Why Do My Headlights Flicker When I Drive?by Richard Rowe
Headlights are power-hungry devices. Combined with the headlight's inherent visibility, this attribute will often make a car's running lights the first components to suffer from electrical system faults. These faults can range from a bad ground, to an failing alternator to a simple abraded wire. The problem is worth looking into, since flickering headlights can indicate a serious fault in your electrical system.
If your headlights just seem to flicker on and off at random -- particularly when driving over rough roads -- then the problem is almost certainly a loose connector or bulb. Loose connectors could range from the wiring harness on the switch, to those on the headlights to the one on the fuse panel. Loose fuses and headlight bulbs are also a possibility.
The current that flows through headlight wiring puts it under a great deal of stress. The stresses of constant high-current flow combined with heat in the engine bay, unintended kinking and bending and abrasion on nearby objects can easily cause a wire either to break internally or to ground out on the frame. This condition is the only other one likely to cause random flickering.
A constant and regular on-off flicker may be the result of a bad headlight relay or headlight switch. Relays are basically electromagnetic switches: When one fails, it can constantly open and close as the current draw through it rises and falls. Internal switch failures can do the same thing, but they are somewhat less likely than a bad relay.
Accessories Drawing Power
Massive stereo systems are notorious for causing headlight pulsation -- some people even like this, since the headlights will dim in response to the bass hit. However, severe faults in other components can also cause excess current draw. Many components on the verge of failure will build up heat and resistance, then shut off as internal resistance overcomes the power input. Engine cooling fans are notorious for this condition, but a bad fuel pump, air conditioning compressor clutch, electric block and cabin heaters, and even windshield wiper motors, can cause an intermittent current draw.
Alternators will generally fail in a gradual way, but that will not always be the case. Most alternators use a built-in voltage regulator to keep output at a steady 14.2 volts or so. A failing voltage regulator of alternator diodes may rapidly spike and drop voltage, leading to either a regular or random flicker, depending upon what's broken.