Dead on side of the road ! ! ! Help !

I believe he is saying the tech signal wire (many times it is green) is grounding somewhere between the coil negative and the Tach itself. Depending on Tach design, it could also be an internal short in the Tach that is allowing it to ground the tach signal circuit.
The tach should be grounded. Typical install:
Red - Key on power
Black - ground
White - dash light dimmer circuit. (To dim the light inside the tach if it is lit internally)
Green - Tach signal from either ignition coil (-) or from your spark control device tach output port.
Like Rusted said above, check the tach signal wire from the end that connects to the coil (-) all the way to the Tach itself. My guess is the same as his, you have a pinched wire or bare wire somewhere in there that is grounding out.
If you inspect and do not find a short, time for a new Tach.