Waste Spark Ignition Systems

A diagram that explains several electrical problems associated with a waste spark ignition system.

This is a diagram of a waste spark ignition system. Each coil fires two cylinders at the same time. Only one cylinder has an air fuel charge the other just inert gas. That’s why it’s called a waste spark system.

The ignition module in the illustration relies on inputs from several sensors including the crankshaft position sensor and the camshaft position sensor to make decisions. Automotive computer modules control actuators like the coils in the illustration based on inputs from these sensors compared to preset tables. A problem with a sensor can easily result in a no-start condition.

An open at point A would result a no-start condition. This is a ground for both coils. The drivers in the diagram ground the appropriate coil’s primary circuit for the next cylinder in the firing order. An open at point B would result in no power to the coils primary circuit. This would deprive the number 1-4 coil of power rendering it useless. An open at point C deprives the other coil of power resulting in no-spark at the 2 and 3 cylinders.

Excessive resistance in a sensors circuit will result in a false signal. This is true about resistance in the crankshaft position sensor’s circuit in the illustration. This would most likely result in a no-spark condition. This sensor provides the crankshafts position to the control module. Inspect the circuit; resistance caused by a pinched or damaged wire or a spaded connector would corrupt the sensor’s signal.