How a Headlight Switch Works
Headlight Switch Headlight switches have become more than just a place to turn on your lights, they are now used for some diagnostic and programming on your vehicle. In this article we will tell you what to look for if your headlight switch is failing and some of the other functions of the switch. Headlight switches used to be a simple pull switch that turned on the parking lamps (running lights) then when pulled all of the way out would turn on the headlights, or with a twist of the knob would brighten or darken the instrument lamps. On today’s vehicles the switch has a few more functions thanks to automatic headlights and the use of on-board monitoring. One of the first differences you will notice is that the instrument panel dimmer has been removed from the headlight switch; it is a stand-alone item on most new vehicles. This is good in that if there is an issue with the instrument lights, you may not need to replace the entire switch for just a dimming problem. Another thing you will notice is that there are several different positions to choose from for the switch depending on the options on your vehicle. There is the usual “off”, “on” and “running lights”, if equipped with auto-headlights you may see an “auto” position, when in this position the system uses a light sensor called a “Twilight Sentinel” (usually in the top center of the dash) to detect if the light levels require the use of headlights. Some GM vehicles have a temporary on, this means that when turned to this position it will spring back to the off position when you let go of it. This is used to enter the diagnostic/programming mode for the Tire Pressure Monitor, consult an automotive service manual for the exact procedure. One thing that is still common though is that a separate switch controls the high-beam lights; on older vehicles it was on the floor (foot controlled) and newer vehicles use the turn signal lever (multi-function switch) to turn them off and on.
If you suspect your headlight switch isn’t working properly, the first step as with any non-functioning electrical item is to get a test light and check the fuses, usually located in the under-hood power relay center. If they test ok, you may need to consult a repair manual to look at the wiring diagram. Some new vehicles incorporate headlight fuses, relays and use the BCM (Body Control Module) to do the actual controlling of the lights, that way they can take advantage of battery run-down protection and shut the lights off should you forget to. On older vehicles though, the switch does all of the work, including the instrument lights. Auto headlight switches tend to last longer than manual light switches due to the fact they simply aren’t used as much, they turn themselves on and off. Since manual switches are only used at night, they will usually last several years. When replacing your headlight switch always use an OEM (Original Equipment Manufacturer) or similar high quality part, cheaper switches are made of inferior material and wont last as long as the high quality replacement. If the switch turns out to be faulty, most of them can be replaced with a few simple tools, screwdrivers and small sockets (¼ inch drive) to remove the interior panels to gain access. Many of today’s vehicles use clips to hold these panels in, so take a good look before starting; it may save you some headaches. If further assistance is needed, our certified car repair technicians are ready to answer your car questions. Related Car Repair Information