Headlights intermittently turning off and on

  • 300,000 MILES
My headlights keep flickering on and off. I changed the switch and it stopped for a while, but now it is doing the same thing again. There are times when I can drive eight hours at night and they are good, but there are times when I drive for about an hour and they start turning on and off. Any help as to what it might be thanks.
Monday, November 14th, 2016 AT 6:00 PM

1 Reply

  • 33,733 POSTS
The dimmer switch is another good suspect, but in both cases, the high current causes the switch contacts to overheat and become arced and pitted, and that heat transfers to the terminals. Look at the old head light switch to see if two terminals are blackened, and the switch body around those terminals is charred. When you see that, the switch must be replaced, and the overheated terminals must be replaced.

You do not have to replace the entire plug. Instead, cut out the melted parts of the connector body with the two bad terminals. Reconnect the plug that way with the two terminals missing. You'll also find the two bad wires are hardened for about the first four inches from being hot. Solder will not adhere to that section of wire, so cut those four inches off.

Splice two pieces of wire of the same gauge to the two wires. I do that by sliding the strands into each other and pressing down any sharp ends that are sticking up. Solder those joints, then seal them with heat-shrink tubing. Never use electrical tape in a car because it will unravel into a gooey mess on a hot day. Use a pair of crimp-type generic terminals to replace the two burned ones, but also solder them to their wires. Plug those two terminals in individually where you cut the melted switch body away.

The same thing can happen at the dimmer switch, and it is usually possible to do the same repair there.

The head light switch also has an auto-resetting circuit breaker built in. It trips when it gets hot from too much current, but it will also trip from the heat of those overheating contacts, and the circuit breaker's contacts can become pitted and arced which will also cause it to trip, and it can cause flickering lights. Those circuit breakers cause more problems than they prevent. If you installed a new head light switch, the circuit breaker is not the cause of the problem. If you installed a used switch, and you cannot find any other obvious cause for this problem, consider the switch may have a bad circuit breaker. We can do some voltage tests to verify that if necessary.

One clue to watch for is when the flickering occurs, if the dash lights and tail lights also flicker, the cause has to be the head light switch or the circuit feeding it. Flickering tail lights can't be caused by the dimmer switch. When the dash and tail lights do not flicker, the cause can be anywhere in the system, including either switch.

A charging system problem can cause flickering lights too, but that will affect every light on the car. If that becomes suspect, the easiest way to verify it is to unplug the three-wire connector on the rear / side of the generator while the problem is occurring. All of the lights will dim noticeably, and the engine will be running on the battery. That will only last for fifteen to thirty minutes before it runs down, so do this test just long enough to verify the flickering has stopped.
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Monday, November 14th, 2016 AT 6:42 PM

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