How to Use a Timing Light (with Video)

by eHow ContributorUpdated June 06, 2023

Modern cars have electronic, distributorless ignitions that are completely governed by the car's computer. Older cars, on the other hand, have ignition systems with distributors that need to be manually adjusted from time to time. A timing light is a device that is used to determine an engine's ignition timing, or the timing of the spark plugs' sparks, relative to the position of the pistons during their power strokes.

What you'll need

  • Timing Light
  • Tool, such as a combination wrench, to loosen Distributor
  • Duct Tape

1. Turn the engine off

Turn the engine off and remove the key from the ignition. Make sure the battery terminals are clean enough to allow for a good connection.

2. Clamp the red clip

Clamp the red clip on the timing light to the positive terminal of your car battery.

3. Clamp the black clip

Clamp the black clip to the negative terminal of your car battery.

4. Clamp the largest clip

Clamp the largest clip (the one with the thickest insulation) around the No. 1 spark plug wire.

5. Locate the timing marks

Locate the timing marks near the lower crankshaft pulley, and the mark on the pulley itself. Rotate the pulley, if necessary, so that you can see the mark.

6. Look to the auto's spec or emissions placard sheet

Your auto's spec or emissions placard sheet should tell you what your timing degree should be. For example, a 1969 Ford 429, it's 6 Degrees BTDC @ 550 rpm in drive gear - with automatic trans. All this means is that you want to line up the pointer on the timing light with the 6° timing mark when the engine is idling in gear at 550 rpm. As you can see, it also says something about "BTDC" for this Ford. There is also an "ATDC." "Before Top Dead Center" and "After Top Dead Center" - top dead center is the point where the piston reaches the highest point in the cylinder and that is when the compression is the greatest. Now, your timing marks will have a line labeled "0" with marks above and below it, and depending on which way the wheel rotates, you will see the lines before the "0" (BTDC), or after the "0" (ATDC).

7. Use some chalk to mark the timing marks

If you want, you can use some chalk to mark the timing marks to make them more visible. Then, on the side of the distributor, disconnect the rubber hose from the vacuum advance and put a piece of duct tape on the end of the hose to seal it off.

8. Start the engine

Start up your engine and let it warm up.

9. Aim the timing light at the timing marks

Aim the timing light at the timing marks, and press the button on the light. The light will strobe. Because of this, the timing marks will appear to be standing still. If the pointer is pointing at the correct mark, you do not need to adjust your timing. If it is not, turn the engine off and proceed to the next step to adjust your timing.

10. Loosen the distributor

Loosen the distributor hold-down clamp by turning its bolt counterclockwise. Only loosen it enough so that you can turn the distributor body with some resistance, not enough so that it's loose. Start the engine and adjust the idle to the specified rpm.

11. Rotate the distributor slightly

Rotate the distributor slightly, then aim your timing light back at the timing marks again. If it is farther away from the marks, then move the distributor in the opposite direction. Then, check again with your timing light. Continue doing this until the pointer is pointing at the correct degree point as required.

12. Re-tighten the distributor

Once the timing is correct, re-tighten the distributor hold-down clamp bolt, making sure you do not move the distributor. After re-tightening this fastener, check your timing again to make sure -- tightening the distributor may change your timing.

13. Turn off the engine

Turn off your engine, replace the vacuum advance hose, and disconnect the timing light.

Tips and Warnings

Use some type of fender cover so you do not scratch the paint. Do not forget to remove all your tools from the engine when you are finished.

Be careful to keep the timing light's wires away from moving parts in the engine bay.

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