VDO Tach to Edis Ignition Wiring

[Speedster]

My ACVW 4-cylinder Megasquirt-II with Edis ignition has been working for years now, but I never figured out how to wire up my VDO tach that was fired from the the negative side of the old ignition coil. The Edis Ignition Control page says:


"For your dash tachometer, you have a few choices, :

You may be able to use the IDM (Ignition Diagnostic Monitor) output to drive the tach. IDM a signals that the primary side of the ignition coil has fired. Simply wire the IDM output (pin 2 on all the EDIS modules) to the tach input, OR

If your tachometer was previously driven by a single coil, and the IDM signal doesn't work, your tachometer will need input from two coils to indicate properly. The trick is that you need to tie all the coils together, without allowing them to ground each other. Below is a simple circuit (with Digi-Key part numbers) showing how to interface the EDIS system with your tach:"

I would like to make the circuit shown, but cannot locate any connection points on the Edis coils that could be the negative sides. Anyone ever do this successfully?

Chuck

[trakkies]

It's usually easier to connect at the EDIS unit - if it is closer to the tach. Connect the diodes to pins 10&12 as per the schematic - those are the negatives of the EDIS coils.

[Speedster]

Dave, thanks for the tip on wiring from the Edis controller. I will use the suggested circuit and wire into Edis.

Chuck