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What are the 4 wires on a tail light?

When it comes to automotive electrical systems, tail lights play an important role in vehicle safety by providing visibility to other drivers. Most modern tail light assemblies contain 4 wires that power the various lighting functions. Understanding what these wires do is key to properly diagnosing and repairing tail light issues.

Overview of Tail Light Wiring

There are a few common wire colors and functions found on most tail light assemblies:

Wire Color Function
Black Ground
Red Brake light power
Brown Turn signal power
Yellow Rear running light power

The ground wire provides a ground reference for the bulb filaments. The other wires deliver power for their specific lighting functions. Let’s look at each one in more detail:

Black Wire – Ground

The black ground wire gives the bulbs in the tail light assembly a completed circuit to light up. Current flows from the vehicle’s battery, through the applicable power wire for that lighting function, then passes through the filament of the bulb and uses the ground to return to the battery.

Without a proper ground, the tail lights would not illuminate even if they have power coming through the other wires. A faulty ground can cause issues like:

  • No tail lights at all
  • Dim or flickering lights
  • Intermittent operation

Problems with the ground wire typically occur in the wire itself. It can break inside the insulation due to fatigue from vibration or rubbing. Corrosion at connection points can also cause an intermittent ground. Testing the ground wire and cleaning connections are ways to troubleshoot ground issues.

Red Wire – Brake Light Power

The red wire provides power for the brake lights. It runs from the brake light switch on the brake pedal up to the tail light assembly. When the brakes are pressed, the switch closes to send power through the red wire and illuminate the brake bulbs. The wire is coated in red insulation for easy identification.

Common problems with the brake light wire include:

  • No brake light illumination – wire shorted, broken, faulty switch
  • Brake lights staying on – short in the wire, stuck closed switch
  • Flickering brake lights – loose connection, short in wire

Tracing the red wire from the pedal switch to tail lights and testing connections with a multimeter can isolate the source of these issues. The wire also tends to break inside the insulation like the ground wire.

Brown Wire – Turn Signal Power

The brown wire supplies power to the turn signal bulbs in the tail light. It leads from the turn signal switch and flare harness back to the rear corners of the vehicle. The brown color identifies it as the turn wire.

If the turn signals on one side or the other side are not functioning, the brown wire for that side is the first place to check. Faults can include:

  • No blinking on one side – broken turn signal wire
  • Fast blinking – short to ground in wire
  • Dim blinking – partial short or high resistance in wire

A blinking problem that occurs on both sides likely indicates an issue with the turn signal switch, relay, or front wiring harness. The wire also tends to fracture inside the insulation without visible damage.

Yellow Wire – Rear Running Light Power

The yellow wire provides power for the rear running or parking light bulbs. It runs power from the headlight switch back to the tail lights anytime the headlights are activated. This illuminates the smaller rear running light bulbs to increase the vehicle’s visibility at night.

Problems with the yellow running light wire manifest themselves as:

  • No running lights with headlights on – broken wire
  • Running lights stay on – short to power in wire
  • Flickering lights – damaged wire insultation

Since the running lights work with the headlights, be sure to check the headlight switch as well when troubleshooting. Testing for continuity in the yellow wire will help isolate the problem.

Additional Wires

Some tail light assemblies may also contain:

  • Green wire – Separate rear turn signal power
  • White wire – Backup light power
  • Blue wire – Antilock Brake System (ABS) signal

Including these extra wires allows for more specialized functions from the tail lights. The general diagnostic process remains similar for checking each wire due to a lighting malfunction.

Wire Gauge Size

The wire size used for tail lights can vary depending on the amperage drawn by the bulbs. Typical wire gauges include:

  • 16 AWG for ground and running lights
  • 14 AWG for brake lights
  • 12 AWG for turn signals

Larger bulbs or additional lights often require thicker 10 or 8 AWG wires to handle the increased current. Always check a vehicle wiring diagram to determine the correct wire size for replacement.

Finding Wiring Issues

Some common steps for diagnosing tail light wiring problems are:

  1. Visually inspect wires and connections for damage
  2. Check for voltage at tail light connector with lights switched on
  3. Test ground wire connection with multimeter
  4. Use a circuit tester to check for power at each wire
  5. Pull on wires along the length to feel for internal breaks
  6. Look for pinched or pierced wires in the harness

Finding the exact location of a wiring fault requires carefully testing the circuit step-by-step. Often this means starting at the power source for that wire and working back towards the tail lights.

Repairing Wire Damage

Damaged tail light wires typically require splicing in a new segment of wire. This involves the following steps:

  1. Cut out the damaged portion of wire if possible
  2. Strip back insulation on both wire ends
  3. Select replacement wire of the same gauge
  4. Overlap stripped ends and twist together
  5. Apply heat shrink tubing over the splice
  6. Seal connector with electrical tape

Soldering makes the most reliable connection when splicing wires. Crimp connectors and wire nuts can also be used in some applications. Adding a protective wrap of electrical tape completes the repair.


Understanding the function of each tail light wire provides a great starting point for troubleshooting issues. The four wires power the various lighting functions through individual circuits. Checking each wire and connection systematically helps pinpoint the source of electrical problems.

While tail light wires can break over time, they are relatively easy to splice and repair as needed. Proper diagnosis of issues along with materials like heat shrink and electrical tape allow defective wires to be restored.

With robust preventive maintenance and prompt repair of problems, the wires feeding the tail lights can provide thousands of miles of safe vehicle operation. Knowing the fundamentals of these vital circuits helps keep tail lights shining brightly.