Forced Slowing Down

[david roberts]

I wonder if any of you experts could comment on something I'm experiencing on my E320 CDI W211 (55 Reg).

Sometimes when I am slowing down, say at a junction, the car automatically slows down even faster (faster than i want it to and sometimes quite jerkily) - it feels like the gears or the brakes are pulling it up. Could this be the braking-by-wire system or is it the gearbox- normal or not?

Many thanks David

 

[dave.robbo]

I guess you could pop it into neutral for a test when slowing down. If the retardation is reduced, then it's the gearbox that's slowing you down. If not, it must be the brakes, or wheel bearing, etc..

 

[brandwooddixon]

These cars are fitted with a brake assist which could be cutting in.
If the gearbox has set itself to match your spirited driving then it too will change down earlier when braking is detected.

I wouldn't recommend popping it into neutral at any speed though as it won't like it and will tell you so.

 

[dave.robbo]

I wouldn't recommend popping it into neutral at any speed though as it won't like it and will tell you so.

Why's that?:?

 

[television]

There is no harm in just popping into N for short periods to test this, I do it on my own car, and when testing others, it is no different to towing, and you are allowed to do that for up to 30 miles at speeds not in excess of 30 MPH. The reason why one should not over do this is because these boxes do not have a rear pump

It could make a difference when driving in W or S.

If going down hill the car may choose lock up to assist braking

 

[louwdj]

I suspect the car is gearing down. Keep an eye on the rev counter and you'll see. My W210 does the same.

 

[david roberts]

Thanks to all. I get the impression that this is fairly normal and no particular investigative action needs to be taken.

 

[The Rock]

My wife's SLK350 (R171) was very jerky in the final few feet when stopping, from new. It turned out to be software related and the dealership knew about it. After new software was loaded it was much better. I've noticed recently that it seems to have become jerky again. The latest service invoice states that "any new software was downloaded free-of-charge". Now whether any actually was I don't know, but I suspect so and may ask them to check it out. The trouble is that I only drive it once in a blue moon so I don't get much chance to notice these anomalies. My wife, bless her, wouldn't have a clue what they did. If they painted it pink she might notice but I wouldn't put my money on it.

 

[jberks]

Are you new to the car?
Not sure if it's a merc quirk but you do get extra engine braking as you pull up, I imagine it's when the gearbox drops into 1st. I find I compensate (subconsciously) by easing off the brake slightly as it draws to a halt. I dare say we all do, only its so subtle we aren't really aware of it.

 

[david roberts]

Thanks again, guys.

JBerks, I've had it since last May, but only really noticed it fairly recently. It doesn't happen with my trusty old 1994 C Class 200 (W202). It's probably as you say - just a wee bit disconcerting.

 

[television]

The 124 box behaves very different in this respect to the 772.6 boxes.

Go down hill in N in the older 722.3,4,5 boxes and pop it into D and the engine will speed up to the road speed of the car. If you do the same with the 722.6 box,when you pop it into D the engine speed will not change until lock up has occurred, and I am thinking now is there a fault on this box, I will test tomorrow when the sun comes out

 

[djb]

just to say I believe there might be an induction air throttle which is closed on slowing.

I am not certain it shuts allways on slowing or only with fast closing of the throttle & braking.

I can say its effect is much less noticeable since having a gear box software update in Nov.
The 7 speed 320cdi does not roll on with closed throttle the distance my previous c sport did, and thus saves a lot of disc & pad wear.

 

[television]

just to say I believe there might be an induction air throttle which is closed on slowing.

I am not certain it shuts allways on slowing or only with fast closing of the throttle & braking.

I can say its effect is much less noticeable since having a gear box software update in Nov.
The 7 speed 320cdi does not roll on with closed throttle the distance my previous c sport did, and thus saves a lot of disc & pad wear.

Thanks for the above djb I wonder if the op has the 7G box as that could explain it

 

[The Rock]

My wife's SLK350 (R171) was very jerky in the final few feet when stopping, from new. It turned out to be software related and the dealership knew about it. After new software was loaded it was much better. I've noticed recently that it seems to have become jerky again. The latest service invoice states that "any new software was downloaded free-of-charge". Now whether any actually was I don't know, but I suspect so and may ask them to check it out. The trouble is that I only drive it once in a blue moon so I don't get much chance to notice these anomalies. My wife, bless her, wouldn't have a clue what they did. If they painted it pink she might notice but I wouldn't put my money on it.

Forgot to mention it's 7G-Tronic.