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.
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.
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