To learn how to use a timing light, clamp its pickup on the #1 spark plug wire, power the light, and aim it at the timing marks while the engine runs. The light flashes each time the spark plug fires, so the marks look still when the timing sits at the reading you are checking.
Before you learn how to use a timing light, check the timing spec in your vehicle service manual. On many modern engines the engine computer controls timing, so the light is mainly a checking tool.
This is general information. The ignition system carries very high voltage, so follow the vehicle service manual and the light maker’s instructions for your model.
What a Timing Light Does and When to Use One
Knowing how to use a timing light starts with understanding that it flashes at the moment the spark plug fires, so the timing marks look still when ignition timing matches the reading you are checking. Ignition timing is the point, measured in degrees before top dead center, where the spark plug fires relative to piston position. Ignition timing problems can cause misfires, as misfire diagnosis explains.
An inductive timing light uses a pickup clamp that sits over a spark plug wire and senses the spark, so you do not need to cut into the ignition wiring. Haltech’s support article explains that the light flashes when the marks line up, which means the engine is firing at that exact degree.
Learning how to use a timing light for checks is simple: compare the reading with the spec in the service manual, or set timing on an engine with an adjustable distributor. A timing light cannot diagnose fuel, compression or sensor faults on its own.
What You Need Before You Connect It
Before you learn how to use a timing light, gather the timing light, the vehicle service manual for the timing spec and procedure, and a way to read engine speed.
- A timing light with an inductive pickup clamp and red and black battery clips.
- The vehicle service manual, which lists the timing spec and the preparation steps.
- A tachometer, or the RPM mode on a digital light, to read idle speed.
- Insulated pliers, safety goggles and gloves.
- A safe work area with the transmission in park or neutral and the parking brake set.
The OTC timing light manual tells you to follow the preparation list for checking timing in the vehicle service manual. It also says to warm the engine to operating temperature before you read the marks.
Check First Whether Your Engine Timing Is Adjustable
When the engine computer controls ignition timing, there is no timing adjustment, and a timing light only verifies the timing. Before you learn how to use a timing light on it, look for an adjustment procedure in the service manual.
On outboards such as the Mercury 25 hp four-stroke, the service manual says the engine control module controls ignition timing electronically, so the light only confirms that timing stays inside the timing windows.
A car is not an outboard motor, so open the service manual for your engine and look for a timing spec and an adjustment procedure. If the manual gives no adjustment procedure, do not force a change, and use the manual to check for fault codes and sensor problems instead.
How to Use a Timing Light Step by Step
Turn the engine off, clamp the inductive pickup on the #1 spark plug wire, then connect the red clip to positive and the black clip to ground before you start the engine. Connecting cables with the engine running can cause serious injury, so do the hookup with the vehicle off.
The OTC timing light manual gives this order for how to use a timing light, which you should match to your own model’s guide.
- Turn the vehicle off and set the parking brake, with the car in park or neutral.
- At the distributor, disconnect the vacuum line and plug it.
- Clamp the inductive pickup on the number one spark plug wire, and close the jaws slowly.
- Keep the pickup away from the hot exhaust manifold and other hot parts.
- Connect the red clip to the positive battery post and the black clip to engine ground on a negative ground system.
- On a digital light, press the mode button to select 2-cycle mode on 2-cycle engines and on waste-spark distributorless ignition systems.
- Start the engine and warm it to operating temperature before you read the timing.
Coil-on-plug engines
If you are unsure how to use a timing light on a coil-on-plug engine, note that some of these engines have no plug wire to clamp. Haltech’s support article lists other spots, such as the ignition output between the engine computer and the ignition module, or the top of a coil. Check your service manual for the exact location before you clip on.
If the display flashes and does not hold a steady reading, the light is not getting a signal. The OTC manual says this means the engine is not running or the number one plug is not firing.
How to Read the Timing Marks
Once you know how to use a timing light, aim it at the timing marks on the crankshaft damper or the transmission bell housing, then read the degree where the moving mark lines up with the stationary mark.
Haltech’s support article says the light flashes when the marks align, which means the engine is firing at exactly that degree. It also warns that aiming at the crankshaft alone cannot show whether the engine is on the compression or exhaust stroke. Aiming at the camshaft can show both the ignition angle and the firing stroke.
When you know how to use a timing light, run the engine at the speed in the service manual, press and hold the button on an inductive light, then count the marks on the engine. Write the reading down before you change anything, so you can compare it with the spec.
Reading the degrees on advance and digital lights
An advance light shows the degree on a dial. Turn the advance knob until the moving mark lines up with the stationary 0 degree mark, then read the dial.
Whichever method you use for how to use a timing light, on a digital light press the advance buttons until the moving mark lines up with the 0 degree mark, then read the number on the display.
How to Adjust Ignition Timing Safely
If you have learned how to use a timing light for checks, adjust timing only when the service manual gives a spec and the distributor can be turned. The OTC manual describes a distributor adjustment that uses the light to watch the marks as you move the distributor.
- Hold the light’s button so the marks stay visible.
- Loosen the distributor hold-down bolt.
- Turn the distributor until the moving mark lines up with the correct stationary mark for the spec.
- Tighten the hold-down bolt securely.
Read the marks again after you tighten the bolt. If the mark will not reach the spec, stop and check the service manual rather than forcing the distributor.
Common Mistakes and Safety Warnings
The ignition system can carry 30,000 to 50,000 volts at the coil, distributor cap, ignition wires and spark plugs, so keep your hands clear of the wires and moving parts. If timing checks out, try compression test instructions.
- Connecting the clips with the engine running, which can cause serious injury.
- Working near high-voltage parts without insulated pliers.
- Letting the pickup touch the hot exhaust manifold.
- Using a light with damaged cords, which the manual says not to do.
- Reading timing without a warm engine or without the spec from the service manual.
Safe use starts with the basics: wear safety goggles and gloves, and keep loose clothing and long hair away from belts and fans.
Frequently Asked Questions
Why does my timing light flash without showing a steady mark?
A flashing display means the light is not getting a signal, which is common when you first learn how to use a timing light. Check that the engine is running and that the number one spark plug is firing, and confirm the pickup is clamped on the correct wire. The OTC manual lists these as the causes of a flashing display.
Which spark plug wire should the pickup clamp on?
The OTC manual says to clamp the inductive pickup on the number one spark plug wire. Confirm the wire location in the vehicle service manual.
Can I use a timing light on an outboard motor?
Yes, knowing how to use a timing light on an outboard works the same way once the service manual gives a timing procedure. The Mercury 25 hp four-stroke manual is one example, and it uses the timing light to verify timing within the timing windows because the control module sets the timing.
Does a timing light fix a rough idle?
Not by itself. When you learn how to use a timing light, you see only where spark timing sits against the marks. Use the service manual and the fault codes to find the cause of a rough idle, because timing is one factor among several.
Is a timing light safe for beginners?
It can be, if you follow the hookup steps for how to use a timing light and keep your hands clear of the wires and moving parts. The ignition system carries high voltage, so use insulated pliers and stop using the light if its cords are damaged.
Do I need a tachometer?
Some timing lights have an RPM mode, which the OTC digital light uses to show engine speed. If your light has no RPM mode, use a separate tachometer, and set the idle speed from the service manual.
If your engine has no timing adjustment, or the reading is far from the spec, stop and check the service manual and the fault codes before you change anything. Knowing how to use a timing light the right way starts with treating it as a checking tool.
