Secondary Ignition Monitor

Since M3.x, most Systems utilize a shunt resistor to additionally monitor the secondary ignition for misfire.

The resistor(s) is located in the wiring tray on the engine. It is connected in parallel to the ground lead (KL 4a) for the coils. As the secondary magnetic field collapses, a voltage spike is induced in the windings. The control module monitors the voltage drop across the resistor as an indication of coil firing. If a signal is missing, the control module detects the affected cylinder from the camshaft position sensor input.

The control module measures the duration of time it takes the monitored voltage drop for each ignition coil to dissipate below two volts. The time scale constantly changes based on engine speed.
- If there is no feedback signal present (0 volts) there is no spark present,
- If the 2 volt signal is not maintained long enough, the control module detects this as a week spark.