Why do most modern vehicles with LED lighting still use incandescent bulbs in the turn signals?

by Vlen Feng
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Why do most modern vehicles with LED lighting still use incandescent bulbs in the turn signals?

Question: “Why do most modern vehicles with LED lighting (brake, running, interior, headlights, etc.) still use incandescent (filament) bulbs in the turn signals?”


You would have discovered the answer to your own question if you had ever tried to replace the incandescent bulbs in you car or motorcycle headlights with LED’s.

  It is very easy to directly replace the bulbs in every other system of your car or motorcycle. One can be substituted for the other. And with the LED you have a longer lasting, somewhat brighter bulb, that draws a lower amperage. 

That is not the case if you attempt to replace the bulbs in the directional signals with LED’s. When you attempt that you will run into several problems. The first of which will be that the directionals will no longer flash. Why?


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 Well what makes the directionals flash is a sort of automatic on-and-off switch that repeatedly sends current to the bulb, momentarily stops it, and then energizes the bulb’s filament again until you or the mechanism in the steering column moves the directional signal lever to the off position. This “flasher” used to look something like this. 



Internal structure:

Internal structure of flasher

It is a make-and-break relay, a lot like a doorbell buzzer, that uses an electromagnet to open and close the contact points that turn your directional signals repeatedly on and off. The directional signal flasher relies upon the amperage draw and resistance of the directional signal’s bulbs to work. The clicking you hear in cars that used these flashers was produced by the opening and closing of the flasher’s contact points.

A serendipitous benefit of these flashers was that, if the resistance was reduced because one of the directional signal bulbs on the same side of the car burned out, the speed at which those contact points opened and closed would increase and you would see the dashboard directional signal indicator flash at double its normal rate and hear the flasher click rapidly. This warning was not purposely designed into the system. Rather, it was simply a function of how these things respond to different resistances.

If you replace the bulbs in the directional lights with LED’s, which draw a much lower amperage and have much less resistance than incandescent bulbs, those flashers simply will not work.

But on modern cars things have gotten a bit more complicated. The directional signal flasher is now found somewhere in a part referred to by some manufacturers as a body control module that looks more like this.

 body control module

Switching from incandescent bulbs to LED’s would require redesigning the body control module.

  (If your directional signals are controlled by a body control module, rather than by an electro-magnetic flasher, the clicking you hear when you use your directional signals is generated by the module and reproduced by an audio speaker. It effectively imitates a sound that no longer exists which we have gotten used to associating with directional signals. Sort of like programming your iPhone to make a ringing sound when someone calls you.)


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  Of course there would be no cost benefit to redesigning the body control module, which communicates with and is integrated into other computers and systems, merely to accommodate a different type of light bulb. Thus, incandescent bulbs were often retained in directional signals after LED’s had been substituted for other lighting functions.

  However, as models are redesigned and their body control modules are updated to accommodate the low resistance/low amperage LED’s, manufacturers are switching from incandescent directional signal bulbs to the longer lasting and more reliable LED’s.

  The use of incandescent directional signal bulbs in cars that used LED’s for other purposes was an interim solution used until new electrical systems were designed for new models. 


 Source: Quora



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by Vlen Feng


Andy S.
Andy S.

You should do some research before blindly performing a cut and paste form Quora. This article is rubbish. Modern automotive flasher circuits can easily and inexpensively be controlled by an electronic rather than thermal flasher which will provide reliable control of LED turn signal bulbs. Case in point is the EP27 flasher from Novita which is a direct plug-in replacement for the standard thermal relay in many, many cars, and which substitutes electronics for thermal control to allow LED bulbs to directly replace incandescent bulbs.
The cost is similar to the thermal flasher, and it does not require additional resistors to eliminate the phenomenon commonly referred to as “hyper-flashing.”

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