When my taillights are on, my brake lights don't work. How to fix?

October 6, 2007 8:06 PM

While my taillights are off, my brake lights work just fine. When they are on, my left one goes out. How can I fix this? I'm driving a mid 80's Volvo. My car uses the same bulb for taillight and brake-light. Basically, while my taillights are supposed to be on, and I step on the brakes, that entire bulb turns off (on the left side).

Head/Tail lights switched OFF, foot ON the brake, brake light is illuminated. (as it should be)

Head/Tail lights switched ON, foot off the brake, tail light is illuminated. (as it should be)

Head/Tail lights switched ON, foot ON the brake, tail/brake light bulb is out. (broken!)

posted by k7lim to travel & transportation (7 comments total)

Something similar to this happened with my '97 S90. Try putting in new brake lights on both sides. This is apparently a common issue.
posted by J-Train at 8:11 PM on October 6, 2007

To clarify, only the left bulb goes out when I step on the brakes with the taillights on.
posted by k7lim at 8:12 PM on October 6, 2007

Almost sounds like a ground issue. If you have a multi meter you can check to see if the ground wire came loose.
posted by Climber at 8:48 PM on October 6, 2007

Yep, it was my left bulb, too. Still recommend the dual-replacement.
posted by J-Train at 9:11 PM on October 6, 2007

Replace both bulbs at the same time. If the problem continues, you may have a bad relay. Find the relay panel, swap the right and left relays, and see if the problem remains.

If it afflicts the opposite side, you have a bad relay.

If it afflicts the same side, you have a wiring fault, or a bad "control unit" (if your car even has a "control unit", many/most do not).
posted by Kwantsar at 9:18 PM on October 6, 2007

Seconding Climber - my trailer does this when it doesn't have a good ground.
posted by rfs at 9:21 PM on October 6, 2007

This era of Volvos is known for flaky relay soldering that goes bad over time. If you're technically inclined, you can even pull the relay apart and test/resolder the connections. Free DIY!
posted by trevyn at 11:26 AM on October 7, 2007