Wilbo,
in my experience just a 12v square wave won't always work.
I've tested both my stock MA61 analogue tachos by hitting them with a 12v square wave of varying duty cycle. One of them worked, the other didn't.
I stuck a CRO on the -ve lead of the coil and saw back emf spikes as large as 400v.
So we are stuck with either constucting a booster circuit or modifying the input circuitry of the existing tacho.
Maybe Witzl's mod is the way to go - I'll send him an email.
Mon, 05 September 2005 05:06
