A timing light is a

stroboscope A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces br ...

used to dynamically set the

ignition timing In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke. The need for ...

of an

Otto cycle An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what happ ...

or similar

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...

equipped with a

distributor A distributor is an enclosed rotating switch used in spark-ignition internal combustion engines that have mechanically timed ignition. The distributor's main function is to route high voltage current from the ignition coil to the spark plugs ...

. Modern electronically controlled passenger vehicle engines require use of a scan tool to display ignition timing. The timing light is connected to the ignition circuit and used to illuminate the

timing mark A timing mark is an indicator used for setting the timing of the ignition system of an engine, typically found on the crankshaft pulley (as pictured) or the flywheel, being the largest radius rotating at crankshaft speed and therefore the place w ...

s on the engine's

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

pulley or

flywheel A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...

, with the engine running. The apparent position of the marks, frozen by the

stroboscopic effect The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples (as opposed to a continuous view) at a sampling rate clos ...

, indicates the current timing of the spark in relation to piston position. A reference pointer is attached to the flywheel housing or other fixed point, and an engraved scale gives the offset between the spark time and the top dead centre position of the piston in the cylinder. The distributor can be rotated slightly until the reference pointer aligns with the specified point on the timing scale. Fuel-injected engines, or engines with microprocessor controls may require special procedures to allow basic spark timing to be observed without control effects from the engine computer. On most automotive engines, the timing is set based on the #1 cylinder.Mark Schnubel ''Shop manual for advanced engine performance'', Cengage Learning, 2005 , page 52-53 In few cases an engine is timed off another cylinder, such as the

International Harvester The International Harvester Company (often abbreviated by IHC, IH, or simply International ( colloq.)) was an American manufacturer of agricultural and construction equipment, automobiles, commercial trucks, lawn and garden products, household e ...

V8 engines, which use #8, and the Isuzu 4Z series four-cylinder, which is timed off the #4 cylinder, or the 3 cylinder

Saab two-stroke The Saab two-stroke was a two-stroke cycle, inline, two cylinder, and later three cylinder engine manufactured by Swedish automotive manufacturer Saab that was based on a design by German manufacturer DKW. Two-cylinder The first version was a ...

engine which is timed on the middle (#2) cylinder. Simple timing lights may just contain a

neon lamp A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and Penning mixture, other gases at a low pressure and two electrodes (an anode and a cold ...

operated by the energy provided by the ignition circuit. Timing lights using xenon strobe lamps electronically triggered by the spark provide brighter light, allowing use of the timing lamp under normal shop lighting or daylight conditions. A timing light may be a self-contained instrument, but is sometimes combined with a

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. Ana ...

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

meter, and a

dwell angle A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch ...

meter, or may be incorporated into a more comprehensive instrument such as an

engine analyser An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...

. Self-contained units used to time automotive engines have an inductive pickup that clamps around the proper

spark plug A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...

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 centre conductor of the spark plug.

References

See also