tach not accurate any ideas
 

I have a 2007 42 fountain 525efi's. One tach works properly and the other gradually gets farther off. I thought it might be set for the wrong number of cylinders, but it was correctly set up. I bought a new tach from livorsi and it reads the exact same as the old one. I did see a black box under the dash that says digital to analog with the mercury smartcraft label on it. Is that where the tach is getting the signal from? Can these be programmed incorrectly? I just want both tachs to work together.

I think I found the solution. AGI box under the dash. I will try switching the plugs and see if the problem changes to the other side.