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Dim Light BulbsNotes From the Road

Dim Bulbs: Troubleshooting Auto Lamp Problems
By Wayne Scraba/

Let’s assume your car or light truck is plagued with a lamp or two that isn’t working properly. Where exactly do you begin? Let’s start at the beginning and examine how the lighting circuits work:

The headlight circuit is comprised of the headlight switch, a circuit breaker or, the dimmer switch and the headlights. Simple enough. If the lights fail to switch from high beam to low beam, or if they go out entirely when the dimmer switch is engaged, the problem is in the dimmer switch. In years gone by, the dimmer switch was mounted on the far left corner of the toe-board (floorboard). The backside of the switch was often exposed to the elements and, as a result, it could easily be fouled. On the other hand, most newer cars have the switch mounted on the column stalk (the turn signal does double duty). Where the switch was easy to replace on old cars (a simple remove and replace), you’ll have to consult a shop manual for info regarding repair of a column-mounted dimmer switch.
Headlamps can be of the sealed beam variety (older cars) or more common today, lamps that incorporate replaceable bulbs. Lamps and bulbs can burn out or be damaged. Examine them carefully. The actually headlight sockets are regularly exposed to moisture and corrosion. The reason is (obviously) because they’re at the nose of the car. Examine the sockets for corrosion or damage.
The headlamp switch also controls the park lights, taillights and the license plate light. If the headlights will not operate at all, the first thing to do is to consider the fusible connection—usually a circuit breaker or in select examples, a form of replaceable fuse. Circuit breakers are small devices designed to break the contact like a fuse; however, once the overload is removed from the circuit, the breaker will return to normal operation. In most cases, the circuit breaker is a sealed assembly and as a result, it cannot be repaired.
Obviously, if a circuit breaker fails completely, it must be replaced. Circuit breakers can be located almost anywhere within the lighting system—even attached directly to the electrical component they protect. More often, circuit breakers are separate assemblies rather than grouped together on a panel like fuses. The electrical portion of your vehicle service manual will spell out the exact locations of the circuit breakers.
One area to check before moving forward is the connection on the ignition switch. In many vehicle applications, the power for the lighting circuits comes from the battery terminal of the ignition switch (typically marked “BATT”). The power does not go through the switch. It simply proves to be a convenient place to connect the wiring. If both the headlights and the taillights are out, take a close look at this connection.
Moving back to the actual headlamps: If only one headlight (or pair of headlights) is out, it could be the fault of the light itself (checked previously) or it could be a wiring issue wiring between the headlights. On most cars and light trucks, the wiring harness is arranged so that it runs down the inside of one fender to the headlight, then across the nose of the car (often following the radiator support) to the opposite headlight. Check the wiring harness for any damage.

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