B-250 Tachometer indicates increase engine speed when going downhill and applying brake

[tuggie]

I have a used 2013 B250 and am not too familiar with all the features this Mercedes has
I did notice today when I was going downhill and approaching a stop sign that when I took my foot off the accelerator and applied the brake to slow down that the tachometer suddenly went up to 3000 RPM for a brief moment then returned to normal
Prior to going downhill and applying the brake the speed was about 65-70 km and the tach was indicating about 1400 rpm
Is this normal. All my other cars when you are going downhill removing your foot from the accelerator and apply the brake the tach goes to idle and stays there
Thanks

 

[alexanderfoti]

Did the engine actually speed up?

Sounds like its just going down through the gears. If your older cars were torque converter autos, they would have this behaviour.

 

[V6Matty]

Did the engine actually speed up?

Sounds like its just going down through the gears. If your older cars were torque converter autos, they would have this behaviour.

The little 169 does this for us at times, though I’m not sure how well engine braking works on a CVT lol

 

[Tony Dyson]

Is it perhaps related to a smart alternator function for regenerative braking ?

 

[ajlsl600]

"smart"( real meaning ,normally) considerably more expensive, or non repairable when faulty alternator ! i heard e-nuff . have heard these can be made to behave in a traditional "simple manner" by disconnecting 1 wire at batt head ?

 

[LostKiwi]

Our SL does this too. Older SL and 210 don't. I don't think its anything to be concerned about.

 

[V6Matty]

Is it perhaps related to a smart alternator function for regenerative braking ?

To early for the hybrid or regenerative braking.

 

[phillj]

2013 B class 7g DCT gearbox? if so normal behaviour

 

[tuggie]

Thanks guys for all your help. This is my wifes car so I do not drive it very often. After doing more research I do find this is normal
When going downhill the system Mercedes uses is called the " glide " mode . If the conditions are right when going downhill the transmission is " disconnected " from the engine hence the tachometer indicates idle rpm and the car picks up speed as it is just rolling freely downhill
It continues to pick up speed if on a good hill , until the brake is applied , at that point the engine and transmission " reconnect " and hence the momentary spike in the tach which then returns to normal rpm for the speed you are traveling
This is a Mercedes feature to increase gas mileage
It is only active in the Eco mode
In some of the other Mercedes when this happens you get a " sailboat "
Indicator
On my 2013 B-250 there is no indication
Some people find it annoying and disable it by selecting either Sport or Manual

 

[tuggie]

Evidently the glide mode is also activated when you take your foot off the accelerator . The car " glides " using its momentum as though you were in neutral and the tach and engine drops to idle
Stepping on either the accelerator or the brake pedal cancels the " glide "

 

[John Laidlaw]

I thought the ‘sailing’ mode only applied to hybrids- every day is a school day it seems !