Odd tachometer electrical issue

[aero four]

My recently acquired 83 944's tach doesn't work, and I recently found why, but still can't fix it.

Background: There are two green wires that attach to the first pole of the nine-pin connector in the engine bay right above the brake master cylinder. One of these comes from the tachometer itself and one comes from the DME, but I am not sure which one - the only visible difference is one is larger gauge. On the other side of the connector, a green wire goes to the ignition coil.

The smaller gauge wire was apparently ripped out of the nine-pole connector - I actually suspect it was cut. Normally the car runs happily, starts right up but has no tachometer signal (besides the mpg gauge). If I jump this wire with another small gauge wire, the car will turn on for 3 seconds, the tach will bump up to about idle speed, and then the motor will retch and die.

Does anyone know why this would be? My jumper wire is not a terrific one - could simply grounding out do this? Or might it be a ground somewhere else? Any help is appreciate - I'd love to have a working tach.