Dumb question: need a wire thats hot when ignition is 'hot' only

[SadbutTrue]

Trying to wire up a tachometer, calls for one wire to be connected to a + source thats only hot when the ignition is 'on' and not when its in 'accessories' (or anywhere else i guess). Whats a convenient wire to use?

Also, i'm assuming the dimmer power wire will be one that goes from the dimmer switch to the instrument backing, so it should be obvious if you're looking up at it from under the dash... right? If not, some guidance on finding that one too would be great.

Thanks

 

[D.Hearne]

Run the wire out to the coil. And connect it on the + side. I usually just find a terminal in the fuse box though for that wire.

 

[66HertzClone]

There is a bullet style connector at your ignition switch, this is the 12 volt coil supply mentioned already. Why not make an adapter harness here, a matching male and female connector with a pigtail for your tach power connection. Saves having to run another wire out to thru the firewall.

 

[68RCodeConv]

There is a threaded terminal on the back of the ignition switch that is hot when the key is on.

As for the wire for the light: I did the lazy thing and wired it to the same terminal. The light is on when the key is on.

 

[SadbutTrue]

There is a threaded terminal on the back of the ignition switch that is hot when the key is on.

As for the wire for the light: I did the lazy thing and wired it to the same terminal. The light is on when the key is on.

Nothing wrong with lazy, this sounds good.

 

[SadbutTrue]

Hrmm... i connected the red to the + side of the coil, the green (tach) wire to the - side, the black to the ground side of the battery and left the dimmer wire hanging just to test it. The needle starts shaking and moves almost to the 0 rpm hash, but doesn't quite get there. What am I doing wrong? It is set to 8 cylinders, and I did a decent job connecting it (i thought)...

I do have an MSD, pertonix and slighlty hotter than stock coil... could that be the issue? Do I need to plug the tach wire straight into the MSD? If so... what kind of adapter do I need?

 

[D.Hearne]

Try the red at a hot source in the fuse box that's switched on with the ignition. Maybe the resistor wire in the ignition is screwing with it. Otherwise you've got it hooked up correctly. I always run the red this way, to the fuse box. If that doesn't work, I'd suspect the MSD is doing the deed.

 

[jlangholzj]

my tach isn't as complicated as yours but i just ran the + power for the tach into my radio wires. works just great.

 

[SadbutTrue]

The issue does appear to be the MSD.. the MSD tech said the coil can't be used as a source for the tach with an MSD ignition, so i had to buy a 30$ adapter off summit. I'll let you know if it works.