Automotive Weekly Waveform 11 - How Smart Ignition coils work and scope captures

Automotive weekly waveform #11 will guide you through the process of scoping smart ignition coils. Smart ignition coil typically have the transistor as well as other electronics built into the coil.

WARNING!!! We are working with voltages up to 40,000 volts. Use caution for your safety and the safety of your scope. NEVER connect scope directly to the secondary ignition cables or the coil output.

For this test we will need a COP wand, a HT extension lead or a coin to get a secondary waveform. If using a coil simply put your scope into a 1 or 2 volt scale and touch your scope lead to a coin placed on top of the ignition coil. Some coils are heavily shielded and will require the coil to be removed and a HT extension lead placed between the coil and the spark plug.

For scoping the logic level signal from the PCM to the coil we just need a standard scope lead. This signal is normally just a square wave between 0-5 volts or 0-12 volts. the IGT signal was 0-4 volts on this 2007 Toyota Highlander Hybrid.

You cannot scope the Primary voltage but you can scope the primary amperage by placing a low amp clamp around the power feed to the coils.

My toyota also had a feedback signal to the computer known as IGF. This wire is interconnected between all coils and get pulled to ground when each coil wants to let the PCM know that it received the IGT signal and is charging up.


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