What causes a front blinker on a car to blink fast when the rear blinker globe blows?

Why does the front blinker blink fast when the back blinker globe blows.Is it somthing to do with the load of the circut.

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Aerospaced4 years ago
In older cars, it is a function of the bi-metal timing switch. The missing bulb has changed the resistance and heats the switch at a faster rate. This was not intended as a "feature", it was a happy accident.
It is common among street rodders to use different value bulbs to gain this effect for both front and back simultaneously.
DrRhodes4 years ago
I have had this happen to me on my old car. I know it was a simple fix to simply replace the blown bulb. In my case I want to say it was actually a dual element bulb and I got the doubled rate when one of the elements went.

I haven't actually looked at the circuit so any corrections are welcome but my instinct would be to say that the bulbs are somehow configured in the timing loop of the LM555 or whatever timing circuit is being used. When one of the elements goes it changes the trigger voltage and timing of the circuit and has the effect of speeding up the blink rate.

I can't remember how it was on mine but the duty cycle of the blinker on and off times was about 50/50. Did this ratio change as well as the rate of blinking? For instance, is the one time a short pulse followed by a longer off time?
I did some searching and it looks like I was a bit off on this one. While newer cars use an electronic control system to regulate the blinking operation, older cars rely on a thermal flasher unit. This unit wraps a fine wire around a filament positioned next to a relay contact. When the relay is open, current flows through the heating wire and causes the filament to heat up. In doing so it expands and makes contact with the relay terminal. Once in contact with the terminal, the current path is made much larger and it's resistance greatly decreases. This means that much less of the current flows through the heating wire and the filament is allowed to cool. When it cools to a certain level contracts enough to flip open. This turns off the blinker and puts the current flow back through the heating wire which starts the process over again. A diagram of this can be seen here:


If the load resistance is changed then the currents through the thermal flasher will be affected. This will cause the timing to be different because the ohmic heating and thermal dissipation rates are current dependent. Actually the dissipation is not directly current controlled but cooling rates to depend on the temperature differential between an object and it's surroundings so the link to current flow is indirect.This is a "feature" included to tell you when you have a blown bulb.

Here is another page I found with some diagrams and fixes for the problem:

You are right, in modern cars, the blinker bulbs (or now more often LEDs) are controlled by the micro controllers and switched by semiconductors. But still, most cars have a 'blinker tapper' (not sure if I chose the correct translation) - a kind of relay without any switch part. It is designed to be especially noisy to give the typical tick-tock sound. Some cars even simulate the faster blinking when the circuit detects a broken bulb - something that will get lost in history when bulbs get replaced by LEDs with newer cars.
ss30004 years ago
Don't know if I would call it a feature, simply a nice side effect. The flasher bulbs on incandescent christmas tree lights work the same way. As Rhodes pointed out it's dependent on the load being applied. When the switch is on and voltage flows through the flasher the element heats up until contact is broken, the element cools until the contacts are closed and it starts all over again. The clicking you hear is another side effect, that's the element opening and closing. This is also why you have to add a load resistor when you install led lights on as turn signals, the leds would not draw enough current to operate the flasher. Unless it's a solid state flasher, which might be a neat project. You could make circuit where you could control the flashing and maybe even flash patterns, depending on local laws and regulations though this could be illegal.
It's a feature to let you know one bulb has blown.