What a timing light tells you
A timing light is a strobe. Every time it senses a spark, it fires a very short flash. Aim it at a spinning part with a timing mark and each flash freezes that part in place, so you can see exactly where the engine was in its rotation at the moment the plug fired.
For that to mean anything you need two marks:
- A mark on the part that turns, usually the harmonic balancer or crank pulley, sometimes a cam gear. We'll use a 0 degree (TDC) mark as the example.
- A fixed reference on the engine, like a pointer, tab or a scale on a backing plate.
If the two marks line up under the light, that spark happened at the angle the moving mark represents. In the example, the engine is firing at exactly 0 degrees. If the marks sit apart, the gap between them, read off the scale, is how far before or after that point the spark is happening.
Why this matters on a Haltech install
The ECU calculates spark timing from its trigger setup. If the trigger offset is wrong, the number in your ignition table and the actual timing at the crank don't agree. The table might say 15 degrees while the engine is really getting 25. That's why we always confirm with a light before any real tuning: every timing number you tune afterward depends on this check. Our calibrating ignition timing guide walks through the ECU side of the process.
Crank marks vs cam marks
On a four-stroke engine the crank turns twice for every complete cycle, so a crank mark lines up at TDC on the compression stroke and again at TDC on the exhaust stroke. A light pointed at the crank can confirm the angle of the spark, but it can't tell you which of those two strokes the spark happened on.
The cam turns once per cycle, so if you can get a light on a cam mark, you can confirm both the angle and the stroke. That's especially useful on a first start, when an engine that cranks but won't fire might be sparking a full revolution out.
Getting the light to flash
A timing light needs its own power, either internal batteries or clips that attach to the car's 12V battery. It picks up the spark through an inductive clamp that goes around a spark plug lead; the pulse of current through the lead is what triggers the flash.
Plenty of modern engines have no plug leads. Here are the ways we get a signal on a coil-on-plug engine:
- Clamp the ignition signal wire between the ECU and the ignition module or coil.
- Add a short plug lead. Pull one coil out, fit an old spark plug lead between it and the plug, and clamp around that lead.
- Let the clamp bite the coil. Hold the clamp open and rest its jaws on the top of the coil so it picks up the field.
You'll be leaning over a running engine with a light in your hand. Keep the light's leads, your sleeves and your hands away from the fan and belts, and route the leads so they can't fall into anything that spins.
When the light won't fire, or flickers
An inconsistent flash almost always means the clamp isn't seeing a strong enough spark. The fix is to make the spark work harder by giving it a bigger gap to jump. Keep it sensible, because too big a gap can damage the coil.
The spark doesn't even need to fire inside the engine for this. With a coil out on the bench and the engine cranking, a spark jumping about 5 mm (roughly 3/16 in) to a grounded screwdriver is often enough to trigger the light. If you do this a lot, a spark tester is worth buying. It's a grounded threaded rod you point at the coil output and screw in or out until the light flashes reliably.
Dial-back or fixed?
| Fixed (standard) | Dial-back | |
|---|---|---|
| How it works | Flashes the instant it senses a spark | Lets you delay the flash with a knob or display until the marks line up, then shows the angle |
| Reading it | Read the angle straight off the marks | Read the angle off the light, based on the delay you dialed in |
| Our view | What we recommend: cheap, simple, consistent | Easy to misread, and a mis-set delay gives a wrong answer that looks right |
If you do use a dial-back light, make sure its delay is zeroed before comparing marks, or everything you read will be offset.
Troubleshooting
| Symptom | Usually |
|---|---|
| No flash at all | No power to the light, clamp on the wrong wire, or no spark |
| Flash is erratic | Weak signal at the clamp. Increase the spark gap or use a spark tester |
| Marks jump around | Timing is genuinely moving because the ECU is still controlling idle timing |
| Timing reads off by a fixed amount | Trigger offset needs correcting in the ECU |
| Crank marks look right but the engine won't fire | Could be sparking on the exhaust stroke. Check against a cam mark |
Timing light FAQ
Do I need an expensive timing light?
No. A basic fixed light does the job and is harder to get wrong.
Can I check timing without spark plug leads?
Yes. Clamp the ECU's ignition signal wire, add a short plug lead to one coil, or rest the clamp on top of the coil.
What should I do after checking timing?
Once the ECU and the engine agree, you can move on to building the tune. Our base map basics and ignition timing vs air/fuel ratio articles are good next reads.
