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Sometimes when waiting for a traffic light, I see the reversing lights on the vehicle in front of me briefly flicker on and then off again. Most often, this is on Mercedes people vans (short buses).

This startles me, since if the vehicle were to be put in reverse by accident, it would hit me when trying to drive away when the light turns green. But it always turns out to be a brief flicker of the reversing lights.

What could be the cause of this?

Is the driver doing something wrong, accidentally putting it in reverse? Is it faulty wiring?

psmears
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SQB
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4 Answers4

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It's simply because some cars will illuminate the reverse lights when the drive moves the gear selector from D, through R on their way to P when stopping for extended periods.

My Porsche, and my Range Rovers before only showed the reversing light when the gear was actually engaged. Other cars seem to have the light wired to the gear selector position.

It's only a cause of concern if the light stays on, at which point you try and work out whether the car in front is more expensive than the car behind you while leaning on the horn.

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As a (former) automotive software engineer, I've spent some time working on car engine controllers.

As @Snow says, the gear lever on an automatic selects whether you're in Park, Reverse, Neutral, Drive or Low gear. The gear selection is arranged in that order, so in the industry you will frequently find this called the PRNDL, to specifically identify the lever (or the sensors monitoring the lever's position) and not any sensor within the transmission itself.

Older cars used a separate switch within the PRNDL or transmission to control the reversing lamp(s). These days, most things are under software control, and there is an increasing drive to reduce sensors for reasons of reliability and cost-saving. On a modern car therefore you will find the reversing lamp driven by software control. The PRNDL position is measured over its travel (usually using a potentiometer, but sometimes a rotary encoder), and using this measurement the reversing lamp is turned on when the PRNDL is in the reverse-gear position. This is just one part of the software finding what gear the car is in, of course, which is used for pedal demand maps and various other settings.

Of course with the PRNDL gear layout, you have to go through Reverse to get to Park. The software therefore uses a timeout mechanism to decide whether to light the reversing lamp, to reduce the effect you describe. The driver needs to have been in Reverse for a short time (maybe half a second, perhaps longer) before the reversing lamp is lit. If the vehicle calibrators have not been conservative with this timeout, it's quite possible that the reversing lamp can flash inadvertently. There can also be a problem with old/worn/sticking gear levers where the user cannot move it through gear positions so easily, meaning that it spends longer than usual in the Reverse position.

Graham
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It will only happen with automatic gearboxes. as already said, the lever needs to go, on some vehicles, through R to get into D. It's that moment that the reverse lights flick on and off again. If they stay on, prepare for some fun. If it happens in a manual vehicle, the driver has messed up, and gone past the gate, and selected reverse instead of first. If it's a short flash, you're lucky. as they've realised, and found the correct gear!

Tim
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Depending on how sensitive the reverse gear detector switch is, in the car in front of you, this may happen with manual transmissions as well. While we learn at driving school to change to the lowest gear and use the motor brake to decelerate, pushing the gear to 1 is actually quite hard while the car is still moving, with many cars.

As a result, I typically just press the clutch and brake, and when the car has come to a stop. switch to gear 1. Sometimes, a few seconds later, I wonder if I'm really in gear 1 - about twice per year I forget and try starting in gear 3, much to the dismay of everybody behind my. So I may switch the gear again just to make sure I'm really in gear 1.

With my current car, even switching the gear from 3 or 2 to 1 while standing triggers the reverse lights for a split second; at least, that's what a colleague told me when he drove behind me and met me on the company parking lot.

Guntram Blohm
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