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Brake lights don't work when headlights are on

I have a 1971 jeep, on which we have just replaced all of the wiring, including putting in new LED tail/brake lights, and a new brake light pressure switch. If the headlights are off, the brake lights and directionals work fine. If the headlights are on, the tail lights work, directionals work, but the brake lights don’t work. Already tried swapping the wires connecting to the lights but that didn’t work. Any thoughts?


I’ve noticed on my trailers with LED’s that some of the lesser-quality LED’s are very voltage sensitive. If you’re testing the brake lights with the headlights on but the engine not running (and obviously the alternator isn’t charging), the brake lights may not work. That would explain why they work with the headlights off and the battery voltage is closer to 12v.

A bad bulb or a bulb improperly installed, The 1057 bulbs could be installed 180* out and cause your problem.

They do work, though; just not when the headlights are on. Would that still be the case if they were installed wrong?

You have a bad ground somewhere, probably in the headlight circuitry. Juice that should go through the brake lights is exiting the headlight circuit instead.

I had this same problem years ago with a '72 Fiat. Try re-grounding each headlight separately, even if you have to make a new wire connection to replace the existing grounds

I agree with SteveF. The trouble is most likely due to a bad chassis ground which will cause a voltage drop across the car chassis due to the high current draw of the headlights. Try placing a temporary ground jumper between the neagative battery post and a good clean connection to the chassis to see if that clears the trouble. If it does then clean the ground connection to the chassis.