912ULS tacho and mag drop puzzles

I always love a challenge.

Assuming it worked at one time... Try twisting the adjuster a few times.

It might just be sitting on a bad spot and needs a little exercise after all the years.

There is nothing to lose.

Yes as you say, Bill.

However I keep thinking about the fact that it's exactly x2, not just any old error, and suspect this must be linked to how the double pulse - shown in the manual - is filtered/blocked/rectified. Opening the case to look inside for a diode problem is not going to be easy with risk of destroying it. First I want to try reversing the wires, and then an external diode in the line.

Before you do anything else; Confirm that the adjust on the back of the case is a variable resistor and not a multi-position rotary switch.

Bill, have a look at sections 3.7 & 3.8 of the instructions you posted. It's a calibration trim if the accuracy drifts.

Mike

Yeah, but the knob twister in me just wants to tweak that pot.  

The perfect 2:1 error is the clue.

I am not sure what you are referring to when you say "Double Pulse".

There will always be a NEG pulse following the POS pulse from any trigger coil.

Analog Tachometers do not count pulses; they compare the timing of the pulses to an internal Reference Pulse and filter the difference of the Pulse widths to produce a voltage that drives the needle.

Are we certain it connects to the Wht/Yel and Blu/Yel wires from the engine?