CC: Two-wire ignition coils such as this Mazda coil, use an externally mounted module, or power stage, for controlling the primary current.
To scope this signal, you can tap into the circuit, but make sure you use an attenuator, so the inductive kick does not damage the scope. You can also use a current clamp on one of the wires to see the switching of the driver circuit and whether the primary current is reaching saturation. On two-wire systems, chances are the ignition module is measuring dwell and burn time through the power and ground in the module.
If you have a three-wire coil such as this Honda coil, you will have power, ground and, depending on the manufacturer, a third wire called the command signal. In this type of coil setup, the power stage is inside the coil.
If you measure the power and ground, you will see 12 volts with the key on. With the engine running, you will see small voltage drops as the coil fires. If you use a current clamp, you will see the switching of the coil with greater definition.
The signal wire will typically have a voltage of 5- to 7-volts that will switch on and off, commanding the internal module to open and close the primary circuit.
If you are using a two-channel scope, graph the current and signal wire.
If you have a coil with four wires like this Audi coil, use a wiring diagram to figure out what the manufacturer is doing with the fourth wire. The majority of the time, it is a ground. But some ignitions have an inductive coil to measure coil performance by the engine control module.
Always check the wiring diagram to confirm the connections to the coil. Also, with the wiring diagram you can determine the module with the ignition coil driver. In some cases, you might find that all the coils are fed ignition voltage with the same power source. Using a current probe on that wire will allow you to view the current drawn on each coil from one connection. This will allow you to see “known good” signals as well as the faulty one you are trying to diagnose. As a bonus, you can find the ground connection or connections for the coil. This can be a big help during a no spark diagnosis.
This video is sponsored by Standard Motor Products.
