The third brake light, or center high mount stop lamp (CHMSL), is a vital safety feature on most modern vehicles. When it stops functioning, the vehicle may fail a safety inspection and reduce signaling visibility to drivers behind. This article explains where to find the fuse for the third brake light, how to identify the correct fuse, and practical steps to diagnose and fix common issues. It covers typical fuse locations, common causes of failure, and safety tips for DIY repairs.
Where To Find The Fuse For The Third Brake Light
In most cars, the fuse for the third brake light is housed in the main fuse box or a secondary fuse panel located under the dash, in the glove box, or in the engine compartment. It is often labeled as CHMSL, CENTER BRAKE LAMP, THIRD BRAKE LAMP, or SRS depending on the vehicle make. If the label is unclear, consult the owner’s manual or the fuse diagram on the fuse box cover for the exact designation. Some vehicles separate the CHMSL circuit into its own fuse block near the battery or fuse panel to ease maintenance.
Common Fuse Locations In Vehicles
Understanding where fuses typically reside can speed up the repair process. Common locations include:
- Interior fuse box: Usually found under the driver’s side dash or behind a panel near the glove box. Many CHMSL fuses are here for quick access.
- Engine compartment fuse box: A secondary box that protects high-current circuits. The CHMSL fuse may be placed here if the circuit runs through the engine area or shares power with other exterior lights.
- Rear fuse panel: Some vans and trucks have a dedicated fuse area near the rear cargo area for tail, stop, and CHMSL circuits.
- Unlikely locations: In rare cases, late-model vehicles place the CHMSL fuse in a relay pack or on a mini fuse block behind the instrument cluster. If you cannot locate it, check the manual or wiring diagram.
How To Identify The Correct Fuse
To identify the right fuse efficiently:
- Check the manual: The owner’s manual or a vehicle-specific wiring diagram lists fuse numbers and amperage ratings for CHMSL circuits.
- Inspect the fuse box cover: Many covers include a diagram showing each fuse’s function. Match the CHMSL label or circuit color to the fuse.
- Fuse rating: Third brake lights typically use a fuse in the 10-20 amp range, but this varies by vehicle. Use the exact rating shown in the diagram; replacing with a higher rating can risk wiring damage or fire.
- Visual check: A blown fuse has a broken metal strip or appears burnt. If the fuse looks intact but the CHMSL still doesn’t work, move to other tests.
What To Do If The 3rd Brake Light Isn’t Working
When the CHMSL fails, follow a step-by-step approach to determine whether the fuse, bulb, wiring, or switch is at fault.
- Replace the fuse: If the fuse is blown, replace it with a fuse of the same amperage rating and test the light. If it blows again, there is a short or overload in the circuit that should be diagnosed by a professional.
- Inspect the bulb: Some CHMSL assemblies use a separate bulb or LEDs. For bulb-based lamps, check for a burned-out filament or corrosion. Replacing the bulb may restore operation.
- Check the wiring harness: Look for signs of damaged insulation, pinched wires, or loose connectors at the CHMSL housing and the tailgate area. A corroded connector can cause intermittent or permanent failure.
- Test the switch and control circuit: The CHMSL is controlled by the brake light switch at the pedal. If the switch fails, the CHMSL may not receive power even with a functioning fuse and lamp.
- Use a multimeter: With power off, test for continuity across the fuse. With power on, check for voltage at the CHMSL connector to confirm power delivery.
Safety Considerations And Tools
Working with electrical systems requires caution. Always:
- Disconnect the battery before handling wiring or fuses to prevent shocks or shorts.
- Use the correct amperage fuse; never substitute a higher rating to “fix” a blown fuse.
- Wear eye protection and work in a well-lit area to inspect for burnt components or melted insulation.
- Follow manufacturer guidelines in the owner’s manual for any special CHMSL installation or replacement steps.
Diagnostics Beyond The Fuse
If replacing the fuse does not restore function, broaden the diagnostic scope:
- Check the ground connection: A poor ground can prevent the CHMSL from lighting even when power is present. Inspect the ground strap and chassis ground near the light assembly.
- Inspect the CHMSL assembly: LED assemblies can fail individually. Look for water intrusion or condensation inside the housing, which can short circuits.
- Test with a known-good power source: If possible, temporarily power the CHMSL from a separate battery or power source to confirm the problem lies in the vehicle wiring rather than the lamp itself.
- Check for recalls or TSBs: Some CHMSL issues are related to defective wiring, connectors, or modules. Look for recalls or technical service bulletins for the specific vehicle model.
Maintaining The Third Brake Light
Prevention helps avoid sudden failures. Simple maintenance steps include:
- Regularly inspect the CHMSL for cracks, moisture, or fogging inside the lens.
- Clean connectors to prevent corrosion and ensure a solid electrical contact.
- Periodically test the light during routine checks or when performing other tail-light maintenance.
- Keep the vehicle’s battery in good condition; voltage fluctuations can affect lighting performance.
Common Questions
- Is the CHMSL fuse the same as other tail light fuses? Not always. Some vehicles group exterior lighting fuses, while others separate the CHMSL circuit. Always verify the exact diagram for the vehicle.
- Can a bad CHMSL affect other electrical systems? In modern vehicles, a faulty CHMSL can sometimes trigger onboard diagnostics that illuminate warning indicators, but it generally does not directly affect other circuits unless tied to a shared relay or power source.
- Should I replace the CHMSL fuse with a bigger one? No. Using a higher-amp fuse can hide a wiring fault or cause overheating and fire risk.