic Tach problems

[Noldisx]

I have a Doral 310 se with 2 Mercruiser 4.2l D-Tronic. On the starboard engine, the tach starts working when engine reaches about 2500 rpm for the first time. After that, it works fine until I stop the engine. (even work on idle speed)
In addition, batt indicator lamp does not work when I turn the key into run position. These are exactly the symptoms for a broken alternator, but wiring diagram shows that grey wire from tach goes into ECM and not to the alternator. Voltage measuring shows about 7,4 volts on the good tach (port engine) and 0,7 volts on the starboard tach. Think engine would not start with a broken engine speed sensor.
Any help would be appreciated
Noldi