I have a 2003 Ford Focus. Recently, I've noticed that when I accelerate (step on the gas), all the lights on the car flicker. This happens to both the dash lights in the car and the headlights outside. They will pulse bright and dim while I'm accelerating, and when I let up on the gas, the lights return to normal.

What might cause this problem?

  • A wild guess would be that your alternator or battery might be faulty. Have an auto electrician run a few tests. If your electronics fail in just the right way, it could burn out your wiring loom, which is expensive. Jan 8, 2015 at 12:36
  • 2
    This could quite possibly be your alternator going out. If one (or more) of the diodes are failing, it can cause the pulsing you are talking about. I recently replaced the alternator in my truck for the same symptoms. You can take your alternator down to AutoZone or the like and have it tested, but I'm not sure their tests would pick up a failing diode. Jan 8, 2015 at 16:36

1 Answer 1


There are three phases of a charging system. The charging system frequently will create AC current and diodes within the alternator will act as a rectifier and convert the AC current to DC where it can be stored in a battery.

Here is a diagram

enter image description here

If one or more of the diodes fails in the alternator you begin to see the pulse of the charging in your lights. It's especially apparent when two legs of the three legged system are down.

This is what the waveform looks like

enter image description here

The top wave is AC the lower wave is DC.
If you you take wave 2 and 3 out and only have the signal from number 1 then you can begin to see why you have the pulsing affect.

The reason it does it when you get on the gas is that your rpm's increase and the transmission and the engine engage, when you get off the gas most automatic transmission cars drop their rpm's and disengage from the transmission. Hence the pulsing while on the gas. Hope this helps, good luck.

  • 1
    Replacing the alternator fixed the problem. The mechanics tested the alternator and battery and decided that the alternator was fine and replaced the battery. However, the problem remained. They then retested the alternator and replaced it, and now the problem is gone. Thanks for the explanation.
    – Ben Miller
    Apr 8, 2016 at 16:34
  • Excellent! Glad it's fixed! Apr 8, 2016 at 16:34

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