Click, Click, Click – an acoustic signal three times in two seconds indicates that the car’s indicators are working. One or two control lamps in the instrument cluster light up at the same time in the same rhythm. You might, however, some day hear a fast clickclickclickclickclick while the green lamp starts to flash frantically. Most people know what this means: A bulb has burnt out, or there is something else wrong with one of the indicator lamps. The other drivers should at least know about this, it is a topic discussed at driving school.

Both groups of people may wonder what the increased speed of flashing is all about. A specification, perhaps? In principle, yes. ECE Regulation 48 states that the driver must be able to visibly see that the indicators are functioning properly. However, the internationally valid regulation allows car manufacturers a wide scope of implementation. Green indicator lights and/or an acoustic signal, the clicking sound, are all allowed. If one of the indicator lamps fails, the control lamp and the sound must change their rhythm. They may either speed up or slow down. The green lamp is even allowed to light up constantly without flashing. However, no-one actually makes use of the creative freedom provided. Flashing rate and sound always accelerate. No car manufacturer attempts any other pattern. Which raises the question of why?

Inspired by technology from days gone by

The technology may be old, but that does not make it obsolete. Its roots stretch back into the past. For decades, a so-called hot-wire flasher was responsible for the flickering lamp. It was one of those round metal boxes. It contained a relay and the eponymous hot wire. The wire interrupted the flow of the current whenever electricity passed through it, and then expanded as a result of the heat generated. When it had cooled down, the cycle started all over again in a flashing rhythm. The whole process depended on the power consumed by the light bulbs. The less power, the faster the process. For physical reasons, the hot-wire flasher had no other choice than to flash faster in the event of a bulb failure.

Drivers became accustomed to this effect and it was therefore adopted in the first electronically controlled flasher relays that appeared in the 1970s. In those days, they were mostly square plastic boxes. Nowadays, the flashing frequency is usually one of many functions in a control unit. The sound comes from a small loudspeaker in the dashboard. The old days do not end with the behavior of control lamps and sounds. For example, the ECE regulation, which is now exclusively binding for this function, prescribes a flashing frequency of 1.5 hertz, i.e. three flashing signals per two seconds. However, it allows a very large tolerance of half a hertz faster or slower. In other words, one flash per second is also permitted, or four flashes in two seconds. This generous tolerance range is also a result of hot wire technology, which reacts very sensitively to voltage fluctuations and also to the aging process. Strong differences in the flashing frequency were common, especially in six-volt on-board power systems.

Some things last for a very long time in automotive engineering and lighting. And for this reason, hardly anyone knows the reasons why things are the way they are. In these days of LEDs in indicators, a failure is a very rare occurrence anyway. And there are always ways and means in the case of conventional technology. Long-life bulbs such as Osram’s Ultra Life indicator bulbs often last for a car’s entire service life. Otherwise, they only need to be changed once. The fast flashing has become a rarity.

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