A timing light is a device used to set an automobile's ignition timing. The hand-held device attaches to the vehicle's battery and to the engine's No. 1 spark plug wire. With the engine running, the timing light has a strobe light that lights every time the No. 1 spark plug fires. The light is aimed at the engine's harmonic balancer, which has a series of numbers engraved into it. The numbers represent the amount of degrees before and after the No. 1 cylinder's top dead center position. A chalk mark is placed in the appropriate spot and is illuminated when the light flashes, allowing the timing to be advanced or retarded until the desired reading is obtained.On distributor-equipped engines, there is often a vacuum advance module on the distributor that must be disconnected prior to using a timing light. The timing must be set with the advance disabled, though the vacuum hose must be plugged to prevent a false reading from the timing light. It is important for people to take the reading with the timing light while the engine is operating at the proper speed. The vehicle's manual will contain information pertaining to the correct engine revolutions per minute (RPM) to set the engine timing at.On some performance timing light housings, there is an adjustable advance knob feature. This advance feature allows the timing light to be set at the desired timing degree based on engine RPM. This feature is used when setting a racing engine's timing; it is able to represent the amount of timing that is set in the distributor for any given engine speed. This feature also makes it possible to determine if a distributor is working and is equipped with the correct weights by flashing and signaling the firing of the spark plug and matching the flash to the timing marks on the balancer.
The typical timing light design takes the shape of a pistol. This shape makes it easy to point at the desired area within the engine bay in order to illuminate the timing marks on the bottom of the engine. Some non-traditional light designs are simply straight housings with the wires coming out of the end of the light. These lights are often difficult to use in an automobile engine compartment; however, they are useful when being used on an exposed engine, such as a tractor. While some of the older lights use a wire spring that is actually placed inside the spark plug wire terminal, most modern timing light designs use an inductive pickup that is merely clipped onto the spark plug wire to take the reading.