Having replaced my tranny (and engine) in my SD, I can say with some certainty that there isn't any electrical tach signal from the transmission, though that does seem awfully coincidental in the timing.

What I did to get mine working again was to clean the face of the sensor down at the 2 o'clock position of the harmonic balancer, and then took a very small screw driver to the back side of the socket that the tach amplifier plugs into, using it to tighten each of the sockets that the pins on the tach amp touch.

Once I had done this, I've had no issues with the tach other than a couple brief jiggles of the needle.

I suspect that the cleaning of grease and grime off of the front of the sensor did almost as much good as the contacts being tightened, as dirty grease plays havoc with the magnetic sensors on the airplanes that I work on occasionally.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...