Ignition coil diagnostics

© gazeo.comCorrect voltage trace in an ignition coil's primary winding. When the coil is charged to approx. 200 V and the electrical circuit of the primary winding is interrupted, voltage of up to 45 kV is induced in the secondary winding, causing a spark on the spark plug. Voltage in the primary winding drops to ca. 20 V. The sequence (lasting 3,5 ms) ends in voltage vibration and the coil is charged again

Winding resistance
Basic coil diagnostics consists of its primary and secondary winding resistance control. Primary winding resistance may range from below 1 Ohm (in newer cars) up to a couple of Ohms (older type coils). Secondary winding resistance is much higher and should range from 8 to 19 kilo Ohms. These are merely approximates, but if the actual values are remarkably different, there might be a short-circuit between the windings. And such short-circuits are the most common causes for ignition systems to periodically malfunction. The ultimate symptom is irregular ignition, particularly visible when the engine works under load. In general, symptoms occur when the coil has warmed up – as long as the engine is cool, they may not reveal themselves.

Oscilloscope measurements
An oscilloscope can "tell” much more in terms of coil diagnostics. Such a tool is often integrated into other devices used in repair shops, like testers and diagnoscopes. An oscilloscope allows the mechanic to visualise voltage traces in the coil's primary and secondary winding, thus making it possible to diagnose those defects which cannot be detected by resistance measurement.

Primary winding
The crucial thing as far as voltage trace in the primary winding is concerned is voltage vibration once the spark plug discharge is complete. Lack of vibration suggests a short-circuit between the coil's windings, which in turn may cause engine roughness.

© gazeo.comA special probe is used for measuring voltage traces in individual coils

Secondary winding
Checking the secondary winding's high voltage trace is particularly helpful in ignition coil diagnostics. It can be done with a special probe (capacitive or inductive), which is placed on an ignition cable. Measuring secondary voltage in individual coils (placed directly on spark plugs) may be troublesome, though. A probe put against a coil's housing could be of great help for doing that. Anyhow, a probe makes it possible to visualise voltage trace on the secondary side. Analysis of the trace allows to diagnose a gap or an inter-winding short-circuit. Measuring voltage trace on the secondary side (ignition voltage) allows to determine its value on the engine's particular cylinders.

Summary
Due to the physical and chemical characteristics of LPG and CNG, all defects of the ignition system's elements in an engine powered by either gaseous fuel become visible earlier. This is why autogas system installers often have to deal with malfunctions of the ignition system. An oscilloscope definitely comes in handy, as it visualises crucial parameters and helps assess their correctness, thus making it possible to locate the source of defect, which would be impossible to detect without it.