I have been watching this site for some time but only recently joined. I have found many if you to be very helful and knowledgeable. That said, after going from a 2004 C32 to a 2005 C55 I have noted a problem with the transmission. After completing my break in mileage I noticed that the car will not always shift up to the next gear when the engine reaches red-line. This happens when the transmission is in sport mode and shifting on its own. The engine will rev to red line and hit the limiter and cut-out momentarily. My C32 never did this even when driven as hard as I could drive it. I have not had many auto transmissions prior to the Mercs. I was accustom to bouncing the tach off the rev limiter from time to time in a Z06 but did not expect this in an auto trans.

Is this a major problem or just a software problem that is not properly shifting the trans? I am afraid that at some point I will grenade the motor. I will take the car into the dealer (reluctantly) but wondered if any of you have experienced this issue. Thanks for the help.

 

It could be holding the gear as a learned response to your driving style. There are a number of adaptive and self-learning functions in the transmission controller unit.

Just lend the car to your mild driving partner and when you get back into the drivers seat you will notice that it drives totally differently.

That being said .... did you try giving the "+" button a nudge ?

 

OMG! This sounds familiar! Is it on the 1-2 shift? Under full throttle? After an any gear to first downshift?

If so, there is no cure.

 

That sounds like the microprocessor is too slow (does not sample the RPM often enough). I had the same problem with a modified 99 Mustang, the only fix would be a faster CPU and RPM data sample rate. After a downshift, would run to readline in about 1 second, even when setting the WOT shift point to 3800RPM. The Rate of acceleration (Jerk) was too quick for the uP Data acquisition system in the PCM.

On the 99 mustang the ECU/PCM runs at only 16MHz and handle both Engine and auto trans. The Audi S4, uses two seperate computers.

Has the ECU/PCM been flashed by an aftermarket company?

Don

 

The problem is very specfic in which it only happens on a 1-2 upshift after an any gear to first downshift. The car does not downshift at speeds which put the engine at an excessive RPM where it needs to upshift immediately. Even downshifts at 10MPH cause the problem. Lastly, it never happens from a dead stop so it is not likely a sampling issue. However, MB (and a few others) attempt to mitigate this potential problem by lowering the 1-2 shift point a couple hundred RPM. After considering the parameters surrounding the problem, we tried to lower the shift point further to 5600 RPM in first without any improvement. Fuel cut off was at least 500 RPM higher. BTW, raising the shift point up to 6000 did not have any negative or positive effect on the problem.

MBUSA for their part, tried to solve the problem on and off (I didn't mind my car being a test mule but I didn't want it to live in the shop either) for a couple of years by replacing (with upgraded parts where available) ECU, TCU, shifter, valve body and other misc parts. In the end, the problem only improved marginally. MBUSA and AMG finally decided last year to give up since "the problem was insignificant, non-damaging and effected such a small number of cars.

 

 

MaxC55- I have, on occasion, experienced what you are describing. However, my interpretation was that the trans is shifting from 1 - 2 as normal, breaking the tires loose (ie: losing traction) and then ESP intervening and causing the momentary cut-out. It happens most pronounced on slippery or glazed asphalt surfaces. Are you certain that you're getting the cut-out before trans shift, and not after?

 

Easy way to tell the difference. ESP alone will not cause your neck to snap forward like cutting the fuel abruptly as the rev limiter does. Normal 1-2 ESP intervention on even the slipperiest of surfaces feels very faint. Fuel cut out feels like you hit a brick wall.

 

So you're not talking about down-shifting, but rather the transmission holding a gear in manual mode until you upshift manually....is that correct?

 

[QUOTE=MAXC55]I have been watching this site for some time but only recently joined. I have found many if you to be very helful and knowledgeable. That said, after going from a 2004 C32 to a 2005 C55 I have noted a problem with the transmission. After completing my break in mileage I noticed that the car will not always shift up to the next gear when the engine reaches red-line. This happens when the transmission is in sport mode and shifting on its own. The engine will rev to red line and hit the limiter and cut-out momentarily.
I have experienced this exact scenario as well. It only happens from gear 1 to 2 and only when I "floor it" from a stop and do not ease on the accelerator. It will run at red line for a second, then cut out momentarily until the engine upshifts to 2nd.

 

this has happened to me a few times as well, I drive an E. It seems kind of normal although its never really happened more than once every few weeks or so.
I think what happens to the car when it goes from 1-2 is that it needs to cut the fuel altogether in order not to break the tires loose. But that is mostly when there isn't quite a neckbreaking snap like you have described.
my .02

 

Does it happen with ESP off? I did a quick skim through the thread so I don't know if that was suggested but that'd be the first thing I'd try to see if it is actually hitting the rev limiter or just ESP intervening.

 

Thanks for the great feedback. I never really noticed if this was occurring on 1-2 upshifts only but based on the speeds I was going (50 plus??) I would think that it happens at a 2-3 upshift as well. I have never had any mods made to the car and do not think that what I am experiencing is related to ESP. It seems to be bouncing off the rev limiter and having a more abrupt cut out. Based some of your own experiences it seems there is little MB can do to make the upshift happen before redline. I will ask when I go in for my initial service and let everyone know what they say.

I only hope that I do not slam a valve into a piston the next time it happens.

 

Try a full throttle start (with the selector in Drive) through the 1-2 shift then slow down to about 20-25MPH and floor it again so it downshifts back to first and hold it until it shifts back into second. This is the method I use to duplicate the problem with or without ESP on.

BTW, the pistons won't hit the valves. That's one reason why they cut the fuel to prevent the engine from overspeeding and floating the valves.

 

I have the same "problem" and I usually don't drive too hard. I can recreate the problem by being in S (ESP on or off) slow to 3 - 5 mph so that the car hasn't downshifted into 1st, hit the gas... even at 1/4 to 1/2 thottle the car will downshift to 1st and hang onto the gear. Since I don't floor it when this happens, the RPMs climb up to about 5k (which is really high when you are not hitting the gas too much) and it kinda hangs onto 2nd for a while too.

It's really annoying when you are just rolling through a stop sign and the transmission acts all aggressive and such when you are driving normally.

I'm going to the dealer tomorow and maybe I'll ask someone about this.

Scott

 

Doesn't sound ike a problem at all