Does a brake light switch need a relay?

No—on most vehicles a brake light switch does not require a relay. Traditional systems route power through the switch directly to the lamps, and modern cars often use the switch as a low-current signal to a body control module (BCM) that drives the lights electronically. A relay is typically only needed when you add significant extra load (such as trailer lighting or additional aftermarket brake lamps) or when protecting older hydraulic pressure switches in classic/custom setups.

How brake light circuits are typically designed

In many vehicles built through the 2000s, the pedal-mounted brake light switch completes the power feed to the rear bulbs. Two standard 21-watt bulbs draw roughly 3.0 amps at 13.8 volts, and even with a third high-mounted stop lamp, the total is commonly under 5 amps—well within the rating of most mechanical brake switches (often 5–10 amps). With LED brake lamps, current draw is lower, further reducing the need for a relay.

Modern vehicles with modules and LEDs

Newer vehicles frequently use the brake switch as a low-current input to a BCM or lighting control module. The module then supplies power to the lamps (often solid-state), and sometimes a factory “stop lamp relay” is integrated into the module or fuse box. In these systems, adding your own external relay is unnecessary and can confuse bulb-out monitoring or trigger fault codes. Always consult a wiring diagram if you’re unsure.

When a relay is useful

There are specific cases where adding a relay can protect components, improve brightness, or handle added load. The following situations explain when a relay makes practical sense.

• Additional or higher-wattage brake lamps: Extra incandescent lamps or high-output bulbs can push current beyond the switch’s rating.
• Trailer lighting without a powered module: A dedicated relay or powered trailer light module prevents overloading the vehicle circuit.
• Classic cars with hydraulic pressure switches: These switches can wear prematurely under full lamp load; a relay can extend their life.
• Long wiring runs in custom builds: A relay closer to the rear lamps can reduce voltage drop and improve brightness.
• Circuit isolation: When integrating aftermarket controllers or third brake light upgrades, a relay can keep factory and custom circuits separate.

In all of these cases, the relay acts as a high-current switch controlled by the existing brake signal, taking the heavy load off sensitive or aging components and maintaining proper voltage to the lamps.

When you should avoid adding a relay

Adding a relay isn’t universally beneficial. In the following scenarios, a relay can introduce faults or simply isn’t necessary.

• Vehicles with CAN bus/BCM lamp monitoring: Additional relays can change sensed current and trigger warnings or DTCs.
• Factory LED systems under warranty: Modifying circuits can cause warranty issues or inspection failures.
• Vehicles that already incorporate a stop-lamp relay or solid-state driver: Duplicating that function adds complexity without benefit.
• When fixing a fault: Dim or inoperative brake lights are usually due to bad bulbs/LED assemblies, poor grounds, misadjusted switches, or blown fuses—not the absence of a relay.

If the vehicle already manages lamp power effectively or is designed to monitor circuit integrity, inserting a relay can create more problems than it solves.

How to add a relay correctly (if you must)

If your application truly needs a relay—for example, to power extra lamps or a trailer—wire it safely and in a way that respects the factory system.

1. Select a 12 V automotive relay rated at least 20–30 A and use wire sized for the load (typically 16–14 AWG for up to ~10 A; consult local standards).
2. Identify the brake signal wire at the switch or BCM output; use this low-current signal to trigger the relay coil.
3. Wire relay terminals: 30 to fused battery power; 87 to the added brake lamp load; 85 to the brake signal (positive trigger); 86 to ground. Reverse 85/86 if using a ground-triggered system.
4. Place a fuse within 6–12 inches of the battery feed to terminal 30; size the fuse 125–150% of expected load (e.g., 7.5 A fuse for a 5 A load).
5. Use a diode-protected relay or add a flyback diode across the coil to suppress voltage spikes that could upset modules.
6. Weatherproof all connections, mount the relay securely, and strain-relieve wires to prevent chafing.
7. Test operation, confirm full brightness, and verify that no warning lights or diagnostic codes are triggered.

Proper relay integration ensures the factory brake switch only provides a trigger while the relay and new fused feed handle the added current safely.

Quick troubleshooting checklist

If your brake lights aren’t working or seem dim, run through these basics before considering a relay.

• Check the STOP/Brake fuse(s) and any related BCM or lighting fuses.
• Inspect bulbs or LED assemblies and sockets for corrosion or failure.
• Verify brake switch alignment and operation at the pedal (many are adjustable).
• Clean and tighten rear ground points; high resistance causes dim or intermittent lights.
• Unplug aftermarket trailer modules to rule out backfeed or overload issues.
• Scan for BCM codes on newer cars; a stored fault may point directly to the issue.

These steps resolve the majority of brake light complaints and help determine whether the problem is electrical load, a control module issue, or a simple component failure.

Bottom line

Most brake light switches do not need a relay. In stock configurations—especially with LEDs or BCM-controlled systems—the factory design handles the load. Add a relay only when you’re introducing additional current draw or protecting older switch designs, and wire it with proper fusing and weatherproofing.

Summary

A relay is not normally required for brake lights: older cars route lamp current through the switch, and newer cars let a BCM or solid-state driver power the lamps. Use a relay only for added loads (trailer lights, extra lamps), voltage-drop mitigation in custom builds, or to protect aging hydraulic switches—and avoid adding one in monitored CAN/BCM systems unless you follow the vehicle’s wiring strategy.

How to tell if a brake light relay is bad?

You know your brake light relay might be bad if your brake lights don’t work at all, work intermittently, or stay on constantly, especially when other symptoms like a gear shift being stuck in Park or a disabled cruise control also appear. To diagnose a bad relay, first check fuses and bulbs, then test the relay itself by listening for a “click” when the brake is pressed, swapping it with a known-good relay of the same type, or using a multimeter to check for proper continuity and voltage.
 
This video demonstrates how to test a brake light switch: 58sCars SimplifiedYouTube · May 12, 2018
Symptoms of a Bad Brake Light Relay

• No or intermittent brake lights: This is the most common symptom, where the brake lights don’t illuminate when the pedal is pressed or only work sometimes. 
• Brake lights stay on: A fault in the relay can cause the lights to remain on even when the car is off. 
• Inconsistent pedal responsiveness: The brake pedal may feel unresponsive. 
• Cruise control malfunction: The cruise control may not work correctly because it relies on the brake light switch and relay system to deactivate it. 
• Stuck in Park: The vehicle’s gear selector might be unable to move out of Park, as a faulty brake light switch or relay can prevent the safety interlock from activating. 

How to Test for a Bad Relay

1. Check other components: Opens in new tabBefore testing the relay, make sure the problem isn’t with the brake light bulbs, fuses, or the brake light switch itself. 
2. Listen for the click: Opens in new tabWhen you press the brake pedal, the relay should make a distinct “click” sound as it’s activated. 
3. Swap with a matching relay: Opens in new tabFind another relay with the same part number in your vehicle’s fuse box (such as for the horn) and swap it with the suspected brake light relay. If the brake lights work, your original relay is bad. 
4. Use a multimeter: Opens in new tabA multimeter can test the relay for continuity between its terminals. You can also check for proper voltage at the relay socket. 

This video demonstrates how to test a relay with a multimeter: 57sEricTheCarGuyYouTube · Aug 31, 2012
Important Considerations

• Safety: Always ensure your vehicle’s ignition is on when checking the circuit. 
• Intermittent problems: A relay that works sometimes and not others is harder to diagnose, making the swapping method or a multimeter more useful. 
• Professional help: If you’re unsure or uncomfortable performing these tests, consult a qualified technician. 

Do brake lights need a relay?

No, most factory-installed brake light circuits do not use a relay; they are powered directly from the brake light switch through a fuse. However, a relay is a good addition to a brake light circuit when a significant current is drawn, such as with incandescent bulbs or when installing supplemental braking systems for towed vehicles, to protect the switch and improve the circuit’s reliability and voltage.
 
Why a relay is often not needed (stock configuration)

• Direct Power: Opens in new tabIn most standard vehicles, the brake light switch directly powers the bulbs after receiving power from the fuse box. 
• Lower Current Draw: Opens in new tabMany modern vehicles use LED bulbs, which have a much lower current draw, eliminating the need for a relay to protect the switch from high amps. 

When a relay is a good idea

• Incandescent Bulbs: Opens in new tabOlder vehicles or those with incandescent bulbs, which draw more current, can benefit from a relay to prevent the brake light switch contacts from burning out. 
• Towed Vehicles: Opens in new tabA brake light relay is essential for towed vehicles to ensure proper operation of the braking system and to prevent over-current to the vehicle’s original brake light switch. 
• Increased Reliability and Voltage: Opens in new tabRelays offer improved reliability and can reduce voltage drop by taking the high-current load off the switch, ensuring brighter and more consistent light output. 

How to tell if you need one

• Check the voltage: Opens in new tabMeasure the voltage at the battery and at the bulb connector when the brake lights are on; a difference of more than 1 volt indicates a voltage drop that a relay can fix. 
• Check your fuse: Opens in new tabIf your brake lights aren’t working, first check the fuse and bulbs before assuming you need a relay, as a failed fuse or bulb is a more common issue. 

Where does the brake light switch get power from?

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.

Do I need a relay if I have a switch?

If you need to activate a heavy load, high current device using a small, light load switch – you will need a Power Relay!