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.

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    \$\begingroup\$ It's like airplane engines: The remaining blinker is doing twice the work, obviously! \$\endgroup\$ Commented Sep 27, 2022 at 15:37
  • \$\begingroup\$ "the brake lights double as turn signal blinkers" What? Are they red or orange? Or do they change color somehow? \$\endgroup\$
    – AndreKR
    Commented Sep 27, 2022 at 18:00
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    \$\begingroup\$ 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. \$\endgroup\$ Commented Sep 27, 2022 at 20:36
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    \$\begingroup\$ @AndreKR It's just an American thing which sounds super confusing to Europeans. Technology Connections has a video about it. \$\endgroup\$
    – gronostaj
    Commented Sep 28, 2022 at 6:09
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Sep 28, 2022 at 13:47

5 Answers 5


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.

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    \$\begingroup\$ 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. \$\endgroup\$ Commented Sep 26, 2022 at 2:11
  • \$\begingroup\$ @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. \$\endgroup\$
    – RussellH
    Commented Sep 26, 2022 at 2:21
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    \$\begingroup\$ Electronic flasher units were coming along in the mid 80’s and a few cars still had the thermal units. \$\endgroup\$
    – Solar Mike
    Commented Sep 26, 2022 at 6:08
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    \$\begingroup\$ @user253751 is correct. On some cars advanced diagnostic tools can even configure the level of current needed to trigger "hyperflash" \$\endgroup\$ Commented Sep 26, 2022 at 23:29
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    \$\begingroup\$ @user253751 Modern cars use LEDs so there's less heating anyway. And don't call me Shirley. \$\endgroup\$ Commented Sep 27, 2022 at 11:12

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'.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Jay M
    Commented Sep 26, 2022 at 10:54
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    \$\begingroup\$ Incidentally, was this ever mandated, or did it happen de facto? \$\endgroup\$
    – 2e0byo
    Commented Sep 26, 2022 at 13:42
  • \$\begingroup\$ Old flasher unit without any electronics did the same. It never was a side effect, it was stipulated. \$\endgroup\$
    – Uwe
    Commented Sep 26, 2022 at 14:04
  • \$\begingroup\$ @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. \$\endgroup\$
    – supercat
    Commented Sep 26, 2022 at 16:13
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    \$\begingroup\$ First fast blinking existed as a desirable side effect, then it became a deliberate feature, then it was made a requirement. \$\endgroup\$
    – barbecue
    Commented Sep 26, 2022 at 19:07

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.

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    \$\begingroup\$ His videos are always amazing and educational! \$\endgroup\$
    – Doktor J
    Commented Sep 26, 2022 at 17:46
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    \$\begingroup\$ 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. \$\endgroup\$ Commented Sep 26, 2022 at 19:17
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    \$\begingroup\$ +100 for linking that video! \$\endgroup\$
    – FreeMan
    Commented Sep 27, 2022 at 13:08
  • \$\begingroup\$ @FreeMan bring those points, my man! \$\endgroup\$ Commented Sep 27, 2022 at 13:31
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Oct 9, 2022 at 0:33

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.

  • \$\begingroup\$ 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 \$\endgroup\$ Commented Sep 26, 2022 at 19:22
  • \$\begingroup\$ 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. \$\endgroup\$
    – Barmar
    Commented Sep 27, 2022 at 16:37

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

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    \$\begingroup\$ Welcome! Please explain "circuit is breached and flow running without resistor" with a schematic. \$\endgroup\$
    – winny
    Commented Sep 27, 2022 at 12:10
  • \$\begingroup\$ 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 \$\endgroup\$ Commented Sep 29, 2022 at 10:08
  • \$\begingroup\$ That didn't explain "circuit is breached and flow running without resistor". Try with a schematic. \$\endgroup\$
    – winny
    Commented Sep 29, 2022 at 11:07
  • \$\begingroup\$ mediafire.com/view/8jyjablybeqw69f/signal_3_3rd_light.gif/file \$\endgroup\$ Commented Oct 3, 2022 at 7:49
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    \$\begingroup\$ 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. \$\endgroup\$
    – winny
    Commented Oct 3, 2022 at 8:56

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