Put a small Sun tach in my rally dash (70 Dart)There are 4 wires, red ( hot) I ran to the main dash power (red) black to the main ground, green tru the bulkhead connector to the coil and white to the input for the instrument lights. Tach doesn't work and light stays on always. Any hints on what is wrong?

Do you have electronic ignition?

yes, Mopar performance.

green wire on the positive side of the coil?

Yes,positive side.

put it (green wire) on the neg pri coil terminal

My Sun wouldn't work with that, older ones where made for points, got a transmitter box from MSD to make work if I remember right.

You didn't mention what model/number tach you have but Cudaman 1969 is correct. If the tach is an older model, it needs a tach adapter.

Pick up (green wire) belongs on the negative side of the coil. You are feeding it 12v constant voltage and that is why it's staying lit and not reading.

you'd think that the coil neg post (circuit) would be getting opened/closed (to ground) the same whether it points or ECU doing the job but yeah swap the wire & if that dont do it then spring for the adapter

It is a newer model, bought a few year back. I found the booklet for my other Sun Super tach and it says run it to a positive that only gets power when ignition is on, and to engine or chassis ground, so having it run to the dash may be the problem. It was on the negative side before and didn't work there either.
If anyone can remember where you ran tour wires ( e.g red to fuse block accessories terminal)please post, it would be helpful.

Wire the red to something like the radio fuse that is only hot with the ignition on. You may have fried the tach by having it hooked up to the coil positive.

I'd T into the heaviest switched 12V wire (hot only when key is on) such as the heater that you can see in the fuse box & tap into (T) before the fuse (such as heater) on the back side & then solder a wire out from the T then run it on to the tack red wire. This keeps the heater porotected with the 20A fuse with no extra draw on that circuit & you are protecting the tack with an inline seperate fuse (ask Sun what they want/maybe 5A)

mostly, its : red - 12v key on
green - coil neg. [-]
black - ground
white - lights [usually tied into orange dash lite
circuit harness]

We wired another Sun into the Duster,to see what would work,red to fuse block,black to dash frame,green thru firewall to coil,works like a charm. Will play with the white later. I got thee yach in the Dart unwired and I am going to wire it same as the Duster later this week. Thanks again for all your help everyone.