Did continuity testing of the 3 wires to the tach reader go all the way from the board's 7-pin power connector to the contacts on the tach reader board?
Did you additionally confirm no shorts between adjacent wires? In other words you tested continuity from A-A, B-B, C-C. But did you also confirm NO shorts between A-B, B-C, C-A.
I don't have that test fixture or service notes that MTH apparently provides to Authorized Techs but what I'd do next is measure the voltages at both ends. IIRC, one contact is a fixed reference voltage. Then measure this relative to the LED transmit voltage which should be stable at just over 1V DC. Then measure the sensor receive DC voltage which should wiggle up and down by 1/2V (or so) as you manually rotate the flywheel.
