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CRUISE CONTROL ENGINE BRAKING???

9K views 31 replies 16 participants last post by  Kajtek1 
#1 ·
Was driving the other day and set the cruise, then I decided to see what it did when I pressed the stalk to decelerate

I was amazed at the higher rate of deceleration than just from letting go of the pedal....

Now when I do this is the engine braking itself? or is it somehow slightly engaging the brakes when I am using the decelerate feature??:thumbsup:
 
#2 ·
It's engaging the brakes.

You don't even have to use the lever to decelerate, you can accomplish the same by acclerating past the set speed and then removing foot from accelerator.

Al night in low light conditions you can see the brake lights.
 
#8 ·
The expectation of Cruise Control is that you set it to "maintain" a specified speed, so it does. How it accomplishes that mandate is not important.

The problem is people were accepting of the older systems that could not maintain a set speed in all driving conditions.

The car should manage the set speed using everything available to the computer including the brakes.

If an owner does not want to maintain a set speed without application of the brakes, increase the speed or shut the cruise control off.

I doubt that MB will design a "Soft Limit" cruise system, of course it would be REALLY easy to do but it would require a significant software change.
 
#9 ·
What I'm after is the system as it is today, including engine braking applied but brakes not being used when engine braking is not sufficient.

I agree that the current implementation is closer to what CC by definition should do but I just find the older approach better, as it actually was on a pre-facelift 211.

I don't think the SW change would be significant, just flag out the request to apply brakes where it is done today.
 
#10 ·
I agree engine braking first then if speed exceeds a specific % wheel brakes. But not wheel brakes first.

Remember the police officer saying,, "the speed limit is the same up the hill as going down:eek:".

In the 50's Mississippi was a great place for a ticket at the BOTTOM of the hill. (hill not mountian) :)
 
#13 ·
WIS explains that the car reduces engine power, applies brakes and shifts gear if necessary to keep the speed at the set value (mentioned in this order). But I cannot believe this would be exactly accurate. If brakes were applied first, there would never be a need to shift gear to reach engine braking because brakes can handle the speed far better than the engine could.

My experience on the way to work supports the opposite order, on the way I have long section of slight downhill and a 50 km/h speed limit. I approach the downhill section at CC set to 50 km/h and the car runs at about 1000 rpm on the fifth gear (7G-tronic). Once the slight downhill starts, the engine is not able to brake in practise at all at 1000 rpm and the first thing the car does is it downshifts. This again is quite sufficient to keep the speed at the set value.

Theoretically the car could momentarily apply brakes and release them immediately when engine braking appears sufficient but I have not noticed even this (next time I can try feeling the brake pedal movements, non-SBC cars "brake with the pedal" (while an SBC car can brake without the pedal moving at all).
 
#14 · (Edited)
It would be mandatory for engine braking ie overun to occur first before application of the brakes. However in the case of a diesel engine braking is near useless due to the lack of a throttle.

On the diesels they must reach the engine braking threshold as often as possible without braking. During Overrun, the ECU/fuel system calibrates the injectors during these periods to provide a "trim" value to compensate for injector differences and normal wear that occurs to insure perfectly uniform fuel delivery.

Another issue is that there is no driver feedback in respect to throttle position even on the gas cars since the floor pedal doesn't move. Unless you are reading actual throttle position or IQ on the diesel you can't determine if the engine is in overrun.
 
#15 ·
sorry to revive this old thread...just to add a point which I think most have forgotten: Safety

If the car (in cruise control) in front of me suddenly applied engine braking as it approach a down hill portion of the road while I have yet to reach, the sudden deceleration without the braking lights being lit cause me to ram into his back if I am not paying enough attention to the road condition.
;)
 
#16 ·
Engine braking in a car is only lettng off the gas and allowing the compression to slow the car much as taking your foot off the gas. There is no current normally installed "Engine Braking" using the Exhaust system as in a diesel truck. So I would expect anyone following to be able to very comfortably slow down with out effort following a car that used engine/compression braking going down hill.

I have even shifted from OD to 3 going down hill to prevent speed build up in some mountain areas. There are no brake lights for this and I don't exceed the speed limit going down hill (at least not by much). This also should be no issue for someone following. I would expect anyone following to be paying attention. But then I have been rear-ended 5 times in the past 40 years while completely stopped at red lights, so perhaps people in the rear based on my experience don't pay attention.
 
#17 ·
It's a German car. What other kind of cruise control would you expect from a German engineer? He's given a task: "Design a system that allows the driver to select a speed at which he wants to drive. Allow for driver overrides and varying terrain features."

So Herr Engineer produces a product: you select a speed ... say, 55 mph ... and activate cruise control. You selected 55; the system produces 55. Not 56. Not 54. No slacking off going uphill. No overspeeding going downhill. 55 is 55.

Next engineering challenge.
 
#19 ·
Is it possible to disable braking downhill when the cruise control is enabled? It is really annoying in the area we live (up and downhill 90%) to have the vehicle constantly braking when on cruise control. It is hazardous since all other vehicles accelerate downhill making it look like the ML driver is an ass braking on a simple downhill. is also uses more fuel and wear the brake pads unnecessarily.
 
#21 ·
Yes that is what I currently have to do but it really takes the essence of cruise control convenience away. Driving manually is the same or less effort/concentration as fiddling with the cruise control lever all the time. One would expect that it can be adjusted or eliminated maybe in the 'menu system' or by the dealership computer??? Braking on highway speed on a downhill is irritating and frustrates the motorists beyond you. My question remains - can it be adjusted/ changed to no braking downhill while on cruise control?
 
#22 ·
I invite someone to confirm with some form of acceptable MB documentation that the brakes are activated in maintaining set cruise speed during downhill runs.

I have driven behind my wife in her W211 while it descended some fairly steep and long mountain roads in the Appalachian Mountains. Her car maintained the CC-set speed precisely, but I never saw the brake lights come on. It appears that speed decreases are controlled by our cars' transmissions, rather than the brakes. This is the same impression I get driving my R230 ... the transmission controls all the speed decreases.

Break out the book and settle this once and for all.
 
#23 ·
I invite someone to confirm with some form of acceptable MB documentation that the brakes are activated in maintaining set cruise speed during downhill runs.
Break out the book and settle this once and for all.
Attached is from the owners manual. I do not know if this is acceptable MB documentation for you.
Mike T.
 

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#26 ·
In mountain areas where we live it is really annoying to ourselves and other motorists when the cruise control brakes the car just over the hill at descent where ALL other cars just free past the set limit if they are also on cruise control. The Merc uses much more fuel since it keeps braking and suddenly it has to accelerate real hard and change gears to do the next hill up while it would have had the momentum if it did not brake. According to law a speed trap may not be at the bottom of a downhill so that argument is senseless. I wish there was a way to DE-activate this stupid braking in other countries than EU. I say - Silly to make this braking standard MERCEDES!!!!!! Nearly everything can be adjusted, why not this???? I hope someone at Mercedes can read this and give it a thought, or come and drive here where it is NOT flat area and feel the madness when the car brakes and accelerates like a mad programmed toy electric car...
 
#30 · (Edited)
Cruise control was developed exactly for that purpose! Cruising. Safely considerations for cruising include traffic volume, road condition, contour(curves) & flatness.

My 2 cents. Works well for me. :D


If the cruise control is that maddening, you could trade it in for a Ford. They work the way you described.

Cheers,
 
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