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Finding acurate dwell time

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Hi everybody! I'm dealing with dwell time. I have measured with an osciloscope the charge time in an old timer ignition coil from a honda xrv 750 from 1992. Well i guess is not the way as i have blown them. Is it there somo way to measure the dwell time without risk of burning them and also with out any way of measuring torque. Not dyno around...


You can do the dwell time numbers for a coil on a running engine, but it isn't healthy for the engine or coils if you get the numbers wrong.

When I am calibration coils I have a bench test rig the uses a spark plug with a separate ground point for the spark to jump and external power supply, oscilloscope using a probe on the ignition trigger signal from the ECU, and a Current Clamp on the Voltage supply to the coil.

I start with a low dwell time (1ms for COP, 2ms for external coils) and supply voltage (8v). This protects the coils from being over dwelled, I then increase the dwell time in the ECU (with the ECU running in test mode to simulate a running engine) watching the current being drawn by the coil, once this starts to fall over or has an obvious knee in the curve, I stop increasing the dwell and bring it back down to about 80% of the time before the knee happens. I then work through a range of voltage to get the dwell curve for the coil.

Hi all,

In this instance, dwell time is not the correct term. What you are looking for is saturation time. The coil stores energy in the form of inductance. You can think of this as a bank of electrical potential energy. The measurement you should be looking at is coil current. If possible, hook up an oscilloscope with an inductive probe (wraps around the wire). Start with a small dwell time as Stephen said and what you should see is a "saw tooth" wave which indicates the current draw increasing as the physical coil saturates. Once the coil saturates fully, the current will no longer increase and will result in a trapezoid shaped wave that will maintain current until the coil burns out.

At this point, you should increase the dwell time slowly (micro-seconds at a time) until you reach this saturation point. The coil won't burn out with slight dwell over saturation, so measure this point and keep your max dwell around 90% of this saturation time.

If you have any questions feel free to ask!

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