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On a COP system, usually I would attach an extra section of HT leads between the ignition coil and spark plug to hook up the inductive clamp of a timing gun to measure the base ignition timing.
However on certain vehicles such as a boxer engine this could sometimes be extremely difficult. I am thinking of splicing off the ignition signal for cylinder one from the ECU harness and attaching the inductive clamp to it to get a rough idea of the base ignition timing. I know the measured timing will be slightly off due to ignition dwell time and the timing light would flash based on a rising edge rather than a falling edge signal but it is better than nothing. At low RPM the difference is not that significant anyway.
How should this be wired after splicing off the cyl 1 ignition signal? My guess would be to ground it through a resistor. But what resistance should I use so I won't draw too much current from the ECU output or weaken the signal going to the coil?