I assume that a timing light functions by some kind of inductive pickup and the signal is amplified by power from the battery and turned into a flash at the bulb.

But I was wondering, is it picking up the strength of the current in the wire, or the voltage, and will the strength of the flash vary according to the strength of the power in the wire? I read that you can detect misfires by looking directly into the timing light. I happen to be fairly certain that I've got a misfire on a particular cylinder when I rev the engine, and noticed that the strength of the flashes got noticeably weaker when revving.

So I'm just trying to understand all what's going on here.

  • You're looking directly into the strobe? I would highly suggest you not do that unless you've got some heavy duty sunglasses on. Jan 3, 2017 at 15:16
  • @Pᴀᴜʟsᴛᴇʀ2 Not holding it up to my face or anything- I hold it away so it's not too bright, and only for a few seconds. Jan 3, 2017 at 15:17

3 Answers 3


A timing light works like a camera flash.

Inside the timing light there is a section that steps up battery voltage. That high voltage is applied across a gas discharge tube bulb filled with xenon gas. The tube is long enough that the high voltage won't arc over.

The pickup detects when the plug fires in an inductive fashion. That inductive kick so to speak is channeled to a coil wrapped around the bulb. When that coil becomes energized it ionizes the gas inside the bulb.

The ionized gas is conductive and the high voltage discharges across the bulb. This cases a flash. When the ignition event is over the ionizing effects of the coil subside. This intern causes the arc in the bulb to collapse.

A timing light could used in a comparative manner across all the cylinders but not in an absolute manner.

  • So what could cause the brightness of The Flash 2 vary when the engine is revved? Jan 5, 2017 at 5:03
  • @RobertS.Barnes The duration of the ignition event affects how long the pulse of light occurs for.
    – vini_i
    Jan 5, 2017 at 10:39
  • And thus it's perceived brightness? Jan 5, 2017 at 16:19
  • @RobertS.Barnes That is correct.
    – vini_i
    Jan 5, 2017 at 16:50

Most timing lights have a magnetic pickup that will 'detect' current flowing through the spark plug wire. This triggers the strobe to flash. The signal is not amplified, it closes a switch. I do not think the brightness of the light is dependent upon how much current flows through as the light is powered from the battery. I believe the flash is less bright because it is being turned on/off faster.

You can see a misfire with a timing light when the light does not light when it should. You will get used to the gap of time between when it lights. If you see a double size gap, then the plug did not ignite for whatever reason. I don't look directly into the light, I just shine it at whatever so I can see the flashes.

  • So it's basically operating like a multimeter's current clamp and it's detecting the DC current ( amps ) in the wire? en.wikipedia.org/wiki/Current_clamp Jan 4, 2017 at 14:52
  • Yes similar, except that it does not care about measuring how much passes through, only that the amount passing through is above a certain threshold.
    – rpmerf
    Jan 4, 2017 at 16:03
  • This is not how a timing light works at all.
    – vini_i
    Jan 5, 2017 at 1:34

It depends on the design of the timing light. Some piggy back in to the HT lead, others have an inductive pickup that wraps around the plug lead. Looking at the design of the timing light should make it obvious which design it's using.

The purpose of a timing light is to fine-tune the ignition timing event. I'm not sure where you read that looking into the timing light can let you see a miss-fire but it sounds like dangerous advice and certainly not a good working practice.

Should your question be about the specifics of how a timing light picks up the presence of a spark or should it be about techniques to detect a miss-fire?

  • It's about the specifics of how a timing light picks up the presence of a spark. Jan 3, 2017 at 18:40
  • Then refer to the first paragraph. It should be obvious from the way the unit attaches to the HT lead. Jan 3, 2017 at 18:41

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