I put this here because it's not about TMCC.
Two times now out of the blue I've had the above start acting erratically after several years of trouble free operation. What I have found out is that the scotch tape that is recommended be put over top the tach tape deteriorates and causes the pick up not to see every black line. An easy check is to turn the flywheel slowly by hand and watch for the red led to blink at all black spaces. Both of my issues were due to the red led not blinking all the way around the flywheel. A new tape without scotch tape solved both issues.
Romiller