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 light bulb socket
    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 cannot return. That means that the metal portion of the bulb is not making good contact with the bulb socket. Moisture and corrosion can also interfere with bulb connections.

  • 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).

  • Diesel-Electric supplies tail lamps – contact an outlet near you.

We recommend should you be in doubt on how to change or suspect any issues you take it to an authorised auto dealer or mechanic to make sure all is operating properly.