Mariner Tach not working

[GTM73]

Hello,  I am new to the group.   I recently purchased a 1997 215DC with a 1997 200hp Mariner Offshore.  Engine is working great however the Tach is not working it is stuck a 8000rpms (all the way to the right) and doesn't move.  Does anyone have any advise on what I can do to troubleshoot this issue.  I would replace the tach however I would like to know it is a faulty tach before I spend the money on a new one.  Thanks in advance for any advise.

[seabob4]

A pegged tach usually means it's history.  A couple things you can check.  First, make sure you have 12V between purple and black on the back side of the tach (I and G).  Then, with the engine running, you should see around 5V between gray and black (S and G).  If you have both, you need a new tach...

If not, make sure all the nuts on the backside are good and tight.  You've got a '97, a 15 year old boat.  Stuff DOES crap out...