HomeTips & GuidesBrake Light Relay: When do you Need one, and How do you Install One?

Brake Light Relay: When do you Need one, and How do you Install One?

The brake light relay is a crucial component of any vehicle as lighting up the brake lights signals the other drivers that you are slowing down. 

Earlier, the brake line system based on hydraulic pressure was unreliable. However, introducing the brake light relays has made this system reliable and long-lasting. 

This article will discuss brake light relays and how you can install them in your vehicles.

Table of Contents

What is the function of a brake light relay?

The brake light wiring system is quite simple. Bulbs in their sockets are connected to the wiring harness at one end and the brake switch at the other. 

When you press the pedal down, it creates a contact, completing the circuit. This entire wiring system draws its power from the battery of the car.

As the hydraulic brake system was unsuccessful, manufacturers switched to a more reliable system,i.e., a mechanical brake light switch system. 

In this system, a brake switch rarely fails as it has a relay that allows a low-volt circuit to control the primary power supply circuit for the brake switch.

Innumerable wires run from the battery to the fuse block, the fuse block to the brake light switch, and then from the switch to the brake lights. 

You may also find the wires running through the body controller at times. All the wiring remains protected in a harness that runs through the whole vehicle.

Relaying your brake lights:

  • First, you must get a Bosch-type relay and its socket from any local spare parts store. You can also get a relay with a built-in mount. 
  • Then, get some wires and match the color codes. 
  • Also, get your soldering iron ready or get a good crimper to make the connection.
  • Before connecting a relay, let’s understand the relay itself. 

Look at the terminal end; there will be four or five terminals. Out of these, three are in one direction with the edge; one is at 90 degrees to the edgel. Hold the relay, keeping this terminal at the bottom. Now, look closely.

The terminal end of the relay

The terminal end of the relay

Did you notice number 30 at the bottom terminal? There will be terminal 86 to its left upper side, and opposite will be terminal 85.

 Lastly, at the top is terminal 87, which is 90 degrees to the rest.

Sometimes, there is another terminal in the middle (87a or 87b); ignore it. Most square relays come with this wiring, but not all. 

You will find similar-looking relays in BMA and Ford, but as their wiring is different, you cannot swap. However, the numbering means the same for all.

So, when you know the relay terminals, it will be easier to wire the relay.

In the present example, we take the Ford Falcon car (1960), where the hydraulic brake switch is mounted near the main cylinder. Now, follow these steps:

  1. Pull all the wires off the brake switch.
  2. Cut wires closer to the brake switch, leaving 1-inch wires after the connectors.
  3. Identify the hot wire in the harness with a test light.
  4. Connect this wire to the numbers 30 and 86 of the relay terminals.
  5. Connect the wire going to the brake lights to the relay terminal no. 87.
  6. In the brake light switch, connect one lead to the ground and the other to the relay terminal no. 85.
  7. With this, the relay wiring completes.
  • Now, it’s time to mount the relay and plug in the connectors. 
  • Test the brake lights by pressing the pedal. As you press, you will hear a click sound from the relay, and the brake lights turn ON.
Relay switch wiring diagram

Relay switch wiring diagram

Other conditions when you need a brake light relay:

If you tow your vehicle with a six-diode system connected to an RV, the indicator lights will turn ON when you use the RV’s turn signals.

However, in the case of a 4-diode system, the car’s brake signal overrides the turn signal when the RV braking system presses the brake pedal of the towing car.

As only the brake lights turn on, you only know that the driver is slowing down but cannot understand if he is turning.

However, if you install a brake relay in the towed vehicle, the car’s brake signal cannot override the turn signal of the RV. As a result, towed car lights always work according to the RV signals.

Relay installation:

Mostly, towed cars use the normally open method of installation. However, Lincoln and Ford models come with neutral tow sets, and most vehicles with push-button start need normally closed installation.

For example, we will take the Roadmaster’s Brake-Lite Relay Kit (brake light).

Normally open installation:

The following steps demonstrate the normally open relay installation. However, you must also read the instructions that come with the Relay Kit for the detailed process:

  1. Firstly, set aside the blue wire from the kit, as you do not need it in normally open installations.
  2. Find out the brake light switch wire of the towing vehicle.
  3. Eliminate the brake light fuse.
  4. Cut the brake wire from the brake light switch’s cold side, leaving a few inches.
  5. Connect the relay to the RV braking monitor wiring.
  6. Connect the relay’s black wire to the brake light wire near the brake light with the butt connector (blue).
  7. Look for a fuse that gets power only when the ignition is in the “RUN” position but does not get power in the TOW position. Pull this fuse and insert it into the closet near the fuse blade. Insert the fuse tap into the fuse panel’s empty slot.
  8. Connect the brake relay wire to the fuse tap wire using the blue butt connector.
  9. The white wire from the relay should be grounded to the towing car’s frame.
  10. Finally, secure the relay with the help of wire ties.

Normally closed installation:

The following steps demonstrate the method of normally closed relay installation. 

Vehicles with neutral tow kit and push-button features that do not start when you remove the brake fuse need normally closed installation.

However, you must follow the instructions mentioned in the relay kit. The normally closed relay installation may need more wires and connectors than included in the kit.

  1. Firstly, set aside the green wire from the relay kit. However, if you use the Roadmaster brake braking system as a supplement, you may use this wire later.
  2. Find out the wire of the brake light switch in your towing car.
  3. Eliminate the brake light fuse.
  4. Cut the brake wire from the brake light switch’s cold side, leaving a few inches.
  5. Connect the relay to the RV braking monitor wiring. Here, you may require a wire.
  6. Connect the relay’s black wire to the brake light wire near the light with the butt connector (blue).
  7. Using a voltmeter, find the wire in the aftermarket brake light harness that carries the turn light signal.
  8. Connect a part of the 14-gauge wire to the red wire using a 16-14 gauge butt connector. Trick this new 14-gauge wire and attach it to the aftermarket harness.
  9. Now, turn on the headlights after starting the engine. Here, test your tail light wire voltage using a voltmeter. If there is no voltage, you can move to step 10. If it registers 12 volts DC voltage, you must install a single diode (RM790 separately sold diode).
  10. The white wire from the relay should be grounded to the towing car’s frame.
  11. Finally, secure the relay with the help of wire ties.
Bosch-type relays

Bosch-type relays


Installing a brake light relay in both conditions becomes easy with these steps. 

However, you can do the job much easier if you have high-quality products like a relay kit or custom relay socket/pigtail wiring harness.

For superior products, contact Cloom, as we manufacture high-quality wiring harnesses for automobiles and other electronics. 

Get in touch with our engineers for a detailed discussion regarding your requirements.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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