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Troubleshooting car lighting problems

Dim car lighting while driving in a tunnel

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.

Car headlights
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 floorboard. The backside of the switch was often exposed to the elements and, as a result, it could easily be fouled.

Most 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.

Car headlamps
These can include the sealed beam variety (older cars) or, more common today, lamps that incorporate replaceable bulbs. Lamps and bulbs can burn out or be damaged. The actual headlight sockets are regularly exposed to moisture and corrosion because they’re at the nose of the car. Examine the sockets for corrosion or damage.

If only one headlight (or pair of headlights) is out, it could be the fault of the light itself or it could be a wiring issue 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.

Car headlamp switch
This switch controls the park lights, taillights and 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 a replaceable fuse.

Circuit breaker
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.

Ignition switch
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’s simply a convenient place to connect the wiring. If both the headlights and the taillights are out, take a close look at this connection.

Taillights and brake lights
Typically, there are two different types of taillight bulbs on a passenger car or light truck. On older vehicles, the taillight and brake lights are combined assemblies that make use of a single bulb with two filaments. One is for the taillights, the other is for the brake lights. Modern cars and light trucks incorporate separate brake and taillight bulbs. The light switch that turns on the headlights controls the taillights on all vehicles. The brake lights are controlled by the brake light switch, which functions when the brake pedal is depressed.

There are two different types of brake light switches:

  • Simple mechanical switch: The most common switch, it is usually mounted on a bracket near the brake pedal (usually the pedal arm). When the brake pedal is depressed, the switch button is released and completes the circuit to turn the brake lights on. When the brake pedal returns to the normal position, the pedal arm makes contact with the switch and the brake light goes off.
  • Hydraulic switch: This type of switch is mounted in the brake line somewhere – most often on or near the master cylinder. In operation, the hydraulic switch senses an increase in brake fluid pressure as the brakes are applied and completes the circuit to turn the brake lights on. Once the fluid pressure lowers, the switch returns to “normal” and the lights go off.

On most cars, the rear of the taillight (and the brake light) housing is easily accessed from the inside of the trunk. Bulb-wiring harness connectors are clipped in place and can be easily removed in order to gain access to the bulbs.

Four common car taillight and break light issues

1. A loose lightbulb socket
The most common wiring problem for taillights (brake lights) is a loose lightbulb socket. If it’s loose, the ground path is broken and the current can’t return. That means that the metal portion of the bulb isn’t making good contact with the bulb socket. As you can imagine, moisture and corrosion can also wreak havoc with bulb connections. If you have a pickup truck with inoperable rear lamps, this is the first place to look.

2. Earlier style dual-filament bulbs
Taillights with the earlier style dual-filament bulbs can actually lose one filament without causing harm to the other. This will eliminate the taillight function while still allowing the brake light to work (or vice versa). There’s more: In some early cars, it is entirely possible to install a dual-filament bulb incorrectly. This allows the brighter brake light filament to function as the taillight. When the brake is applied, you won’t be able to see the brake light since the lower-powered taillight filament is on. To fix it, simply remove the bulb and reinstall it in the correct position.

3. Electrical disruption
If the vehicle has no rear lights at all (the license plate light and the side marker lamps will also be out), the problem is an electrical disruption. Check the fuses first and then check all connectors in the wiring leading to the taillights. If the brake lights (only) are not functioning but the taillights are operational, the problem is the brake switch, the brake lamp fuse or wiring from the light switch, which operates the lights. On vehicles that incorporate a switch on the brake pedal arm, there’s a good chance the switch is simply out of adjustment. See your vehicle service manual for adjustment procedures.

4. Out-of-adjustment switch
Another common problem is when brake lights remain on even though the brake pedal is not depressed. This is most often caused by an out-of-adjustment switch. Or, in the case of the hydraulic pressure switch, internal brake line corrosion may be causing residual pressure, which in turn allows the switch to stay closed (effectively turning the brake lights on).

Troubleshooting lamps

For more info on troubleshooting lamps, check out the following photos and steps:

Car light switch control and switchgear turned off
1. Always check the car's light switch
One place few people consider when they’re tracking down faults in the lighting circuit is the actual switch. Switches can eventually wear out; however, it’s more likely that a fault can be traced to a loose connection. The bottom line: Don’t discount the switchgear.

Checking the light dimmer switch on the turn signal stalk
2. Check the light dimmer switch
High and low beams (bright and dim) are most often controlled by way of the turn signal stalk. On older cars, the dimmer switch was positioned on the floor and was consequently operated by your foot. At that location, it was vulnerable to road debris, water and general wear (on both sides of the car). In both examples (floor and column), be sure to examine all connections carefully when troubleshooting lamps.

Checking the front bulbs of blue car headlights
3. Make sure the headlights have not burned out
Headlights, whether sealed beams such as this or lamps with replaceable bulbs, burn out. That should be no surprise. In both cases, headlamps are exposed to the elements, and you can be assured their connections can often become a source of trouble.

Circuit breakers and fuses for the headlamp circuits
4. Research where to find the fuses and circuit breakers
Plenty of vehicles use relays, circuit breakers and heavy-duty fuses for the headlamp circuits. Before tearing into the electrical system on your passenger car or light truck, consult the owner’s manual and determine the locations of the appropriate fuses and circuit breakers. They can be the source of your trouble.

Checking the brake light switch mounted by brake pedal
5. This brake light switch is easy to locate, but not all are
There are a couple of different brake light switches. This arrangement is the most common. Here, the switch is a mechanical device mounted ahead of the brake pedal. As the pedal is released, the switch opens and the brake light goes off. The switch is usually easy to access and relatively simple to adjust. Consult your vehicle service manual for the exact process.

Checking taillight bulbs on the backside of blue car
6. Check the taillight bulbs and connections 
Taillight bulbs are accessed from the backside. You’ll find the bulb and bulb connection simply clip into the back of the lamp housing. In many cases, the taillights live in harsh environments (for example, a pickup truck). Check the bulbs and pay special attention to corrosion on the connectors.


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