Timing lights are available in several different types. The very low-end timing lights are powered by the ignition spark itself. These lights have two wires. One has a clip that clamps to the battery ground, and the other inserts between the distributor cap and the spark plug wire. This type uses a neon bulb, which is not very bright and must be used in a dimly lit area. It's not possible to use this light with many of today's cars because of direct ignition systems.
The DC-powered timing light amplifies the 12-volt power from the vehicle battery to power a Xenon strobe bulb. This type has a very bright bulb, making it usable even in sunlight.
A third type of timing light is the AC-powered light. This light is very similar to the DC-powered light with the only difference being the power source. AC-powered lights use 110-volt household power and must be plugged in to work.
The better timing lights have three wires. Two of these wires connect to the vehicle battery, and the third has a spring-loaded clamp that clamps around the spark plug wire. This clamp picks up a weak inductive electrical current every time the spark plug fires. This weak current is the trigger that causes the circuitry in the timing light to discharge a higher current to the strobe light bulb. This circuitry boosts the 12-volt current to many times that because in modern timing lights the strobe light bulb is a xenon bulb that requires 1,000 or more volts to illuminate. These bulbs are very fast, requiring milliseconds to illuminate fully with their bright white light.
Timing lights work by illuminating the timing marks on the harmonic balancer on the front of a vehicle's engine. The timing marks are calibrated to be representative of the position of the piston in the first cylinder of the engine. The 0-degree mark indicates that the piston is at top dead center of its stroke. Connecting the timing light's inductive pickup to the spark plug wire for cylinder one allows the timing light to flash on the timing marks and the relationship between the piston position and the spark to be determined. Adjusting the position of the engine's distributor changes the ignition timing.
