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Timing light on COP effected by dwell?

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When measuring timing on a COP system, I usually use an extesion HT cable but I couldn't find one a pinch today so I attached the clip to the wires going to the 1st coil instead.

After getting home I started thinking, coils have a dwell time where they need to charge up prior to firing so the ECU will send and maintain a signal telling the coils to charge up and the coils will only fire once the signal from the ECU is off. I am not sure how the pickup on a traditional timing gun works but my guess is, once it senses voltage, it will flash; so the light is only telling me when the coil is receiving a charge signal rather than when the coils are firing?

I was reading the signal on cranking so let say it was at 300 rpm, then it should take approximately 0.00056 seconds or 0.56 ms (1/(300 x 360)) for the crank to rotate one degree. Given a typical dwell time of 3ms, my reading would be 5 degrees off? At higher rpm the error would even be way larger if my assumption above is correct!

I just wanted to verify whether attaching the timing clip to the wires going to the COP would work, or the dwell time would throw everything off. Thank you!

Wait does it work? I expect the wire gauge is too small for the inductive clamp to detect anything.

If it does work, I think you're indeed measuring dwell since the power wire is on the primary side.

The dwell will throw the timing off. If you do a timing test at idle, and adjust the dwell, you will see the timing move around. Flipping the inductive clamp backwards will often show a different reading (depending on the clamp, ive had a bunch do it) and sometimes if you just crack the clamp open you will get a different reading.

Only way to accurately check timing is with a HT lead.

Just did it with a HT lead to reconfirm and the timing seems to be off by 1 degree or so, not much but then, the RPM wasn't that high during cranking to get thrown off by dwell too much.

Still, yes I agree HT lead should always be used if you want a accurate/consistant reading.


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