Tachometer problem

[Gaia]

Hi there , 2004 Mariah 3.0l mercruiser. Tachometer not working . Befor I buy a new tach I want to rule out all other problems that may cause it not to work . I have a fairly good knowledge about how it all works , BUT , I NEED TO KNOW , 1. Should there be a voltage current on the sender wire Grey one that comes off the coil to the tach , if so what should it be . Currently on checking my circuit 11.5v with ignition on and 11.5v engine running and rising voltage with increase on throttle. Any body help?

 

[alldodge]

The Gray wire should only see pulses Red should be ALT/BAT volatge Black Gound You should see 14.5V from the alternator while the motor is running

 

[wrench 3]

If you put a test light on the gray wire you should get fast flicker at idle. It works with an old incandescent light but I've never tried an LED.