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Mallory

Is this car supposed to be this "special" in reponse to driver input?

12 posts in this topic

So we ended up biting the loan bullet and picking up a cute white on white with orange interior passion. Looks cute, drives cute, we drove around it for days and the fuel indicator barely slipped below where it was when we got it. However it's got a few alarming rude habits and I'm wondering if I should be taking it in for a warranty claim.

 

1. It takes a full "one-mississipi" before the fat little car takes off from a full stop (such as a stoplight). The dealer who sold it to us (not MB, it was a trade-in) said that it was because "the car shifts to neutral every time it's stopped and only shifts to 1st when you throttle". I dunno if that's true or not, I thought this car was supposed to have a normal but scaled-down automatic transmission. That being said, between the size of the car, the size of the engine, and the turbo I'm willing to believe that being this lackadaisical off the line is normal, and it's consistent enough that if you set yourself to never be in a hurry it's fine. Just figured I'd ask in case there's a way to make it be a little bit more responsive (and also not roll backwards on hills. I thought we were supposed to be over this).

 

2. This is the more alarming one. This car will just sometimes randomly decide that it will continue driving in the opposite direction from where you indicated you want it to go - i.e. it will roll forward a few extra seconds/inches before finally switching to reverse, and do the opposite when switching from R to D. It's less predictable and seems to be less severe if you mash the throttle like a race driver, but when navigating a tight parking spot that's a bit of a hair-raising proposition. So far it seems to happen more often if you use the throttle gently, and less if you are more aggressive. I've never seen anyone point out this car as a self-minded vehicle when it comes to parking, so I figured I'd ask. It certainly sucks if this is normal and I might have to start carrying a towel in the back to put between the cars if someone doesn't leave me any room to pull out from where I'm parked.

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Assuming it's under warranty, get it into M-B pronto for a warranty check.  Neither of those things is normal.

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Thanks MikeT, you're always here for us! Yes it's still got more than two years of warranty left and previous owner only serviced it at a Merc dealership. Thanks, will definitely call them later today.

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Posted (edited) · Report post

1. The clutch would have to be engaged on take-off, to do so smoothly, it will not be instant.  If the car is rolling backwards on small inclines, there may be something wrong with the hill hold assist.

 

2.  It is not a traditional automatic transmission, it is an automated dual clutch unit.  Having very little driving experience with the 453 DCT.  The 451 will not always change from R to D or D to R if you are moving too fast.  You may have to actually stop before selecting a different gear.

Edited by Huronlad

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Your car has a turbo.  I have heard of some folks experiencing "turbo lag" when pulling away from a dead stop ..  Almost like what you have described in your first post..  I am not an expert on the newer models so I may be completely wrong.  If so, I am sure someone will jump in and correct me, but it's food for thought.

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Reading the OP I get the idea that it's a more than one second lag, and there should be zero lag when stabbing the accelerator pedal, hill holder or not.  The car should move off the line smartly immediately.  The idea that the car shifts to neutral at a stop is just wrong.  First is selected and the clutch is disengaged, unless the driver shifts the car to neutral.  The full stop is a good reminder for everyone - no "California Stops" where you roll through a stop sign.  That could lead to some weird laggy situations when shifting to reverse in particular!

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Driving another 453 with a DCT could determine if the issue is only with the OPs car.  M-B would know if there are any software updates to improve transmission function.

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Posted (edited) · Report post

We went and drove the car again today to try and replicate the situation with me behind the wheel this time.

 

The best way I can describe it's performance is it feels like the car is winded. It will not take off from a full stop for a full second which is super weird when it comes to determining what's a big enough gap for you to pull through in traffic. Maybe we just have to get used to it because it felt fairly consistent but... weird. And the other car we're used to it an old Nissan Versa Note (so... not an acceleration beast). Like I said I'm willing to believe it's turbo lag because after that initial hesitation it moves along just fine. It's just strange because neither of us remember it being this slow off the line when we test drove it.

 

In regards to issue 2., it's completely different than what I got from my partner's wide-eyed explanations. Basically the sequence of events goes like this: you pull up to a parking spot that happens to be on an incline; press your brake to stop, switch gears from R -> D or D -> R, whatever you need. When you take your foot off of the brake, for less than a second, the car is stationary - clearly with hill assist engaged. Then the hill assist disengages, but the car rolls in the direction of gravity for about a second and a half before finally engaging the proper gear and driving in the indicated direction through idle power. If at any point in time, you press the brake pedal, there's a reset on the "sequence" where the car will, once again, hang out for a split second on hill assist, roll in the direction of gravity, and then engage the transmission and move in the correct direction. The reason why aggressively mashing throttle seems to "reduce" this effect is because in doing so you move your foot faster between the pedals. Giving the engine power seems to snap it out of whatever sleepy state it's in when it's not immediately engaging the right gear when hill assist disengages. It's kind of problematic because the natural reaction to "what the hell, why is my car going there?!" is to step on the brake, which... will just make your situation worse.

 

We have a time scheduled with an MB dealer already and I think we'll take Huronlad's advice and go drive another new DCT or two and see if either of them do the same thing.

Edited by Mallory

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Posted (edited) · Report post

The DCT may not be programmed to creep as an old automatic would, it causes unnecessary wear on the clutch.  The 450 model never did this and the 451 did this poorly when first sold, many software updates later it was better. 

 

I was watching some videos taken from a 453 earlier and did not notice any noticeable delay moving from a full stop.

Edited by Huronlad

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I test drove one with auto a couple of years ago and there was no 1 second lag.  The only complaint I had about it was the lurching into first gear when decelerating in heavy traffic.

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Can I ask, and sorry to semi hi-jack....but my 450 2005 has this 1 second lag from a full stop also...yes totally different years but is it the same issue, a turbo type lag?  As said above by OP it also is nippy so to speak once on the move, it's just this initial step on the gas and it feels like "it" decides when it should step off the line......lol It's the same lag every time so I have adjusted my driving to suit it. But when picking a gap in traffic you must either time it very well or wait  for a larger gap...lol

 

If normal, I can live with it also but if it's tunable to get that out, even better.

 

For the fuel economy I can accept the lack of accelleration and live with it, but.....you just have to add that lag into your normal driving techniques....

 

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I have found with 450s and 451s, if you are experienceing any kind of "lag" between application of accelerator and the car moving off the line from a full stop, that performing a clutch teach-in using STAR has always resolved the issue. When the ECU stores a "drag point" for the clutch, and then you pile on the miles, the ECU still thinks the drag point is at XYZ position of the clutch actuator but in fact wear and tear will result in the actual drag point being before or after that XYZ setpoint. The other symptom of this, which I have noted on 451s particularly, is that when you let off the brake at a stop, the car will lurch forward in a neck-snapping kind of way, very uncomfortable. I've also noticed a symptom of a poor drag point setting is that when starting on an uphill, the car will rev up from idle but not actually move forward and not give enough revs to build enough power to overcome the grade, basically just feels like it doesn't have enough power to get going but in fact it's just a matter of the clutch being out of adjustment.

I have no experience with 453s but I could envision that the DCT transmission might need a similar periodical teach-in of the clutch drag point(s).

Edited by booneylander

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