Troubleshooting Auto Light Problems

How to isolate and repair headlight, brake light and tail light trouble

Let's assume your car or light truck is plagued with a lamp or two that isn't working. Where exactly do you begin? Let's start at the beginning and examine how the lighting circuits work (we're going to leave the turn signals out of this equation; they're complex enough to demand their own troubleshoot):


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 tail lights 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.

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 for 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. The most common is a simple mechanical switch, which 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.

The other type of switch you might encounter is a 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 obviously, the brake light) housing is easily accessed from the inside of the trunk. Bulb-wiring harness connectors are clipped in place are easily removed in order to gain access to the bulbs. The most common wiring problem for taillights (brake lights) is a loose light bulb 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 pickup truck with inoperable rear lamps, this is the first place to look.

Taillights with the earlier style dual-filament bulbs can actually lose one filament without causing harm to the other. This will eliminate the tail light function while still allowing the brake light to work (or vise-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.

If the vehicle has no rear lights at all (the license plate light will also be out along with the side marker lamps), 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.

Another common problem is where brake lights remain on even though the brake pedal is not depressed. This is most often caused by an out-of-adjustment switch (as above). 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).

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

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