There are two things to look for, (three if computer controls are part of the circuit). In the past, head light switches have had internal auto-resetting circuit breakers in case a wire became shorted. Often the contacts for that circuit breaker became pitted or arced, and that caused them to overheat to the point the breaker would trip, then quickly reset. Those built-in circuit breakers caused much more trouble than they prevented. The clue typically was the flickering occurred more often when using the high beams because they draw more current. On most older cars, arcing of the circuit breaker also caused the tail lights and dash back-lights to flicker. If you're fortunate enough to see that, the cause of the problem has to be before the dimmer switch.
Another common cause is burned terminals at the head light switch or at the dimmer switch. You need to look at the connectors, and if you find one or two blackened terminals, that switch must be replaced, and the black terminals must be cut out of the connector, then they are replaced with individual terminals that can be plugged in separately. When terminals overheat, you will also find about four inches of their wire is hardened. That must be cut off too because solder wont stick to that. Splice in new wire, crimp the new terminals on, but solder them too. Seal the splices with heat-shrink tubing. Never use electrical tape in a car because it will unravel into a gooey mess on a hot day.
If you cannot find a bad switch or connector terminal, we will need to connect something that lets you see where the problem is occurring. Typically we would connect a test light on the wire that goes from the head light switch to the dimmer switch. If the test light flickers when the head lights flicker, the cause of the problem is before that point. If the test light does not flicker, move it further down the line, then watch what happens the next time the problem occurs.
Some newer cars use relays to turn the head lights on. Those will be in the under-hood fuse box. If you have those, swap the head light relay(s) with one of the other ones like it. If the relay was the cause, the lights will no longer flicker.
Wednesday, November 30th, 2016 AT 10:49 PM