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On my 1999 Ford Escort, the brake lights double as turn signal blinkers. A left brake light burned out recently and my blinkers went much faster than normal when signaling a left turn. This is a convenient (and embarrassing) way to be alerted to a problem with my brake lights. Is this something that the designers had to add extra components to make happen or is this a natural failure mode of a simple circuit?

More specifically, what makes the blinker cycle go faster when one of the lights has burned out?

I have no background in electronics. I tried to research this and guess that the brake lights have a low resistance shunt wire that decreases resistance when the bulb burns out (as opposed to leaving an open circuit like a normal light bulb). This decreased resistance would increase current, causing a bimetallic strip in a thermal flasher unit to heat up faster, making the blinker go faster. However, I don't see a shunt wire in the light bulb and this doesn't explain why the bimetallic strip cools off faster when a bulb has burned out. So the only thing my research shows is that I don't understand electronics.

jsotola
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David Costanzo
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    It's like airplane engines: The remaining blinker is doing twice the work, obviously! – Peter - Reinstate Monica Sep 27 '22 at 15:37
  • "the brake lights double as turn signal blinkers" What? Are they red or orange? Or do they change color somehow? – AndreKR Sep 27 '22 at 18:00
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    When both sides light up, that's the brake. When one side lights blinks, that's the blinker. I see them all the time, they're not super uncommon. We just don't think about them. – Mooing Duck Sep 27 '22 at 20:36
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    @AndreKR It's just an American thing which sounds super confusing to Europeans. Technology Connections has [a video about it](https://www.youtube.com/watch?v=O1lZ9n2bxWA). – gronostaj Sep 28 '22 at 06:09
  • @AndreKR there are two red lights brake lights on each side. When you signal left, both of the left red brake lights blink (and remain red). It's so common to have the red brake lights blink in the U.S. that I never thought there was anything weird about it until yesterday. In my case, one of the brake lights on the left had burned out. – David Costanzo Sep 28 '22 at 13:47
  • @DavidCostanzo In most of the world, turn signals are required to be yellow and brake lights red, so they can't share the same light. The US is weird in that regard. (Many American cars also have separate yellow turn signals too, but for some reason a lot of them don't.) – Hearth Oct 04 '22 at 14:24

5 Answers5

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The current when two lamps (front and rear) are on is higher than when one is burnt out. This heats up the bimetallic strip to a higher temperature causing it to bend more. The strip keeps bending after the switch opens, but because the energy stored is higher it takes a long time to cool down. When only one lamp operates, the temperature of the strip is not as high and does not keep on bending as much after the switch opens so the on time is shorter and the off time is shorter making it blink faster. It was mechanism to indicate that one lamp was burnt out.

Your misunderstanding was not about electronics but about the thermal-mechanical mechanism.

Update: The 99 Ford escort uses an electronic flasher module. It will use current flow to determine the condition of the lamp, then adjust the flash rate accordingly. Here is the wiring diagram. Check page 25.

RussellH
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    Surely modern cars are not still using bimetallic strips. They might be designed to simulate the same effect electronically because people are used to it. – user253751 Sep 26 '22 at 02:11
  • @user253751 I don't know when bimetallic strips were phased out. As long as the filament lights were used, there was no reason to change. LED lights just starting in the late 1990s. Electronics took big leap in cars in the early 2000s. So now a small micro with current sense could be used. – RussellH Sep 26 '22 at 02:21
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    Electronic flasher units were coming along in the mid 80’s and a few cars still had the thermal units. – Solar Mike Sep 26 '22 at 06:08
  • A bimetallic flasher will only sense if any of the blinker lights are out. – Oskar Skog Sep 26 '22 at 07:47
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    @user253751 is correct. On some cars advanced diagnostic tools can even configure the level of current needed to trigger "hyperflash" – Selali Adobor Sep 26 '22 at 23:29
  • My car has an audio cassette player, window cranks, and a stick shift, so I'm disappointed it has a modern flasher unit. I'm more interested in the thermal flasher, though, since computers defy all logic and do whatever they want. I can somewhat understand the explanation of the wire continuing to heat up after the switch is thrown, but why is there more current when one lamp is out? If they're wired in parallel, then when one lamp blows, resistance should increase, which should decrease current, right? Is there a shunt that makes the opposite happen? Can you elaborate on that aspect? – David Costanzo Sep 27 '22 at 05:10
  • @DavidCostanzo you've got it backwards: the current does indeed go down when a lamp blows, but a higher curren causes a *slower* flash rate. Think of it like a swing: if you push it harder, it goes further past the netural point, and the frequency actually goes *down*. – 2e0byo Sep 27 '22 at 09:17
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    @user253751 Modern cars use LEDs so there's less heating anyway. And don't call me Shirley. – Carl Witthoft Sep 27 '22 at 11:12
  • @DavidCostanzo: Higher current stores more thermal energy. It takes more time to cool down. – RussellH Sep 27 '22 at 17:40
  • Presumably, a bimetallic strip would be affected by the ambient temperature, so your blink speed would likely be very different in cold weather vs. hot. Likely a reason they were phased out. – Darrel Hoffman Sep 28 '22 at 14:53
  • @DarrelHoffman: I think a bigger issue would be that replacing a light bulb on one side of the car with one that has different characteristics, without doing likewise on the other side, would cause the left and right sides to blink at different rates even when all bulbs were working. Historically, there would be no reason not to have bulbs with matching characteristics, but even things like halogen bulbs might not match the behavior of conventional bulbs. – supercat Sep 28 '22 at 17:05
  • @supercat Can't imagine too many people would replace their bulbs with halogens (or LEDs etc.) on *only one side*. If you're just replacing one bulb because it blew out, you'd probably use the same type that was there before. If you wanted to switch to halogens etc., you'd replace both sides, even if one or both of the old ones were still working. – Darrel Hoffman Sep 28 '22 at 18:03
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Modern electronic flasher units flash at a higher rate as a design feature in order to alert the user to a dead 'bulb'.

What was once a side effect is now a design feature'.

ATCSVOL
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    Correct answer, but needs further explanation: The reason used to be that the flasher circuit was just a relay, a 'resistor' and a capacitor or bimetalic strip. The resistor being the repeater lamp inside the cabin. The flasher lamp filament resistance took current from the capacitor/bimetal charge/temp, slowing it down. When the bulb or circuit was faulty that capacitor charged quicker making the repeater lamp flash faster alerting the owner of the fault. – Jay M Sep 26 '22 at 10:54
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    Incidentally, was this ever mandated, or did it happen de facto? – 2e0byo Sep 26 '22 at 13:42
  • Old flasher unit without any electronics did the same. It never was a side effect, it was stipulated. – Uwe Sep 26 '22 at 14:04
  • @Uwe: It's the older units where the behavior would have been a "side effect". It may be that as soon as one vehicle's flasher was found to work that way, efforts were made to mandate such behavior for all of them, but the circuit design predates the recognized usefulness of the feature. – supercat Sep 26 '22 at 16:13
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    First fast blinking existed as a desirable side effect, then it became a deliberate feature, then it was made a requirement. – barbecue Sep 26 '22 at 19:07
  • Huh, a bimetallic strip. I would never have thought that. My first thought was, "probably something to do with RC time constant because when one bulb blows, the circuit's resistance will decrease". A bimetallic strip would have the negative effect of being very sensitive to the climate, wouldn't it? It would have a very different characteristics in the summer/winter. Or if you went somewhere like El Salvador (warm) or Greenland (cold). – Lorraine Sep 27 '22 at 13:43
  • @OmarL: The flash cadence of a bimetallic blinker at +40C ambient would likely be noticeably different from its cadence at -40C, but so what? What was noticeable when a bulb burned out was not so much that the blink on the side with a burned-out bulb was at a weird cadence, but rather that there would be a mismatch between the cadence of the left and right flashers. – supercat Sep 27 '22 at 21:13
  • @supercat and also a difference between the bulb being fine one day and not the next. IIRC the UK spec allows a factor of 2 variation in flash rate (not that we get that kind of temperature swing anyway: -10°C to 30°C would cover nearly everything). Anyway the flasher unit probably isn't at ambient temperatures for long. The one I've handled was accessed from the cabin, plus there's self-heating. – Chris H Sep 28 '22 at 15:43
  • @ChrisH: My point was that if ambient temperature changes by a huge amount between one time a car is used and the next, the blinker cadence might change enough that the driver would notice it, and might wonder if a bulb is out, but if both the left and right signals blink at the same rate that would suggest that either all lights are good or both sides are missing the same number of bulbs (with the former being more likely). If the rates don't match, then one side likely has a non-functional bulb, though someone would have to look at the lights outside the car to determine which bulb is out. – supercat Sep 28 '22 at 17:02
  • @supercat yes, but I don't believe you'd get such massive temperature swings over a short enough time that you'd remember the frequency. 20°C in a day or two is quite a lot and wouldn't have a massive effect. You might get a bit more in a dessert, but still not enough. To get the range you mention, even in the few places it happens, would take months. – Chris H Sep 29 '22 at 05:28
  • @ChrisH: Going all the way from -40C to +40C would be very rare, but many places can have rather large temperature swings between a late after noon one day and the following morning. – supercat Sep 29 '22 at 14:40
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While all previous replies are correct, a clearer (and certainly more entertaining) explanation is this video from Technology Connections.

Bottomline: Properties of bimetallic flashers present slight variations across the same batch. The bulb, being part of the circuit, will also change the overall time constant when its resistance changes due to degradation of the filament.

Daniel Melendrez
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    His videos are always amazing and educational! – Doktor J Sep 26 '22 at 17:46
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    Indeed! They are like _watching_ a page from an Encyclopaedia. The thing I love is the length of his videos and the depth of the info. – Daniel Melendrez Sep 26 '22 at 19:17
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    +100 for linking that video! – FreeMan Sep 27 '22 at 13:08
  • @FreeMan bring those points, my man! – Daniel Melendrez Sep 27 '22 at 13:31
  • Did you notice that the pinned comment on the video says: "I was surprised to learn that the thermal flasher doesn’t change speed depending on how many lamps are in-circuit. I thought that this was just a thing all flashers have always done. But once I worked out how this particular flasher functioned, I realized that of course it doesn’t hyperflash with a bulb out because that’s just not possible with its design." So it seems like the author of the video thinks that a bimetical strip doesn't hyperflash. – David Costanzo Oct 09 '22 at 00:33
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The changing of the flashing rate of the turn signals is a federal mandate in the US and many other countries. In the US, it falls under §571.108 of the Federal Motor Vehicle Safety Standards. Specifically, S9.3.6 states:

Turn signal lamp failure. Failure of one or more turn signal lamps such that the minimum photometric performance specified in Tables VI or VII is not being met must be indicated by the turn signal pilot indicator by a “steady on”, “steady off”, or by a significant change in the flashing rate, except when a variable-load turn signal flasher is used on a multipurpose passenger vehicle, truck, or bus 2032 mm or more in overall width, on a truck that is capable of accommodating a slide in camper, or on any vehicle equipped to tow trailers.

Here is a federal recall covering the tick-tock of the flashers: MV-1 Recall #R1806 TURN SIGNAL BULB OUTAGE – INDICATION TO OPERATOR. It references several others. Another link that may be interesting: General Motors Service Bulletin 07-08-42-006O.

If my memory serves me this started to become law around 1970.

The terms of the Physics/Electronics on why a blinker changes its rate cannot be answered without knowing the model and make of the blinker. The blinker the OP has could be OEM or aftermarket. There are many sources of these. They may be bi-metal; however many are electronic by design. Some of these electronic designs are discrete in design while some utilize an IC. Many of the ICs used are custom and OEM proprietary. There are many designs posted online using the NE555 IC. US law states the tick-tock (blink rate), must make an appreciable change in a fault condition. There have been some changes to this over the years but the basic premise has held. Note the law does not state the technology that must be used. It also states they must be visible to the operator so if the operator cannot see the blinker there must be an indicator.

Experiment: Take either a front or back bulb out and watch the tick-tock rate change.

Toby Speight
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Gil
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  • I think @David Constanzo was asking more in terms of the Physics/Electronics on why a blinker changes its rate. Not a mandate from law to make them blink XD – Daniel Melendrez Sep 26 '22 at 19:22
  • With modern electronic controls, there's no reason why the pilot blink rate would have to be the same as the actual signal blink rate. Faster blinks could be disturbing to other drivers, so it could be reasonable to decouple them. – Barmar Sep 27 '22 at 16:37
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This problem occurs due to increase of flow of current in circuit. When lights are in good working form then circuit is calculated and current flow is also working properly, But when one or more lights are burnt out then circuit is altered as one of resistor not working and flow of current is going on without resistor so turning signals blinks faster then normal condition.

enter image description here

Usman Shabbir
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    Welcome! Please explain _"circuit is breached and flow running without resistor"_ with a schematic. – winny Sep 27 '22 at 12:10
  • resistor is used for blinkers and if light is burnt out then resistor of that light also not usefull for the circuit that's why due to lack or resistance blinks more faster then normal ones – Usman Shabbir Sep 29 '22 at 10:08
  • That didn't explain _"circuit is breached and flow running without resistor"_. Try with a schematic. – winny Sep 29 '22 at 11:07
  • https://www.mediafire.com/view/8jyjablybeqw69f/signal_3_3rd_light.gif/file – Usman Shabbir Oct 03 '22 at 07:49
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    In the future, please embed images in your answer, no external links. Since you are new here, I have done this for you. Your image does not explain "circuit is breached and flow running without resistor". Please show what current is flowing where without what resistor. – winny Oct 03 '22 at 08:56
  • There is a bi-metal switch which bends when current is put through it. A piece of metal acts as a switch. When current is passed through this metal, it heats up until it bends(because two pieces of metal contracts and expands differently when heated. Hence the name bi-metal). When it bends, current stops flowing and it cools until it bends back and makes contact. – Usman Shabbir Oct 04 '22 at 07:37
  • When both front and rear turn signals blink, the metal heats up more because of the increased current, so it takes longer time to to cool down and return to the initial position, where current is allowed through the metal piece. When only half the current flows, the metal doesn't get as hot, so it returns quicker to its initial position. – Usman Shabbir Oct 04 '22 at 07:37
  • This might be useful answer – Usman Shabbir Oct 04 '22 at 07:47
  • The "circuit breached" and "flow (assuming you mean current) without resistor" is still contradictory to me here when we talk about light bulbs in parallel and one of them going open circuit. – winny Oct 04 '22 at 08:49
  • oh my bad English is not my mother Tongue so i took circuit breach as overall problem now i came to know about that what is actual meaning of breach my bad i need to replace breached by disturbed ...... – Usman Shabbir Oct 04 '22 at 09:11
  • No worries. Better now, but "without resistor" in EE is interpreted as short circuit. You mean open circuit. – winny Oct 04 '22 at 09:29
  • Thanks for the correction – Usman Shabbir Oct 05 '22 at 04:59