Why does my turn signal blink faster when one of them goes out?
October 19, 2006 6:30 PM   Subscribe

Why do my turn signals blink faster if one of them goes out?

This my guess is that it's because the blinking is done by a contracting and expanding bimetal contact point that makes and breaks contact periodically as it cools down and warms up, respectively (which could also be making the ticking sound). When one of the lights goes out, it's drawing half as much current, so somehow that is changing the rate at which the bimetal strip conducts heat.

That, or assuming the lights are in parallel, the resistance changes, which then changes the circuit as a whole -- maybe if there's something like a 555 timer controlling the frequency, and the resistor ratio changes it causes the frequency to double.

Can someone please verify this for me? Gruesome details would be awesome.
posted by spiderskull to Science & Nature (14 answers total)
My car's instruction manual tells me that it's a 'feature' to make me aware of the 'hazard' of driving without a turn signal.

Or it's a crazy circuit freakout.
posted by donguanella at 6:38 PM on October 19, 2006

Your heat-tripping relay descriptions matches my understanding of how analog blinker units work.
posted by xiojason at 6:38 PM on October 19, 2006

According to Wikipedia:

It is also required that audio and/or visual warning be provided to the vehicle operator in the event of a turn signal's failure to light. This warning is usually provided by a much faster- or slower-than-normal flash rate, visible on the dashboard indicator, and audible via the faster tick-tock sound.

I don't know the mechanism by which the faster/slower flash rate is caused, but it is a deliberate feature.
posted by katemonster at 6:40 PM on October 19, 2006

Modern cars use an electronic relay. If the relay thinks that a bulb is out (high resistance on one of the circuits), it intentionally gives you the fast blinking as an indicator.
posted by jellicle at 6:47 PM on October 19, 2006

My very old VWs did the exact same thing. I don't know if it was intentional, but I always knew it was time for a new bulb-somewhere.
posted by snsranch at 6:48 PM on October 19, 2006

My old Tempo would just light up instead of blink.
posted by cellphone at 6:55 PM on October 19, 2006

I usually use it as a cue to buy a new car.
posted by drewcopeland at 7:08 PM on October 19, 2006 [1 favorite]

I asked about this yesterday when I was test driving a car, and my mechanic told me that the replaced lightbulbs in the turn signal probably had a different resistance. In the car I was testing the blinker flashed faster when I stepped on the brakes.
posted by terrapin at 7:30 PM on October 19, 2006

terrapin -- doesn't sound like a car I'd want to buy...
posted by spiderskull at 7:39 PM on October 19, 2006

The higher resistance answer was given by the Car Talk guys to a recent caller as well.
posted by of strange foe at 9:32 PM on October 19, 2006

It is, indeed, purposeful.

I put LED taillight bulbs in my 95 caravan while I still had it; same result.
posted by baylink at 10:21 PM on October 19, 2006

IIRC old Volkswagens have something that looks like a Snapple lid to control the blinker. It works liked the the submitted suggested. When it gets hot it snaps out and connects the lightbulb circuit, when it cools down it snaps in and breaks it. Having a burned out bulb screws with the timing.
posted by sideshow at 9:00 AM on October 20, 2006

You can think of blinkers as a light with a fuse attached to it. Current runs through the circuit lighting the turn signal. As the current flows, it heats the fuse to some critical temperature, which then clicks the circuit off. After a short period, the switch closes again and the process is repeated.

When one blinker is out, more current is flowing through only one of the paths, thus switching faster.

How a Blinker works
posted by Andrew Brinton at 3:19 PM on October 20, 2006

How a Blinker Works

now with working link
posted by Andrew Brinton at 3:21 PM on October 20, 2006

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