Skip the auto shop - Our mechanics make house calls

Q: Break light goes out when headlights turn on.

asked by on

I replaced the socket and bulb, but my break light still does not work with headlight on. Works fine with headlights off.
My car has an automatic transmission.

A: This is most often a result of a bad ground...

This is most often a result of a bad ground. Your lighting has common grounds in several spots on the car. In order to locate them you will need a wiring diagram. Keep in mind, not all wiring diagram give ground locations. You may need to access Mitchell or AllData information systems to find this information. Of course, a factory repair manual is the best source of information, but the most expensive.

As unlikely as it seems, another bulb could have an issue that could be causing this. Your exterior lighting uses common circuits. When any part of the circuit has a problem, it causes problems in other areas. Guessing where a problem will show up isn't really possible. Just know that I have seen the wrong bulb installed in the front of a vehicle and it has caused problems at the rear. I recommend checking all of your bulbs. If any of them are not correct or if the sockets, grounds or power supplies are insufficient, problems can and will arise elsewhere. When checking all your bulbs, look for anything else that isn't factory or working as it did from the factory and start there.

Aside from these possibilities, the next component that is suspect is your combination switch mounted in your steering column. When you turn your headlights on, there are more circuits being powered that will interact with each other. Much like a bad ground can affect random parts of the lighting system, so will a crossed wire or bad connection in a circuit of your combination switch.

Often, the most effective method to send you in the correct direction is a little like CSI. Look for anything that just does't seem right. Is the combination switch flopping around, or just doesn't seem like it should? Maybe the high beams don't work anymore. Is there a damaged lens assembly on the outside of the car? Has it been in an accident. More often than not these types of clues are key in finding what is causing the problem.

If this doesn't help you find the problem, I recommend having a certified technician perform a diagnostic of your electrical system to help you figure this out.

Good luck!

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Dash light flicker, light switch hot

Hello there, thank you for asking about your 1984 Oldsmobile Cutlass Calais. There is the possibility of a cross connection inside the switch due to a failure in the assembly or corrosion going across the switch contacts. This may explain...

Q: Problem with brake lights

Hi there. If the brake lights are always staying on, then you most likely have a faulty brake light switch. On the Jeep Grand Cherokee's this is a common as the switch ages. In extreme cases, the switch will always...

Q: Faulty brake lights in a Hyundai Getz

Hey there, thanks for writing in. If you have the center light working, then the brake switch is okay and you most likely have two bad bulbs in the lights. If you need help having this repaired, a certified pro...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.