vdo tach

[slotracer]

i have a vdo tach that has 3 plugs on back positive, ground & sending. but the guy i got the boat from has 4 wires at the gauge looks like he has ing. postive, ground & sending. how do you hook it up?

[Icarus prospector]

That fourth wire is probably for the light.
The positive goes to a keyed hot, the sender goes to the neg side of the coil and the ground to a ground.

[Jetaholic]

the sender goes to the neg side of the coil
Unless you're running an MSD ignition box...the box will have a seperate wire that goes to the sender terminal on the tach. You will fry the tach if you connect the send terminal to the negative side of the coil with an MSD ignition box.

[slotracer]

i am running a msd box. it has a seperate light

[roostwear]

On most newer tachs the wiring is standardized. Here's a pdf for Mallory (http://www.vintagerodcomponents.com/inst/protach.pdf), but it should be the same as VDO (for a stock ignition).