My brake lights only work when headlights are on, and it can be pretty dangerous during the day. What shall I do? Here is the solution to this problem!
The main reason your brake lights work only when the headlights on your vehicle are on can be the wrong bulb fitted in the brake lights. There are two main categories of brake light bulbs, a single filament bulb, and a double filament bulb.
If you fitted a single filament bulb in place of a double filament bulb, then the brake light circuit gets connected to the headlight circuit, and when you turn on your headlights, only your brake lights work.
If you have recently replaced your brake lights and this problem started right after, then there is a high possibility that the wrong bulb is the main reason for your brake lights to malfunction.
Brake Lights Work Only When The Headlights Are On
Using the wrong bulb
Using the wrong bulb in a brake light is the main reason your brake lights work only when the headlights are on. There are two main types of bulbs, single filament, and double filament.
Most of the vehicles have the same bulb for tail and brake lights, and that is why they feature a double filament bulb with one circuitry for brake light and the other for tail lights.
If, while replacing or getting some work done on your vehicle, you have replaced the brake lights with the wrong bulb, then it is the main reason your brake light works only when headlights are on.
Using a single filament bulb connects the tail light circuit to the brake lights circuits, and that is why the light only works when you switch on the headlights. What you can do in such a case is remove the wrong bulb and replace it with the right one to solve this issue.
Using ill-fitting bulbs
Another reason for a brake light to work only when the headlights are on would be using a wrong fitting bulb inside the brake lights. A faulty bulb can also result in your brake light malfunctioning.
One of the filaments on your bulb may be touching the other filament, resulting in the power source from the headlight circuit flowing into the brake light circuit.
Faulty brake light circuit
The brake light circuit being faulty is another possible reason for this issue. The brake light and the headlight circuits may have shorted, and the power from one might be flowing into the other.
So when you switch on your headlight, that can result in your brake lights being on, and the brake lights won’t work on their own.
If the problem started recently, try to find out what might have caused the issue. If you had some work done on your vehicle, or you might have driven your car through water, any replacement parts on your vehicle might be the culprit behind your brake lights working only when your headlights are on.
If fixing all these issues does not solve your problem of brake lights working only when the headlights on your vehicle are on, then the problem may be something else. It would be better to get your vehicle checked out at a mechanic.
Frequently Asked Questions
Why does my brake light only work when my headlights?
One of the main reasons for your brake lights to work only when your headlights are on is using the wrong bulb in the tail light.
If your tail light supports a double filament bulb and you have replaced it with a single filament bulb, the brake light circuit gets connected with the headlight, and your brake lights will function only when the headlights are on.
Another reason your brake lights work only when your headlights are on can be due to a short circuit in the headlight and brake light wirings.
Why would my tail lights work but not my brake lights?
If your tail lights are working but your brake lights are not, then the most common reason would be a faulty bulb. The brake and tail lights are often the same light bulbs, but they have two distinct circuitry for different functions.
So if your brake lights are working and your tail lights are not, it is due to one of the circuits being burnt. There is also a high chance that some of the bulbs in your brake light may be blown. Check for blown light bulbs and replace if any.
Another reason for your tail lights to work and brake lights not can be due to a faulty brake light switch. A broken fuse can also be the reason for this problem. Use a multimeter to check whether or not the brake light switch is faulty.
How do you diagnose a bad brake light switch?
There are a few different ways to determine whether a brake light switch is bad. If your brake lights don’t work when you apply the brakes or if your brake lights are continuously on, it is due to a faulty switch.
Another few symptoms of a faulty brake light switch are the car not turning on or the ABS warning lights on your dashboard. You can check whether a defective brake light switch is the reason behind these issues by checking the switch with a multimeter.
If you get a connection when you release the brake light switch and don’t get a connection when you press the switch, then your brake light switch is functioning properly, and if it gives the opposite reading, it is not functioning properly.
Do brake lights and tail lights use the same bulb?
Yes, in most vehicles, brake lights and tail lights use the same bulb. The bulbs used inside the taillights have a double filament in such vehicles. One filament is for the tail light, and the other is for the brake light.
However, there are some vehicles with separate bulbs for tail light and brake light circuits.
A Few Final Words
The brake lights only work when the headlights on your vehicle are on because of fitting a wrong bulb in your brake lights.
Some of the bulbs are double filament, and if you replace them with a single filament bulb, that can result in incomplete circuitry, and that is why your brake lights will only work when you turn the headlights on and will complete the circuit.