A timing light is a stroboscope used to dynamically set the ignition timing of an Otto cycle or similar internal combustion engine.
The timing light is connected to the ignition circuit and used to illuminate the timing marks with the engine running. The apparent position of the marks, frozen by the stroboscopic light, indicates the current timing of the spark. On multi-cylinder engines, the timing light is referenced to the number one cylinder.
A timing light may be a self-contained instrument, often combined with a Voltmeter, RPM meter and a dwell angle meter, or incorporated into a more comprehensive instrument such as an engine analyser. Self-contained units used to time automotive engines have an inductive pickup that clamps around the proper spark plug wire and serves as the trigger for the strobe. Power for the strobe comes directly from the vehicle's battery. Some older timing lights require the removal of the spark plug boot in order to attach a direct pickup between the wire's terminal and the center conductor of the spark plug.