Cruise Control & Brake Lights

[AndrewCrown]

Here'a a rather daft question...

I like cruise control and use it all the time... especially in my Merc which does most of the miles in our house..

So say I'm on cruise control at 80mph... and one comes up to those long camera controlled 50mph stretches e.g. M3 , M1 etc... or Roadoworks I click it down to 50... and in my Merc at least the car definately brakes..

Question... Do the brake lights come on?

[DrDoofenshmirtz]

Are you sure it applies the brakes?...I don't think it does you know. It just feels like it does.

[Cupramax]

Audi cruise con puts the brakes on if going down hill and yes of course the sodding brake lights come on can you imagine the number of law suits?

[McSam]

Cupramax said:

Audi cruise con puts the brakes on if going down hill and yes of course the sodding brake lights come on can you imagine the number of law suits?

This it's very rare that the car will brake, it'll try its hardest not to (as braking is throwing away energy hard won by fuel already burnt), but if it does the brake lights must illuminate.

Have you never seen HGV brake lights flickering on long downhill stretches?

[saaby93]

What about a Prius where it sticks the energy back into the battery, does it light the brake lights?

[Claudia Skies]

Modern Mercs do feel very much as though the brakes have been applied. Amongst other things, they have a "speed limiter" feature which again seems likely to be able to apply the brakes.

Suggestion: Someone with a Merc estate could perhaps rig a piece of white paper on the back window to see if the centre brake light illuminates....

[Barreti]

Merc Dynamic Cruise Control certainly does brake. You can feel the brake pedal go away from your foot on downhill sections if you hover on the pedal.
And the brake lights do activate.

[Sump]

DrDoofenshmirtz said:

Are you sure it applies the brakes?...I don't think it does you know. It just feels like it does.

Yes they do. Decent cars nowadays do, Saabs don't hence your confusion.

[McSam]

saaby93 said:

What about a Prius where it sticks the energy back into the battery, does it light the brake lights?

It's not required to, and nor are EVs like the LEAF or the i3 which have really quite aggressive regenerative braking in their most "eco" settings. Those systems just use the motors to drag the wheels and run as generators, they don't use the wheel brakes.

I'm fairly confident the LEAF doesn't show its brake lights under any regenerative circumstance, only when the wheel brakes are actually used. Don't know about the others, but it raises an interesting question. I expect that as EVs become more common, it might become a requirement to illuminate the brake lights when the car's decelerating by a certain amount, whatever the reason for it.

[BigBen]

My Merc with radar cruise shows a brake light when it reduces speed, I suspect my other Merc with normal cruise will do likewise, I will check next time I drive it as you can easily see the centre brake light reflected in the tailgate window

[simonh9]

McSam said:

saaby93 said:

What about a Prius where it sticks the energy back into the battery, does it light the brake lights?

It's not required to, and nor are EVs like the LEAF or the i3 which have really quite aggressive regenerative braking in their most "eco" settings. Those systems just use the motors to drag the wheels and run as generators, they don't use the wheel brakes.

I'm fairly confident the LEAF doesn't show its brake lights under any regenerative circumstance, only when the wheel brakes are actually used. Don't know about the others, but it raises an interesting question. I expect that as EVs become more common, it might become a requirement to illuminate the brake lights when the car's decelerating by a certain amount, whatever the reason for it.

Technically the Leaf does show its brake lights with regen - when the brake pedal is pressed - most of.the braking in normal situations with the pedal is still regen and then foundation brakes are blended in as required (I.e. at higher G, lower speeds or when the battery is full and can't take any more charge).

I *think* there's a limit to the deceleration before brake lights have to be shown, but I couldn't tell you what it is.

[fatboy b]

I had to drill a hole in our fridge door to make sure the light went out

[romeogolf]

I've also got a Merc and can confirm it does brake when you re-adjust the cruise to a lower speed (or when going down hill).

It will use up to 30% of the total brake force I think (exact figure is in the manual) and the lights come on.

Asked a friend following me to let me know because I was just as curious as you! As a result, I tend to flick it off cruise to go down-hill and just let momentum carry me a bit.

Also, with the average speed checks, I just flick the cruise off about 200 yards before the limit begins to gently slow down. Usually hit the limit at ~55/60 mph and as it hits 53mph flick cruise back on. No need to brake

[McSam]

simonh9 said:

Technically the Leaf does show its brake lights with regen - when the brake pedal is pressed - most of.the braking in normal situations with the pedal is still regen and then foundation brakes are blended in as required (I.e. at higher G, lower speeds or when the battery is full and can't take any more charge).

I *think* there's a limit to the deceleration before brake lights have to be shown, but I couldn't tell you what it is.

Ah, that's true. I meant for the regen caused by coming off power, particularly in the eco B setting where it can cause quite dramatic deceleration - you can drive the car quite normally without ever touching the brake pedal! I would have been more correct to say that the LEAF can slow quite considerably without you using the brake pedal, but that it only shows the lights when you do.

ETA - I'm very surprised to read above that Mercedes' cruise system routinely applies the brakes, so much so that you have to work around it to get a gradual deceleration. That setup will be much harder on fuel, so it's very odd. Perhaps since it isn't part of the emissions drive cycle so doesn't affect CO2 or fuel economy figures, they don't care about the real-world efficiency and just did what they thought customers would prefer.

Edited by McSam on Monday 12th January 09:50

[loudlashadjuster]

I asked about this on a Merc forum and go a pretty definitive answer that the lights don't come on on non-Distronic cars, since confirmed by me with mein own eyes

www.mbclub.co.uk/forums/general-discussion/132675-...

[AndrewCrown]

Thanks L... looks like you have cleared this one up...
Cheers
A

[BigBen]

loudlashadjuster said:

I asked about this on a Merc forum and go a pretty definitive answer that the lights don't come on on non-Distronic cars, since confirmed by me with mein own eyes

www.mbclub.co.uk/forums/general-discussion/132675-...

Some W211 Mercs have a fancy braking system which can apply the lights when you don't hit the pedal, what I need to confirm is if it is also use by cruise, the linked thread suggests yes.

[GTIR]

Well I can confirm that on my old 13 plate E250 the brake lights did come on when the cruise is applied, when coming down hill, and I don't have Distronic. I often see the same on similar Mercs on the same stretch.

I have high level light (saloon) but I can see them come on as they are so bright!

[Jim AK]

fatboy b said:

I had to drill a hole in our fridge door to make sure the light went out

Fantastic. 12th January & I have my comment of the Year!

[AndrewCrown]

Actually thinking about this... I now have the ideal excuse to go and buy a GoPro to check....