The brake lights are one of the most important of our cars. They are in charge of indicating to the rest of the drivers that we are reducing the speed. If the brake lights are not turned on, we could suffer a range impact, besides that, we could be penalized and, of course, we would not pass the MOT.
Brake lights can fail for many reasons. The most common is that some bulb is blown, but when none of the brake lights work, one of two, or we are very left and we have all blown or have a problem in the electrical installation.
What to do if the brake lights do not turn on
If the brake lights of our car do not turn on, we will have to solve it as soon as possible. If they all have stopped looking at the same time, it would be strange that they have been melted at the same time, most likely there is a problem with the installation.
Brake light fuse
This point is the easiest to review. In the user’s manual of the car we can see where the fuse box is located and what is the fuse for the brake lights. If you do not find the user’s manual or do not clear it, you can always check the fuses one by one.
You have to take the fuse out of its location to check it. A quick glance should be enough to check if the resistor is broken. This is the small connection that joins the two metal plates, which can be seen through the transparent plastic. If you can’t see it clearly, you can also use a multimeter by following these steps:
- Set the polymer in continuity (function indicated by a speaker symbol and/or an arrow with a vertical line at the tip)
- Put the test leads by touching the fuse tabs
- If the polymer beeps, it means that there is a current, if not, it means that it is fused.
If this is the case, it will have to be changed to one of the specifications required by the manufacturer (a fuse costs pennies). That is to say, it must have the same number of amps as indicated at the top. If we put a fuse of fewer amperes, it will blow the next time the brake lights are turned on.
Watch out! It is possible that the blown fuse is not the cause, but the consequence of a larger problem. If there is a short circuit or a bad wire, it could be the cause of the blown fuse. If the fuse blows frequently, the electrical installation should be checked carefully.
Fused brake lamps
If only one of the brake lights has stopped working, it is most often a blown bulb. Usually a P21 24V type. The difficulty of replacing it with a new one depends much on the model and make of car, but in most cases the steps to be followed are always the same. Before starting we recommend you to disconnect the battery. Although this is not completely necessary, you should be careful when you are going to touch electrical components.
- The first thing we have to do is open the trunk to access the rear lights. In today’s cars, the most common thing is to have to disassemble the tail light, but you may also be able to simply reach the lamp holder, without having to disassemble anything. The light is usually attached with screws on the side and inside the cargo space.
- Whether you had to remove the whole light or not, you will have to dismantle the lamp holder. It is usually held in place with a flange or a screw.
- Once you have dismantled the lamp holder, remove the bulb in the same way as with a domestic lamp, by turning it counter-clockwise. In some cases, it may be necessary to press the bulb in order to turn it. This is a system to improve its fixation so that it does not come loose while you are walking.
- Check to see if the bulb is actually fused or has loosened.
- If the car is old you may want to clean the metal contacts of the bulb holder to remove dirt or rust. This will ensure that the new bulb makes proper contact.
- When fitting the new bulb make sure it is securely fixed.
- Reassemble the bulb holder on the pilot
- Place the pilot if you had to remove it.
- Start the car and press the brake pedal to see if the bulb works properly.
The brake pedal sensor
The brake lights come on when the brake pedal is operated. This acts on a button or contactor that closes the circuit of the brake lighting system, making the electricity reach the bulbs.
It is not uncommon for the brake pedal sensor or switch to fail. If we bend over and look at the bottom bracket, we will see the switch in question, with a button that goes in and out. The first check is to see if it has been removed or there is a defect in its placement. The switch must be pressed enough when the brake pedal is pressed. No it does not, you must adjust it so that it touches properly.
If that is not the problem, we can usually remove the sensor by hand by turning it halfway around and disconnecting the wires. Try to clean the contacts well because many times the problem can be solved with this alone. If it still does not work, it is advisable to check it with a multimeter. The steps to follow are:
- Set the multimeter in continuity, which is the option marked with the symbol of a speaker or an arrow with a vertical line at the tip.
- Place the multimeter test leads on the metal tabs of the switch. Many times there are only two, so there will be no chance of error. If you have four, put the wires in the ones that are opposite each other. They are usually easy to distinguish because they are the same color: a silver pair and a copper or bronze pair. In this case, the positive and negative polarity does not matter.
- With the wires placed in the tabs, press the switch to see if it opens and interrupts the current correctly. If there is currently the multimeter should beep. If, as we have told you before, there are four tabs, one pair will emit the beep with the switch pressed and the other will do so when it is not.
Warning: When you set the multimeter in continuity, do not use it in anything that has voltage: neither wiring in which there is current, nor batteries, nor capacitors, nor generators… Otherwise, the multimeter could be damaged.
If the multimeter does not ring, it means that the switch does not work. The best thing you can do is replace it with a new one because it is cheap and easy to replace the part. You can find one for your specific model for about 20 euros. Although with some premium models this figure can go up a bit.
To install it you just have to repeat the steps of its disassembly in reverse:
- Connect the cables firmly, until the clicks on the side tabs sound
- Place it in its slot where the defective sensor was
- Turn until it is fully engaged.
Problems in the electrical installation
If the bulbs, fuse and brake switch are not the solution to our problem, then we must look in the car’s electrical installation. We will have to check the main masses that can affect the brake system, as well as check that the electrical circuit of the brake lights is not cut in any of its parts. This is the most complicated diagnosis to make since the wiring of the car can be very intricate depending on the model. If you have no knowledge of mechanics we recommend that you take it to a workshop.