I asked this over on the S55 board, but didn't much response and I know you guys are more active.


I get an occasional hesitation at about 2000 r.p.m. when accelerating casually. I was wondering if that's when the S.C clutch kicks in and if this normal. I have read about clutch gap on this board, and wondered if it's possible to have too much gap on an 1800 mile car. I do not hear any chirp.The one response I got mentioned the tranny and I have certainly considered the learning aspect of these auto boxes, but these seems to be more of a S.C. hesitation. I have owned several turbo cars, but this is my first blower car.

Any info would be appreciated.

 

I think it depends on what you mean by hesitation. I notice the supercharger kicking in sometimes when I hit the gas, but the car still moves initially. I'm assuming you're not hitting the gas and nothing is happening right. I think that little delay is normal.

 

I am hitting the gas, but not hard. It is a hiccup, as if there is a slight delay in the S.C. clutch hookup and the power delivery. If I hit the gas hard, there is no hesitation.

I wonder if there is an easy adjustment to the clutch.

Is this similar to the old centrifical clutches in motorcycles with weights and springs and what looked like drum brake pads spinning out against a drum?

Does anybody have a picture of the clutch itself ?

 

I think the peak torque for the 55 is at 2750 (? S/C kick in ?) and I always make sure to get to that rev range whenever I want instant get up and go (via kickdown on the steering "paddle"). When I'm at the 2000rpm range there is a slight hesitation in "kick" until I'm around the 2700-2800 rpm sweet spot, then all hell breaks loose!!!

I'm technically-challenged and I'm sure others will post with a better explanation.

 

I am familiar with the torque peak, but the clutch would have to kick in somewhat before that to build boost. You also indicate a slight hesitation at 2000 and that's why I am asking if that is when the clutch acually starts to engage.

If that's the case, then you could adjust the clutch "tighter" which would bring the boost in earlier. ?

 

I've asked Vadim if the SC could engage at a lower "load" threshold to minimize what you are experiencing, but currently I'm adjusting my driving style (more below).

The engagement varies based on engine load. It just so happens that this occurs at ~2000 RPM in normal driving.

What I would really like is to move the light-load SC engagement curve down from ~2900 RPM to 2000 RPM. There are references that the light-load engagement at 2900 RPM is for safety reasons, MB/AMG doesn't want the engagement to occur at too high an RPM.

My theory is that in first and second gear, the delta between the RPM at which the SC begins to engage and when the engagement actually completes is large on throttle-up. So, you get hesitation/stumbling/bucking. You will notice the "chirp" of SC engagement in third and fourth gear at just a little over 1000 RPM, and there won't be much hesitation. It's just pronounced in first and second.

Also, in first and second at any given load you will probably be running a higher RPM when the the load threshold is crossed and the clutch engages -> so it stumbles/hesitates.

My worry is that over the long haul this could be a problem for us. My friendly MB parts guy told me the SC remanu units cost ~$3800, so I guess I'm not too worried ... not as bad as I thought.

Try this - drive around in manual mode to get a "feel" for how much throttle input/load causes the SC to engage . Then, learn how to gently apply the brake when the SC engages, and it will smooth things out. Of course, you don't want to do this with somebody on you back bumper. With practice, you won't be able to feel when the SC engages.

Also, if I know I'm going to do a quick getaway from the light, I always powerbrake just a tad as I leave ... kinda use the brake as a clutch. This preloads it and makes the SC engagement smoother.

If the VW/Audi "Lemmiwinks" tool could be used on our MB's, I would jack up the idle torque offset and play with the SEL factors to force the engagement to occur at a lower RPM ... guess I need to figure out how to use STAR diagnostics.

 

Thanks for the response. It does seem to happen right around the 1-2 upshift, so there could be some tranny learning going on as well.

 

See if you can drive another 55 and see if it's the same as yours...

 

My rationale on why this happens is because the load changes as the upshift completes, which causes the SC to engage. So, driving around in manual mode I try to keep the SC from engaging right after after the upshift by backing out of the throttle a bit.

So ... whew!! ... I like having the SC engaged, but I try to keep it from happening during the upshifts. I either get it to engage in first gear, or I try to get it to engage well after the upshift to 2nd (or sometime 3rd) occurs.

 

When it happens are you in sport mode or comfort mode?

 

I wonder if its just the tranny changing down a few gears.
What happens in manual mode if you do the same thing? If the problem is gone its the tranny, if still present its the engine/supercharger/throttle by wire etc.

 

Sport...I never drive in C.

 

This only happens with a very light pedal when pulling away from stop sign etc... the car will upshift very early, leaving the r.p.m.s quite low in second.If I use a firmer push on the pedal or rev the car a little higher before upshift it is smooth. This could be an area where the new 7 speed will help. My 7 speed SL never did this.

It's not that big a deal, but I will call some of the tuners and see what they think.

I will report back.

Thanks for the replies.

 

I talked to Kleemann today and we both came to the conclusion that this stumble might be the torque converter lockup, which happens at about 1800 r.p.m.

If I rev 1st to 3000 r.p.m. second gear will start at about 2200-2300 r.p.m. and there is no hesitation.

He said it was probably not the S.C. clutch-it engages higher in the range.


So, that's it, time to blow this off as a minor deal.

Thanks again for your replies.