tachometer

by kent
(laplace la.)

I have a 1996 mercury 150 hp and I replaced the tachometer and Iam still nit getting a reading.
Should I have a voltage reading on the sending wire at he back of the tackometer when th e boat is running.
The sending wire is coming out of the rectifier.
The motor runs fine . Could the rectifier be bad?
WHAT Can I TEST TO PINPOINT THE PROBLEM.
Is the recifier stil ok or do I need to replace the rectifer to correct the sending wire?