It does work on the move but not usually for very long. I bought the boat new and it worked until about a year ago..It is a Humminbird and the transducer is a shoot through..It is set for feet.FYI.....I had it on the lake yesterday and the tach quit working. Don't know if there is a connection between the two.

Ok

First off, pull the transducer wire and clean the connections. Second, make sure the transducer is still glued in firmly to the inside of the hull. If neither of those options work, you an buy an equivalent sounder for less than 200 bucks. ( If you can beg, borrow, or buy with the option to return a transducer for the unit, you could stick it in the bilge temporarily using a large glop of vaseline. Run the boat see what happens. If it works, buy a new 'ducer)

On the tach--check the connections, especially the ground, using a multimeter. Good info here: http://www.boatingmag.com/gear/electronics/multimeter-tips

There are also little dip switches on the back of the tach, that set it t read different alternator pulses. if any work has been done behind the panel recently, one of those may have been inadvertantly flipped. You can call your engine company or the tach maker to find the correct dipswitch setting.

If none of that works, the easiest troubleshoot is to get a working tach (lets call it a shop tach) and hook it up with alligator clips; if that works, you know its the wiring.

If you can t source or dont want to buy a tach for that purpose, the gray wire is the signal source from the engine and tach sender. using a multimeter set to low voltage , you can check to see if their is a signal output,. You can also use the meter to check the sender.

A service manual for your engine will be helpful in locating the wires, senders etc

Good Luck

Kevin