A Timing Light is a flashing light used to adjust the ignition timing advance on an automobile by means of markings on the crank pulley and on (or near) the engine block. It accomplishes this by flashing a light every time the number 1 spark plug fires. There are two kinds of Timing Light; inline, and inductive. Inline timing lights are connected by unplugging the #1 spark plug wire and plugging a connector inline (hence the name) with that wire. Inductive timing lights have a clip with an inductive sensor built into it, and are simply clipped around the spark plug wire. They will not work with coil on plug ignition, and some of the cheaper and/or older models will not work on distributor-free ignition systems, or some of the lower-power electronically controlled ignition systems.
A basic timing light is little more than a glorified strobe light, typically using a xenon bulb. The major difference is that they can be powered by (typically) 12 volts DC, and that they can be somehow connected to the #1 spark plug wire. They are exceedingly simple devices.
Timing lights used to be metallic, uniformly pistol-shaped and typically finished in chrome, though they were much larger than any handgun. They featured a trigger which could be used to activate them. However, recent trends towards avoiding things which look like a gun have transformed them into sleeker forms. In addition, a metal coating is conductive and an extremely bad idea when you are messing about with your car's electrical system.
Modern timing lights are often microprocessor-controlled and have various additional features. The most feature-laden models will check battery voltage, dwell, and RPMs. In addition, you can set an advance on them to test your car's vaccum advance (or other automatic timing advance system.) They are generally rectangular in shape and generally have a hinge midway through so that they can be pointed at the more awkwardly-located crank pulleys found on vehicles with cramped engine compartments and transversely mounted engines. They also sometimes have a setting so that they can be used on both two and four-stroke engines. The practical difference is that on a four cycle engine each spark plug fires on every other rotation of the engine.
Theory of Operation
Because the timing light flashes once firing of the number one spark plug, which fires slightly before the number one piston reaches top dead center, and the crank pulley rotates once per revolution of the engine (as it is directly connected to the crankshaft) the light will appear to make the timing mark on the pulley stand still. This is usually a notch or painted (white or one of various flourescent colors) mark which will line up with corresponding marks on the engine block. The timing mark will match up with the "0" mark when piston number one is at top dead center.
On vehicles with a distributor, timing is adjusted by loosening two or more bolts which hold it in place, and rotating it. This changes the timing because of the way a distributor works; It contains a rotor which contacts points on the distributor cap which lead to spark plug wires. The rotor is energized by a coil which raises the 12 volts (actually in the range of 13-15 volts) the car normally generates while running into something to the tune of twenty to fifty thousand volts, the better to jump across the gap of the spark plug's electrodes. Rotating the cap changes the times at which the rotor's wiper contacts the points, resulting in a spark.
Operation and Use
Therefore, on most cars, to adjust the ignition timing with a timing light, you perform the following steps, to a car which has not yet been started:
- Disable the automatic timing advance system. On carbureted vehicles this is generally a vacuum-based system, known as a vacuum advance. Cars with fuel injection, especially those which use a MAF (Mass Air Flow) or MAP (Mass Air Pressure) sensor rather than relying on speed density are more likely to have an electronic timing advance, but may still use vacuum in some cases. If the system is vacuum-based, plug any lines you remove.
- Attach the inductive pickup clip to the first spark plug wire, or connect the inline spark plug adapter.
- Connect the dwell sensor clip to the negative side of the ignition coil.
- Connect the red (positive) clip to the positive battery terminal.
- Connect the black (negative) ground clip to the negative battery terminal.
- Start the car and allow it to run until it reaches normal operating temperature.
- Point the timing light at the top of the crank pulley in order to illuminate the timing marks.
- If the timing requires adjustment, loosen the bolts preventing the distributor from turning, and rotate as necessary to line the timing mark on the crank pulley up with the appropriate stationary mark.
- Re-check and adjust as necessary.
While adjusting your timing, observe the following precautions:
- Remove all metal jewelry. If you should cross a substantial source of current it will be welded to whatever contacts you short circuit and become red-hot in seconds. Anything you cannot remove should be covered with an insulative material such as leather or rubber.
- Remove loose clothing and jewelry so that it does not hang down and become entangled in the belts, pulleys, and fans of your car. If this should happen to a necktie, for example, it could seriously reduce your life expectancy.
- Make sure that none of the leads of the timing light (or anything else) come into contact with aforementioned moving parts, or with any hot surfaces. Leads which are connected to power, upon contact with a hot surface such as an exhaust manifold, will lose their insulator and be grounded to the engine (which is connected to ground, generally the negative battery terminal) with disastrous consequences.
- Don't forget to keep your hands (or other body parts) away from moving parts and hot surfaces, as well.
- Finally, be warned that if your spark plug wires have cracked boots they should be replaced immediately or they may shock you. Approximately thirty thousand volts will find its way through your hand, potentially to the place you have your other hand, which had better not be on the frame of the car, which is grounded, because this could kill you. Even seemingly intact boots can have cracks large enough to let you be shocked. Be smart, wear electrically insulative gloves.