tach won't let car start

[DOHCD15]

i just installed an autometer tach in a 1971 240z, i am using the factory tach harness and i now use the green and white stripped wire as my tach signal. if i try to start the car with the green wire connected to the negative side of my coil my car cranks and cranks with no fire and the moment i remove it the car fires right up, i then attach the green wire back to the negative side of the coil and the tach works again. why wont the tach allow me to start the car but runs just fine when attached after the vehicle is on

[Zs-ondabrain]

That is confusing. Almost sounds like it's back feeding somehow.

I'd try a Diode on that wire. Try is both ways, once with the band on the diode facing the coil and aagain with the band facing the green wire. Couldn't hurt to try.

Just remember, it's a friday night and the guys are out till tomorrow morning. Maybe you'll get a better, more precise answer then.

Dave

[motorman7]

The two tach wires are part of the start/ignition circuit. I removed my broken tach once to get it serviced and the car would not start, it just turns over. I had to jump the two wires in order to get it to start without the tach installed.

I am thinking of tapping into this circuit with a hidden switch in the future to use as a hidden circuit break to make it harder for theives to steal the car.

[alhbln]

This might be a wiring problem, as the ignition does run with the tacho connected, but not while starting.

I've attached a wiring scheme of the ignition circuit and added the Autometer tach, if your wiring is different you might want to try out the one in the attachment.