Coil-trigger timing light

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Coil-trigger timing light

Postby elaw » Sat Sep 16, 2017 5:22 pm

Okay technically this isn't a "Megasquirt success story" but it's Megasquirt-related and I thought it might be of some interest to people here.

Like others I've seen post here, I've had trouble getting a timing light to trigger with coil-per-plug and coil-on-plug setups. My personal theory is that those types of coils produce shorter HV pulses because they're smaller and have less inductance than traditional coils, but that and $1.00 will get you a cup of coffee - at some places. :lol:

Anyhow I recently had a brainstorm about a solution to the problem. I picked up a beat-up old inductive-pickup timing light at a swap meet for 5 bucks, and removed its external wiring along with the pickup. Inside the timing light I added a small transistor inverter circuit, and a cable going to some connectors that would allow it to hooked in-line with the coil.

The result: a timing light that works beautifully! It triggers absolutely reliably and stably, and made checking the basic timing on my engine (and finding it was about a degree and a half off) a breeze. Since the hookup to the coil provides both power and trigger, connecting it is easy too.

For anyone interested, I've included a PDF of the schematic.

This particular light was Sears brand but the PC board inside was labled "Actron". I suspect many other basic lights are similar. Externally the light had two wires going to the pickup and power and ground leads going to clips meant to be attached to the car's battery. All 4 leads went to a circuit board inside the light (the +12 via a trigger switch), but on the circuit board the ground wire and one of the pickup leads were connected together. I suspect any other light that is wired similarly would work.

On the coil side, I'd expect this to work with pretty much any logic-level-triggered coil where the trigger signal goes high to energize the coil, and low to fire the spark. DO NOT try to use this circuit with conventional coils that don't have a built-in igniter... it won't work and you'll fry the transistor in the inverter circuit.

One last note: as some may recognize from the photo, I use VAG COP coils on my car. I was not able to find anyone selling a connector the same as that on the coil itself, so to make my "inline" setup I ended up having to buy a coil and cut it up! That's why the male connector looks the way it does.
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Coil trigger timing light.pdf
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Eric Law
1990 Audi 80 quattro with AAN turbo engine: happily running on MS3+MS3X
2012 Audi A4 quattro, desperately in need of tweaking

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Re: Coil-trigger timing light

Postby Raymond_B » Sat Sep 16, 2017 6:52 pm

Very cool, thank you for sharing!
1995 Ford Lightning. Dart based 427 Windsor, Novi 2000, full sequential, E-85, etc. MS3/3X
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