Here is an email reply from them with some juicy technical details. I can’t wait to open the tach to see what I discover:
Hi Kevin
What you can do is at least take the tach apart to determine what is wrong. The heart of the mechanism the meter coil hoop attached to part that moves with the pointer. the meter coil gets it connection through the clock springs front and back of the movement. You can measure the resistance of the meter coil with a multimeter on ohms at the two tabs where the internal wires attach to the clock spring adjusters. You should see something around 160 ohms give or take about 20 ohms. (it varies from tach to tach) If you are getting a resistance reading of something much lower or higher than that target of 160 ohms the meter coil is likely defective and you will have to send me the tach to have the internals replaced from another tach.
the only other parts of the inside of the tach between the meter coil and the terminals on the back is just some old fashioned resistors and diodes and wires joining the them from the front to the back. the resistors and diodes can be bypassed to test the tach as they are not really necessary with the new sending unit. run new wires front to back keeping the path the same orientation as polarity is important for which wire gets power (yellow) and which terminal is ground (black).
If that doesn't work than tach internals have to be replaced. which I would have to do, If you want to do that please send me the sending unit too so I can calibrate it as a complete system.
Brian Ball
4126 Henderson Rd.
Harrowsmith, ON
K0H 1V0 613-532-2587