lap

I am trying to resolve what appears to be an intermittent issue with my car.

When I park on a slight incline/decline the car will rock forward or backward (+/- 6-8inches) and I will get a KNOCK/THUMP when I move out from Park Also (again intermittently) there are times when I am in Drive stopped on an uphill (occurs from Slight Grades to Significant Slopes) that the car will start & continue to roll back (as if I were in a standard car with the vehicle in neutral and holding the brakes slightly)!!!! Most other times, in the same situation, my car will "brake" itself and not roll back.

Anyone else have this issue??

TIA
lap

rguy

The slight rolling is nothing to be afraid of on a mercedes-benz. I have owned four automatic mercedes and they all do it. My parents have owned many more than me and their's all did it too.

Mercedes just designs the transmissions to be "loose". That is, they have some play in them. I like it because I don't have to come to an absolute complete stop when shifting from reverse to drive.

I think it is a good thing that it also feels a little like a manual. I like catching the car on a hill with a few revs fed in by my right foot.

Bottom line, if you are trying to get rid of it, use your parking brake every time you park. You should be doing this anyway to prevent rust and having your parking brake lock to the rear setup. On a hill, although I don't recommend it, you can hold the vehicle with gas and prevent the rollback.

At any rate, your car isn't going to roll backwards down a hill until it hits something or flattens out, it will catch itself in the first foot or two.

My method for parking is as follows:

(1) Keep your foot on the brake at the final moment of a stop to park.

(2) Shift to park, keeping your right foot on the brake fully.

(3) With your left foot, engage the parking brake fully while still holding the brake fully.

(4) Release the brake under your right foot.

I developed this because I park in tiny single car garages sometimes, and rollback can mean the garage door comes down on the car's bumper and scratches it.

Try it a few times on a hill and I think you will get it easily. I hope this helps. If I am off base, then sorry.

If you are using your parking brake during this whole issue AND doing it the way I described above, then you should have your parking brake checked out. Since you describe the problem driving and parking though, I am guessing this will take care of it.

lap

I still think I have a problem. I went up a hill today & stopped (yes, no traffic behind me) and as the car started rolling back it then STALLED by itself. The car seemed to try to brake itself as it was rolling backward (the c32 was in Drive) then konked out, similar to what would happen with a standard not receiving enough gas. If I would give it gas then it would roll forward & I could "rock-it" back/forth to a neutral stationary position. The car seems to be acting more like a SMG than an auto.

Has this occured with anyone else?

TIA
lap

TC32AMG

An automatic car rolls a little bit after you put the transmission in Park and release the foot brake. This is common when you do not use the parking brake, it is the transmission engaging the parking pawl in the transmission. The weight of the car resting against the parking pawl can make it almost impossible to get the shifter OUT of Park but when you do you get the clunk that you are hearing as you disengage the parking pawl.

It is normal for this sound if you do not use the parking brake when parked on an incline. I have driven many automatics from different manufacturers and this is common for the car to move forward when parked on an incline and the corresponding thump when you shift the car out of park.

You SHOULD use the parking brake when parked on an incline as you can damage the parking pawl if you constantly park on an incline and use the transmission to hold your car.

As for the rolling backwards on an incline with an automatic this is also normal depending on the steepness of the incline and depending on the manufacturers specs on the transmission.

Automatics 'creep' when on level pavement this is, because the transmission uses a torque converter which is a viscous coupling. The transmission disengages when engine rpm's drop to idle speed. But since it is a viscous coupling there is no definite mechanical disengagement of the transmission and the engine like in a car with a standard gearbox. The fluid in the transmission is still 'connected' to the engine via the torque converter. Thus the car creeps very slowly forward or backward when in reverse.

You can rock the car back and forth using the gas on an incline because as you apply gas the torque converter is starting to 'catch' the fluid in the torque converter is starting to turn the impellers in the torque converter. When the amount of gas you apply matches the force that is pulling your car backwards on the incline you are reaching the stall speed of the converter (nothing having to do with the stalling of your vehicle)

So everything you describe is normal except your last post about the vehicle actually stalling, were you mucking around trying to hold the car on the grade with the gas? That can sometimes do it. Should not really do that as it puts strain on the tranny's as you are putting an uncessary load on it. Same goes if you are stuck in traffice for long periods of time with the car in drive. If you are sitting ther going no where then you should put the car in park or neutral.

T

rguy

TC32AMG, thank you for acting as a second voice of reason here. The first time I drove a mercedes the rollback suprised the heck out of me. When I dug into the issue and learned about the dangers of not using a parking brake and other such transmission related issues, I became a true believer in the parking brake.

I don't think mercedes designs their cars for what people should do, but rather what they do do as I engaged the parking brake one time this summer on $60k mercedes and the pedal broke

Anyway, needless to say, they replaced it under warranty. They also said they had never heard of that happening before.

It brought a tear to my eye.

Anyway LAP, yeah ditto on whatever TC32AMG said. It is a characteristic of all autos and especially mercedes autos. As a last resort, you can use just enough brake pressure to hold the vehicle on the hill and feed in about five-hundred revs with your happy foot while slowly releasing the brake, kind of like a clutch. This will give you extra hold and keep you from rolling back while stressing the parts the least of all methods I have run across.

The stalling is not normal as aforementioned by TC32AMG, but if you were on a big uphill slope and you have a slow or unsmooth reaction it can happen. Best of luck!

rguy

As a simplified model of why your car stalled, it was probably for the following reason.

At idle your car puts out only so much torque. Also, your car weighs only so much. The tendency of the car to want to roll back down the hill competes with the tendency of your engine to want to move the car forward in any forward gear.

When the weight of the car accelerating backwards overcomes the capabilities of your engine at idle, it bogs the engine causing it to stall.

That is an oversimplification, but in essence is what is most likely going on with your vehicle.

Secondly, I am assuming you have a winter/comfort or sport transmission. In other words, your transmission has a switch that says either "c/s" or "w/s". If you are driving around in (w)inter mode or (c)omfort mode then the car starts in second gear. 1st has more torque to the rear wheels as it is a shorter gear. If you are getting this problem when in these modes, then try using (s)ummer or (s)port modes. This may fix your problem too.

lap

Let me offer the following insight.
A) with respect to "parking" a car on inclines I completely agree with your observations and have been practising the same for years. This does not seem to be the big issue at hand.

B) the issue that seems to trouble me most is when I am engaged in drive.

1. Yesterday, I took my car to a driveway with a slightly less aggressive grade (+/- 20%) and brought the car to a stationary point. 2 people in car, 'S' mode.
2. I slowly let off the brake (car still engaged in drive) and it started to roll backward
3. the car started to pick up some rearward momentum as it rolled back (+6 feet)
4. the transmission (?) started to attempt to brake itself to stationary position
5. the car started to hop somewhat from the braking action
6. the car stalled!

7. I repeated to bring the car to the same point and as it started to roll backward, I feathered the gas to maintain a 'fixed' position, the car did not stall.

8. I repeated to bring the car to the same point and as it started to roll backward, applied brakes gently to stop the car, the car did not stall.

9. I repeated to bring the car to the same point and as it started to roll backward, applied brakes abruptly, and on several times (after the vehicle had attained some significant rearward momentum) the car stalled!


I truly appreciated your comments, it just seems to me, that in any proper functioning automatic transmission, the vehicle should not roll back that much to a point of stalling by itself.

I also looked into my last service records and it was noted that the tranny fluid was "topped off". My service advisor also told me that there were no apparent leaks visible. The fluid was low & no error codes were thrown, they just picked it up from a visual inspection..

Maybe I can drop in an old Porsche G-50 5 speed instead...

rguy

LAP,

I would say you have either a slipping gear, a line pressure problem, or the transmission control unit (tcu) is whacked out (technical term )

I stand behind what I said as to what is causing your car to stall. But now that you have provided a much more clear statement of your problem, I can more competently answer. 6' of rollback is and definitely not normal in Drive.

I would take it to a tech and reproduce the same experiment you conducted on the driveway. They will know immediately what is wrong, or where to look. If you have an AMG transmission, which I am guessing you do, it may have been dragged or tracked if you bought it pre-owned. Or, if you do some spirited launch techniques this can screw 1st gear as well.

It really sounds like a computer problem though, since you can fix the problem by using the gas. I don't know if the tranny just goes to a back-up gear, like 2nd in that situation.

I would start there and see what happens. Keep me posted, this one is interesting.

Sorry it had to happen to you, you seem like a nice person.

TC32AMG

Lap,

I was curious so I went out today to replicate your issue. First i tried a driveway with an incline in a parking garage. No problem the car held without anything of my doing.

So next I went to a steeper hill that i know that will cause the car to roll in the opposite direction of what gear the car is in. Its a pretty steep hill so I am guessing the driveway you are doing this on is not a slight incline but a pretty steep one (20% grade is steep!! 20 ft change in elevation for each 100 feet in distance)

Car rolled back, feathered the gas car held, no prolems. What I did find was that that the car did not roll back on the same hill if i put it in (W)inter mode. Now that I think about it, when in winter mode the car starts in second but I am guessing that it is also locking the torque converter as you can hear the transmission doing somethinig.

I used to have a switch in my Grand National that could lock the transmission in any gear I wanted and I remember the same kind of thing as when I put the C32 in W mode. (Good for drag racing i think I will try this next time out at the track...hmmm)

Anyways, back to the issue at hand. Locking the torque converter will eliminate the slipping that allows the car to roll back in (S)port mode.

Next I tried letting the car roll and pick up a little speed and adbruptly applied the brakes, car stalled. Then I tried it the other way facing downhill with the car in reverse, let it pick up some speed jam on the brakes, car stalled.

From this experiment I found that you have to have a bit of speed (as you had mentioned above) and jam on the brakes fairly hard to get the car to stall, certainly not something I would ever do on a regular basis no matter how steep the hill as it took several seconds for the car to pick up that amount of speed.

I would imagine this happens to you and is a concern because you park in a spot with a steep driveway that is quite tight and don't bother shifting into drive as you manouever into the space? I have a friend who has a driveway like that, it is murder. I just park in front instead.

Secondly, thinking all this over I am pretty certain that this could not be healthy for the transmission as Drive is for moving forward and letting the car roll backwards while running is basically forcing the transmission to run in the opposite direction of the forward gear (thus the hopping you describe, and i felt it when i was testing too) , very hard on the clutches, valves and seals. It says in the owners manual to avoid using the throttle to hold the car as it causes unecessary heat and I would also imagine wear on the transmission.

Lastly I think that this may even be a safety feature of the vehicle in that if you got into and accident and the car was drifting backwards or forward in the opposite direction of the gear selector that the vehicle would shut down to prevent it from rolliing any further and to stop the fuel pump from pumping fuel. Some cars have a fuel cutoff that cuts power to the fuel pumps in case of an accident. This kind of makes sense since in a serious accident the doors unlock after the airbags deploy.

I would also imagine as the car is drifting backwards and you stop suddenly you are stopping movement in the driveline, like putting a stick in the spokes of a moving bicycle, this could very well stop the motor as the intertia from the engine side of the torque converter is not great enough to overcome the weight of the car and everything else connected to the transmission side of the torque converter. Remember the torque converter is a viscous coupling and has some room for slippage explaining why the stalling does not occur when the brakes are applied slowly, like having your hand in water and moving it slowly versus really tring to move it rapidly.

I don't think that this is a serious problem with your transmission if it seems to work fine in all other instances. I would just not allow to let the car roll like that in everyday driving or hold the car using the gas. Driving automatics all this time I have never had to let the car roll back more that a foot while in drive.

I believe there is a TSB (technical service bulletin) out on leaky transmissions on C32's.

I am not worried about my tranny even though I could duplicate the problem. But for your peace of mind you should go to the dealer and let them check it out. Shouldn't cost you anything as the car should still be under warranty.

Geezz getting long winded sorry. I am no mechanic but I did a few years of shop way back when correct me if I am wrong somebody.

T

FrankW

Normal

rguy

I keep on forgetting that 11.3 degrees is a good deal of hill for a car to encounter. I used to spend a lot of time in the off-roading world and 20% just doesn't register as much to me anymore. In this case, it would be normal, and I thank everyone for pointing that out. I should have just stuck to my guns about it being normal, eh? Anyway, I don't know why anyone would let the car roll back that far, and that probably aided my perception that it was abnormal for a car to roll back 6'. I am going to try it myself tomorrow. I will let you guys know if I am able to duplicate TC32AMG's results. Gold stickers all around people.

benz-tech

It sounds more like a possible mechanical fault in the freewheeling unit inside the trans. i say possible, because i don't think you are experiencing anything abnormal. MB stopped saying in print that their transmissions would hold on a hill. One way to semi self-test your freewheeling unit is to manual hold car in 1st gear drive to about 3k rpm and let off. you should feel the decel as you let off. if the car feels like it starts coasting, then its bad. an other way to identify a weak/bad freewheeling unit is a harsh bang when coming to a stop on the 2-1 downshift. "w" or "c" mode may not hold on a hill in 2nd gear. i've built many 722-6 trans, but never for this complaint. I'm curious to hear what your dealer says about after checking the internal data. HTH

lap

for my pre-winter setup & will let you guys know if anything new arises.

As for the time being, I guess my excuse is "Sorry Officer, I had to mash the pedal so it wouldn't stall!"

Cheers.
lap