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Notes From the Road

Dim Bulbs: Troubleshooting Auto Lamp Problems
By Wayne Scraba/

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 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. 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 and can be 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 taillight 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).

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