In your diagram you have the signal feed to the tacho to the + side of the coil. It should be connected to the - side of the coil.

To work, tachos need:
1) "-" side of coil for rpm pulse/feed;
2) earth;
3) "+" power feed usually via ign switch (although I have a Stewart Warner after market tacho which doesn't require a power feed. It just has earth and "-" side of coil)

The wire to the + side of the coil is simply the coils power feed from the + side of the battery/ignition. By wiring as per the diagram you essentially just have both sides of the coil connected to the + side of the battery.