So I have an 86 sami. I wanted a tach so i bought a sunpro tach. I installed it in the dash wired it up, and blam  nothing happened.   It lights up when i turn the lights on so i know it is grounded and has power. the instructions say to wire it to the negative terminal on the  ignition coil..... tried that nothing happened. tried the other side of the coil nothing happened. now my question is ,  how stupid am i right now. I know it must be something small that im not noticing,   but Io have no idea what it is.   all help is greatly appreciated.

were there any jumpers or settings on the tach that need to be set?
like for different engines like 4 6 and 8 cylinders

that and hooked up to the neg side of the coil and it's supposed to work,
or you might have a defective unit, that's what happens to me all the time

Wild

there is a setting, but its on the 4 cyl like it should be. and if it were defective would it still light up with the lights like it does.  btw thanks for the help.

It should be pretty simple.  One goes to your 12v, another to ground, one for the back light and one to your coil. 

Ya, the lighting up part has nothing to do with the
needle sweeping electronics, and BTW I use a tach
signal off of the back of the instrument cluster for
my MegaSquirt secondary EFI system because hooking
it to the coil directly caused it to not run, must of grounded
it out all the time

Hook it up to positive on the coil i bought the same cheap piece of shit. instruction are wrong

I tried hooking it up to both sides. i had a buddy sit in the car and i touched it to both terminals and got no result.  i also attached them with a connector to the + and - sided and still nothing. at this point im pretty sure it is defectice. thanks for all the help if you have any other ideas that would be awesome.

i connected the red and white to the batt. light is always on but it works, grounded and green to the coil.  got one a couple y=ears ago, worked fine for about a month then just quit.  its like another said, piece of shit.

so I finally fixed the tach, I ended up just running the green wire to the positive terminal of the coil, the red wire direct to the battery, and grounded in the truck and the white to the dimmer like it said. I guess the red wire didnt like being attached to power in the cab so i ran it outside to the battery .  o well it works now.

That just means you didn't find the right power under the dash. I noticed a lot of wires that seem live w a meter are really not. A lot of the lighting wires are open grounds. They wires are hot when turned off by what ever switching means and dead when turned of bc it completes the ground. There is nothing wrong with the way you did it though.

I picked up a wiring block and put it under the dash, then spliced it into the different voltages.  This way I can wire anything inside the cab to a single spot.

12v constant
12v ACC on
12v Ignition on
12v Lights on
Reference Ground