I was behind a Honda today in some traffic, and I noticed its LED tail lights were pulsing a couple of times a second, and I want to know what may have been causing it. It seemed to stop once we were moving again although I was concentrating on the traffic so I may not have seen it.

Was it some sort of modification? The tail-lights were LEDs and seemed aftermarket. Is it a sign of a problem? If it's a problem, what's the likely cause?

  • 1
    If you are a robot that uses an electronic camera system for eyes, LED brake lights do tend to flicker with the frame rate of many such systems.
    – JPhi1618
    Dec 18, 2017 at 16:59
  • 1
    Last I checked I was a human, let me try the Capcha again @JPhi1618
    – GdD
    Dec 18, 2017 at 17:04
  • WAs the car indicating (for a turn) at the time? A dodgy earth shared between the indicator and tail light can cause flicker, and some after-market LED assemblies don't respond to changing line voltages the same way as you'd expect from experience with bulbs.
    – Chris H
    Dec 19, 2017 at 11:52
  • 1
    No, there were no turn signals @ChrisH.
    – GdD
    Dec 19, 2017 at 12:46

2 Answers 2


One issue which may be happening is a problem with the alternator.

To explain, I'll give a brief outline of how an alternator works. The alternator, as the name implies, creates alternating current (AC), then through a set of diodes, rectifies (I believe is the correct term) the electricity to create direct current (DC) which the vehicle can use. The circuitry looks something like the following:

enter image description here

In the center of the image, where you have the three curly cues, are the field terminals where the power comes out of and power is forced through the diodes which are to the right of it. Each field terminal can only "push" electricity through the diodes in the direction of the arrows (representing the diodes). During the alternating process of electricity's creation is a wave process, it only pushes electricity through as DC current during 1/2 of the wave cycle. The reason there is three field terminals is so the three "output" portions of the current being created covers the greater period of time while the alternator is running. If you only had one field terminal and one diode set, you'd have fluctuating energy creation which you'd be able to see in the lights. If one of the diode sets quits working (gets burned out or whatever), the alternator would only be producing electricity during 2/3 the time it would normally. When the engine is slowed down during idle, this dip in energy production becomes noticeable and why you might be seeing the fluctuation in light output from the tail lights as you're sitting still. As the engine speeds up (not at idle anymore), the time period between energy creation gaps becomes shorter and it is much less noticeable.

  • Not sure about the US but in europe some cars (VW group I know of) have different modes for braking - solid = normal braking; flashing = hard braking over 50kmh and below 80kmh; random flashing = hard braking above 80kmh. The lights flash until the brake is released and the vehicle moves again.
    – Mauro
    Dec 18, 2017 at 16:48
  • @Mauro - That's good to know and something we don't see here in the States. It didn't seem like this was what GdD was describing. Dec 18, 2017 at 16:50
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    @Mauro That should be an answer.
    – Bill N
    Dec 18, 2017 at 16:56
  • @Pᴀᴜʟsᴛᴇʀ2 There are many aftermarket modules, especially for motorcycles, that allow programming of brake lights and turn signals. Check out shop.12oclocklabs.com for one example (I'm not connected with them, but I do use their speedometer calibrator.)
    – Bill N
    Dec 18, 2017 at 16:59
  • I should point out that the pulsing was visible even when the brakes were off and the car was moving forward @Mauro.
    – GdD
    Dec 18, 2017 at 19:44

This evening, I saw something similar to what the OP describes. The tail lights don't flash (turn on/off entirely, as with hazard signals), but rapidly pulsate/fluctuate between bright and slightly less bright. In this instance, the pulsating stopped when the vehicle came to a stop at an intersection. I took a short video (https://youtu.be/wedKW9H-bOM) which doesn't show it very well. I think it might be slightly more apparent if you play the video at a slower speed. It was more pronounced/visible viewing directly/in-person.

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