Coil Negative triggering for rusEFI

[Simon@FutureProof]

Been asked to deal with a couple of tricky classic car installs that need to keep the stock ignition systems and use the rusEFI unit in a fuel only mode triggered off the negative side of the ignition coil the way the old megasquirt units used to do it.

Current plan is to make a small module up replicating the optoisolator circuit that MS used to use.

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Will probably find a small box to put them into and pot them in resin as they will have to live in the engine bay but will post up here with whatever design I work out for this as I am sure there are going to be some others with a similar need.

[MHTSOS]

Why not use the BMW sensor that goes around the spark plug wire?

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[AndreyB]

Do you happen to have the part number?

[Simon@FutureProof]

It was used on the M30 and M20, I did consider this but for whatever reason the customer does not want to add to the car, this is a bit of a stealth install.

http://bmwfans.info/parts-catalog/E30-2-doors/Europe/325i-M20/browse/engine_electrical_system/ignition_wiring/#pos_11

That is the part he is referring to, I think it is basically a coil of wire like a current clamp. Unsophisticated but capable of delivering a pulse when cylinder 1 is fired.

I did also think about using a cheap current clamp / inductive current sensor but was not sure how well it would pick up the pulse.
They are cheap enough that I might pick some up just to play with.