# Rear lights go dark when brakes are applied... why? **URL:** \ **Category:** Factual Questions\ **Created:** [May 1, 2013, 5:57am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177 "2013-05-01T05:57:46Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Student\_Driver](https://avatars.discourse-cdn.com/v4/letter/s/3da27b/32.png) [@Student\_Driver](https://boards.straightdope.com/u/Student_Driver)\ **Post date:** [May 1, 2013, 5:57am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/1 "2013-05-01T05:57:46Z")
Luckily not my car, and an answer isn’t needed quickly. I just happened to be behind one of these types of cars while coming home for lunch break, and (since I drive primarily at night), I see these types of cars several times a night. Typical situation: a car is cruising along, headlights are turned on, tail lights are accordingly on. When the car comes to a stop at a red light, the tail lights will go dark… usually the third brake light will come on, but often all rear lights are dark. They come back on when the brake is released. What kind of wiring problem would cause something like this? The lights obviously work in some fashion… what could get messed up such that braking would break the circuit? ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [May 1, 2013, 6:32am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/2 "2013-05-01T06:32:02Z")
Fuse maybe. Or a wonky connection, maybe a ground? ---
**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\ **Post date:** [May 1, 2013, 9:22am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/3 "2013-05-01T09:22:32Z")
Question… have you noticed whether or not the cars have ‘after-market’ type, tail lights? I would hazard a guess that they do, and that said lights weren’t properly installed. I’ve noticed the same/similar conditions on cars in my area. IIRC, they looked like they had non original tail lights. Either that, or just ‘wonky connections’. 😕 ---
**Author:** ![Maserschmidt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/maserschmidt/32/18829_2.png) [@Maserschmidt](https://boards.straightdope.com/u/Maserschmidt)\ **Post date:** [May 1, 2013, 10:30am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/4 "2013-05-01T10:30:03Z")
Is the OP sees these types of cars several times a night, it seems like it would be something more common than a wonky circuit. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [May 1, 2013, 10:43am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/5 "2013-05-01T10:43:33Z")
Rear car lights typically use twin filament bulbs, usually [P21/5W](http://upload.wikimedia.org/wikipedia/commons/7/79/P21_5W.JPG) or some variation. The symptoms the OP describes usually happen when the two filaments inside the bulb come into contact. The solution is very simple: replace the bulbs. They are dirt cheap. Other symptoms might be that the front parking lights are always on, indications on the dashboard go awry, etc. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [May 1, 2013, 11:23am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/6 "2013-05-01T11:23:54Z")
I run into this fairly common on trucks. Broken or loose ground. The tail lights will ground through the brake light circuit until brakes are applied. ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [May 1, 2013, 3:25pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/7 "2013-05-01T15:25:52Z")
And in all likelihood, the driver of the vehicle is completely unaware that there is a problem. ---
**Author:** ![Student\_Driver](https://avatars.discourse-cdn.com/v4/letter/s/3da27b/32.png) [@Student\_Driver](https://boards.straightdope.com/u/Student_Driver)\ **Post date:** [May 1, 2013, 3:58pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/8 "2013-05-01T15:58:25Z")
Considering I live in a state that doesn’t require annual vehicle inspections, I imagine they’re not aware until they get pulled over. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [May 1, 2013, 4:23pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/9 "2013-05-01T16:23:57Z")
Oh, or the relay has gone wahoonie shaped. ---
**Author:** ![JerrySTL](https://avatars.discourse-cdn.com/v4/letter/j/e274bd/32.png) [@JerrySTL](https://boards.straightdope.com/u/JerrySTL)\ **Post date:** [May 1, 2013, 6:12pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/10 "2013-05-01T18:12:33Z")
Like Dog80 said, many brake light bulbs are twin filament and just the brakes light filament can burn out. First start by changing all the bulbs. They are cheap and probably need changing anyway if the car is older. If that doesn’t fix the problem, then worry about wiring, relays, and other expensive possibilities. Oh! If the cruise control isn’t working, changing the bulbs may fix that. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 1, 2013, 9:39pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/11 "2013-05-01T21:39:08Z")
> [@Dog80](#): > > The symptoms the OP describes usually happen when the two filaments inside the bulb come into contact. The filaments contacting each other (other than at their ground side, where they are already in contact) is exceedingly rare and will not cause the symptom described. > [@JerrySTL](#): > > Like Dog80 said, many brake light bulbs are twin filament and just the brakes light filament can burn out. This will not cause the taillights to go out when the brakes are applied. > [@HoneyBadgerDC](#): > > I run into this fairly common on trucks. Broken or loose ground. The tail lights will ground through the brake light circuit until brakes are applied. This is the most likely cause of the described symptom. > [@Student\_Driver](#): > > …what could get messed up such that braking would break the circuit? Braking doesn’t break the circuit. The normal circuit is already broken. Braking applies voltage to a part of the circuit that was acting as a substitute ground; when the taillight filament has 12v on both sides of it then it ceases to light up. ---
**Author:** ![Mikahw](https://avatars.discourse-cdn.com/v4/letter/m/e9bcb4/32.png) [@Mikahw](https://boards.straightdope.com/u/Mikahw)\ **Post date:** [May 2, 2013, 2:40am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/12 "2013-05-02T02:40:19Z")
How does the ground work through the brake light circuit? I’m envisioning that the brake light is attached to +12V via a momentary switch. When the switch is closed, one side of the brake light has 12 volts (the other side is hopefully grounded). When the brake light switch is open, it’s left floating so that there would be no path for the tail light current to travel, to ground or otherwise. Am I missing something? ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [May 2, 2013, 5:33am UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/13 "2013-05-02T05:33:08Z")
> [@HoneyBadgerDC](#): > > I run into this fairly common on trucks. Broken or loose ground. The tail lights will ground through the brake light circuit until brakes are applied. GRound! ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 2, 2013, 3:43pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/14 "2013-05-02T15:43:58Z")
> [@Mikahw](#): > > How does the ground work through the brake light circuit? I’m envisioning that the brake light is attached to +12V via a momentary switch. When the switch is closed, one side of the brake light has 12 volts (the other side is hopefully grounded). When the brake light switch is open, it’s left floating so that there would be no path for the tail light current to travel, to ground or otherwise. Am I missing something? It’s the third (high center) brake lamp that allows this. If the ground for the corner (left and right) tail/brake lamps is open, the current for the taillights travels through the left and right brake lamp filaments and then through the third brake lamp filament to its ground. I would expect the third brake lamp filament to glow dimly, but probably so dimly as not to be noticeable. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 2, 2013, 3:50pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/15 "2013-05-02T15:50:58Z")
**Gary T** nailed it. It’s a bad ground. Probably from a poor body shop repair. I once had a rental car that when you stepped on the brakes the dash lights and radio display went out. Looked in the trunk, yup ground wires not connected. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [May 2, 2013, 3:56pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/16 "2013-05-02T15:56:10Z")
All problems of this nature are known as ‘a short’. They aren’t necessarily caused by a short, but when something is wrong with a car’s electrical system it’s always called ‘a short’. So you have a short, which means nothing, maybe even the opposite of a short. Luckily you’ve received some good information here about the actual problem since it can cost a lot of money to fix ‘a short’. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 2, 2013, 4:01pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/17 "2013-05-02T16:01:01Z")
> [@TriPolar](#): > > All problems of this nature are known as ‘a short’. They aren’t necessarily caused by a short, but when something is wrong with a car’s electrical system it’s always called ‘a short’. No they aren’t called that by people that do this for a living. Only the uninformed call every electrical issue a short. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [May 2, 2013, 4:16pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/18 "2013-05-02T16:16:44Z")
> [@Rick](#): > > No they aren’t called that by people that do this for a living. > Only the uninformed call every electrical issue a short. That was my point. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 2, 2013, 4:19pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/19 "2013-05-02T16:19:16Z")
> [@TriPolar](#): > > All problems of this nature are known as ‘a short’. They aren’t necessarily caused by a short, but when something is wrong with a car’s electrical system it’s always called ‘a short’. So you have a short, which means nothing, maybe even the opposite of a short. Luckily you’ve received some good information here about the actual problem since it can cost a lot of money to fix ‘a short’. > [@Rick](#): > > No they aren’t called that by people that do this for a living. > Only the uninformed call every electrical issue a short. You’re both right. This one appears to be an open (the opposite of a short), but laymen tend to use the term “short” instead of the more accurate and clunkier “wiring problem.” On preview: as **TriPolar** has reiterated. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [May 2, 2013, 5:53pm UTC](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177/20 "2013-05-02T17:53:56Z")
Bulbs will often have grounds that are riveted instead of soldered or they will use one small connector to ground several lights and it will start to build up resistance and eventually stop making contact. Bad grounds can cause some screwy looking problems sometimes. [Next page](https://boards.straightdope.com/t/rear-lights-go-dark-when-brakes-are-applied-why/657177.md?page=2)