How Brake Light Wiring Works

The streets are aglow with brake lights.
The streets are aglow with brake lights.
Stephen Studd/Photographer's Choice/Getty Images

­Brake lights serve a simple but important function -- they warn drivers behind you when you're slowing down. Have you ever been stuck behind a car that has faulty brake lights? Your frustration level can hit the roof as you're trying to predict when this driver is about to brake or turn since his lights aren't working. Or perhaps, his brake lights stay on indefinitely and you must stay a few cars' lengths behind since you have no idea what maneuver he'll be performing next. Without functioning brake lights, the incidence of rear-end collisions would certainly be much higher.

If one of your brake lights goes out, hopefully your first clue isn't a cop pulling you over to let you know. If one of them does go out, you can probably fix it yourself if you know a little something about brake light wiring.

The brake light wiring system in most cars is not overly complex. At one end, you have the lights themselves: bulbs in sockets connected to a wiring harness. At the other end is the brake switch, where the pedal presses down and creates contact that completes the circuit. The system draws power from the car's battery.

If you're planning some brake work or brake repair, need to rewire brake lights or are already in the middle of fixing brakes, this article will tell you how brake light wiring works.

Brake Light Wiring Preparation

This is the tail light assembly from a 1993 Saturn. The cut wire ends would lead to the main wiring harness inside the car if this assembly were still attached.
This is the tail light assembly from a 1993 Saturn. The cut wire ends would lead to the main wiring harness inside the car if this assembly were still attached.
Photo courtesy Ed Grabianowski

Before you dive right into a brake repair job, it will pay to do some brake light wiring preparation. First, find the car's manual and have the brake light wiring diagram handy. All brake work should start with diagnosing the problem. It's very helpful to have a partner working with you, because you'll need someone to press on the brake pedal while you're otherwise occupied. Your partner is vital for the first step -- check to see which brake lights are working and which ones aren't. Have your helper press on the brake pedal, and then watch to find out if just one brake light is out (specifically which one) or if they're all out. 

­If none of the brake lights are working, check the fuse block. Find the fuse marked "brake lights" and check to make sure the fuse isn't burned out. It's not a bad idea to double check the fuse operation using your voltage test light or multi-meter. If the fuse is OK, check the bulbs. An easy way to do this is by activating the turn signal. In many cases, the brake light and the turn signal use the same bulb. If the turn signal works, you know the bulb is fine, so the problem lies elsewhere in the brake light wiring.

The next step is to check the sockets. Again, you can use your test light to see if the sockets are getting power. Then, move on to the brake light switch, located near the brake pedal. There will be a power wire and a ground or output wire (consult the wiring diagram). Your test light should show power from the power wire at all times, even without a key in the ignition. The ground/output wire should show power when the pedal is pressed down. Another point to remember is that the switch may be improperly adjusted. The bracket that holds the brake switch in place could keep the brake pedal from making contact with the switch if it has been moved.

If all else fails, check the diagram and trace the brake light wiring through the entire car. There may be a break in the wire at some point. If the wiring bundle passes through an area that has a rough or sharp edge, the casing surrounding the wire may have been cut causing a short in the circuit. Another possibility is if the wiring has been exposed to wetness, it could have rotted out and then short circuited.

Once you've tracked down the problem, it's time to get down to the business of fixing brakes, or more accurately, fixing the brake light wiring. You'll need the right tools for the job.

Tools for Brake Light Wiring

The brake light socket with the bulb.
The brake light socket with the bulb.
Photo courtesy Ed Grabianowski

Brake repair isn't very difficult as long as you have the proper tools for brake light wiring jobs. Brake work is always easier if you have a test light or a multi-meter, the right size replacement bulbs and a set of wire disconnectors. Wire disconnectors are small metal picks or hooks that pop the wire out of the bulb socket. If you need to do some rewiring as part of your brake work, you may need a wire stripper and some electrical tape, too. If you have access to a diagnostic scan tool, this can make fixing brakes (or at least diagnosing the problem) even easier. In some cars, the brake wires go through the body controller, and electronic control unit separate from the engine controller. A scan tool might give you a better idea of where the problem may be.

You can usually splice wires just by twisting them together, then covering the splice securely with electrical tape. However, if you want a stronger splice, you could use a soldering iron.

For many, seeing is believing. In order to give you a clear picture of where the wires go, visit the next page and see a basic brake light wiring diagram.

Brake Light Wiring Diagram


Connecting to the Brake Light Wiring Circuit

­If you need to replace the socket, it's often easiest to simply cut the wires, then splice on the new socket.
­If you need to replace the socket, it's often easiest to simply cut the wires, then splice on the new socket.
Photo courtesy Ed Grabianowski

If you've determined the problem is with the brake light switch or with the wiring, then you'll need to know about connecting to the brake light wiring circuit. The circuit is essentially the full run of wires from the battery to the fuse block, from the fuse block to the brake light switch and then from the switch to the brake lights themselves. Sometimes, the wires may also run to the body controller. Most of the wiring is bundled in a wiring harness that runs through the entire car.

If your brake repair requires you to connect the brake light switch to the circuit, start at the fuse block. Find the hot side of the fuse block and run a power wire either from the terminal for the brake light fuse or from another hot pin on the block. This wire should lead to the brake light switch. The switch itself is connected with a plug, so you may need to splice your power wire to the power wire coming out of the plug. Once the switch has power, you need to connect the output wire from the switch to the rest of the brake light wiring harness. A wiring diagram for your specific vehicle will show you which wire in the bundle to splice. As long as you're taking a careful approach, chances are that you shouldn't need to replace or rewire entire sections of the brake light wiring circuit -- just find the faulty wire, section of wire or component.

Let's find out more about the brake light switch on the next page.

Connecting to the Brake Light Switch

When you press the brake pedal, remember that you're triggering the brake light switch, too.
When you press the brake pedal, remember that you're triggering the brake light switch, too.
Burazin/Photographer's Choice RR/Getty Images

­If your brake light switch isn't working properly, there's a chance that you could be driving under risky conditions. Your brake lights may not light up when you press on the brake pedal or they may stay lit regardless of whether you're pressing the pedal or not. Or perhaps, some switch malfunction is causing the lights to operate intermittently. So you can see the problem: If you don't regularly check your brake lights for proper operation, you may be driving around town with no brake lights at all; brake lights that are continuously on; or brake lights that work only part of the time. Either way, it's dangerous to you and other drivers around you.

Before you get too anxious about the brake work that you may or may not need, you may want to investigate the problem yourself. Actually, it may be an easier fix than you think, just by investigating your brake light switch. You can see your brake light switch by looking under the dash, near the top of the brake pedal. The brake light switch is usually attached to a small bracket that holds the switch, activated when the pedal is depressed, in position.

Once you've located the switch, check to make sure that the brake light switch's electrical connection is firmly attached and that all of the wires connecting to the brake light switch are in good condition (at least as far as you can see from your position). Then check to make sure that the switch isn't out of position or stuck in the on or off position. Unless there's been a failure somewhere within the switch itself -- a highly unlikely problem, by the way -- it's relatively simple to decide if the switch is at fault.

If you're able to determine that it is indeed your brake light switch that's at fault, replacement is simple. In fact, it's likely that you won't even have to get your hands dirty to complete a brake repair of this sort. Remember, there are several different types of brake switches available, including ­hydraulic brake switches, so be sure to get the one that's right for your specific vehicle. You'll need to know the make, model and year of your vehicle but really that's about all. Your local auto parts store should be able to locate the correct part for you.

It may seem elementary, but remember that it's important to properly diagnose the problem prior to buying any repair parts. It won't do you any good to buy a new brake light switch if the problem is in the wiring leading up to the switch, in the connector itself or in the wiring that goes from the switch to the brake lights at the rear of the vehicle.

So as you can see, sometimes fixing brakes -- or rather, fixing brake light wiring -- doesn't have to be as complicated as it initially sounds. Read the next page to find out about connecting the brake lights themselves.

Connecting the Wiring to the Brake Lights

Even with a diagram, tracing wires through the car can be difficult because the wires from different components are bundled together.
Even with a diagram, tracing wires through the car can be difficult because the wires from different components are bundled together.
Photo courtesy Ed Grabianowski

If your brake repair problem was with the brake light sockets, you may have had to replace one or more of them. Connecting the wiring to the brake lights may be as simple as unplugging the old socket and plugging a new one in. Or it may involve replacing the wiring along with the new socket. If you have the right tools, you could even disconnect the wires from the old socket and connect them to the new socket directly, but this is intricate work and it's usually not done this way. It's more likely that you would cut the wires leading to the old socket, consult your wiring diagram and splice the new socket onto the old wires. 

­Some brake work involves the center brake light, usually mounted somewhere on the rear window. Rather than the more typical incandescent bulbs, center brake lights often use LEDs. They may have a simple plug for a new socket, or you may have to rewire each tiny LED socket individually -- it depends on the make and model of your car.

The brake lights might use a common ground, or it's possible that they may use a separate wire to ground each bulb and socket. This isn't usually an issue if you're just dealing with the brake light bulbs. If you properly connect the lights and sockets to the wiring harness, the ground should be taken care of.

As a final step to help maintain your lights (so you can spend less time fixing brakes), you can apply some dielectric grease to the sockets where the bulbs plug in. This nonconductive grease helps seal out moisture and prevent corrosion.

For more information about brakes, brake light wiring and other related topics, stop by the next page.

Related HowStuffWorks Articles

More Great Links


  • "Stop Light Switch." (11/10/2008)
  • Dodge Forum. "Brake Light Switch." 9/11/2006. (11/10/2008)
  • Grabianowski, Marty. Personal interview. Conducted 11/5/2008 and 11/6/2008.
  • Roadmaster. "Stop Light Switch Bracket and Wiring Kit."  (11/10/2008)
  • VW Vortex. "Installing a new brake light switch (without breaking it!)." 12/19/2003. (11/10/2008)