Mercedes Transmissions - PeachParts Mercedes-Benz Forum

Alan Hamm · #1 07-21-2004, 08:26 AM

Been wanting to ask this for some time.

For some reason, perhaps because I can, I often shut my Mercedes cars off with the cars in Drive. And because I can, I remove the key without putting the car in park. And then the cars starts to roll.

1. If left unchecked, how far would it roll?

2. What caused the transmissions to disengage when the engine is off? Is this controlled by vacuum?

3. How is this good? Why would they be like this? Seems dangerous, unless there is a certain distance where it rolls before it catches. I have not wanted to test this, but I have been half way out of the car and had to jump back in and hit the brakes.

TwitchKitty · #2 07-21-2004, 08:42 AM

My car will roll away if irresponsibly left unattended. It has a manual transmission. It is really no problem to leave it in gear and set the parking brake. I use the parking brake in my vehicles with automatic transmissions to keep the strain off of the parking pawl.

Pete Burton · #3 07-21-2004, 09:25 AM

What you are referring to is actually a design feature. Above a beer hall in Stuttgart, a small group of MB engineers and techs work diligently at introducing such features. Frequently, they take breaks from their work and go downstairs for inspiration. In this case, they intended to empower their cars to run away from owners who neglected them, perhaps finding a new home near a saugage factory

wagonpilot · #4 07-21-2004, 09:34 AM

Yep, them damn Mercedes transmissions are screwed up.

But, then I guess that means ALL automatic transmissions are screwed too.

ANY car with an automatic tranny won't roll *backwards* with the engine running due to the torque transmitted through the torque converter. if you were to loose your torque converter, then yes, the car would roll backwards.

I do agree with you about the safety hazard though. My three year old took our TD for a "drive" across the driveway when she accidently kicked the selected out of park while playing in the front. (car turned off)

Sometime when you you're feeling really smart, start reading up on how an automatic transmission really works. It'll make you dizzy ! I'll bet you a case of cold ones that after reading all the descriptions you still can't explain it to your idiot brother-in-law!

Cheers.

WP

Ted_Grozier · #5 07-21-2004, 09:50 AM

It is now federal law in the United States to have an ignition-shift interlock on auto trans vehicles. Specifically, the key cannot be switched off and removed unless the transmission is in park and, under a separate law, the transmission cannot be shifted out of park unless the key is turned to the run position.

The car will roll forever in drive with the engine off -- it will not "catch" and stop.

Always set the brake!

Alan Hamm · #6 07-21-2004, 09:59 AM

Quote:

Originally posted by Ted_Grozier
It is now federal law in the United States to have an ignition-shift interlock on auto trans vehicles. Specifically, the key cannot be switched off and removed unless the transmission is in park and, under a separate law, the transmission cannot be shifted out of park unless the key is turned to the run position.

The car will roll forever in drive with the engine off -- it will not "catch" and stop.

Always set the brake!

American cars have been this way for years. It seems REALLY dangerous the way MB's are designed. Wierd thing is that now in my wifes Nissan Van I always shut the engine off before shifting to Park.

The other thing I do is turn the key and wait for the glow plug to turn off.........on GAS CARS!. I've created a monster!

P.E.Haiges · #7 07-21-2004, 10:13 AM

Alan,

Its guys like you that causes the govt. to pass laws like mentioned above that make cars more complicated and less easy to use.

What is hard about putting the shift lever in park before stopping the engine. If you start doing it every time, soon it will become an auto-reaction.

I hate these new cars where I have to step on the brake pedal before the car will go in gear. If the car was mine, that would be the first thing I would disconnect.

P E H

Alan Hamm · #8 07-21-2004, 11:56 AM

Quote:

Originally posted by P.E.Haiges
Alan,

Its guys like you that causes the govt. to pass laws like mentioned above that make cars more complicated and less easy to use.

What is hard about putting the shift lever in park before stopping the engine. If you start doing it every time, soon it will become an auto-reaction.

P E H

I assure you the Gov't has never been notified about this by me. So don't blame me for gov't regulations.

I started this thread because I actually wanted to better understand what about the engine running caused the transmission to engage but to release when the car is off. Obviously a different design than "other" cars. Is it vacuum? I also was trying to verify that my experience was "normal" behavior, although having 3 cars that all do the same thing seems to be a pretty good test sample.

But thanks for slapping me around and lumping me in with Gov't regulations idiots just cause I asked a question. Feels good sometimes ;-)

Ted_Grozier · #9 07-21-2004, 12:09 PM

Quote:

I actually wanted to better understand what about the engine running caused the transmission to engage but to release when the car is off. Obviously a different design than "other" cars. Is it vacuum?

All automatic transmissions use hydraulic pressure to engage and disengage the friction elements at each gearset. When the engine is not running, the input shaft of the transmission is not turning and the hydraulic pump in the transmission is not either. Some transmissions, like these, have a sort of tow-to-start capability whereby pump pressure builds if the output shaft is turning, but this is well beyond my understanding...

That the car will roll away in Drive with the engine off is not unique to M-B. Any automatic will do so. The difference is you can't take the key out on most cars unless you shift to park.

Added complexity aside, I think it's a pretty good idea.

Ted

H-townbenzoboy · #10 07-21-2004, 12:22 PM

Quote:

Originally posted by P.E.Haiges

I hate these new cars where I have to step on the brake pedal before the car will go in gear. If the car was mine, that would be the first thing I would disconnect.

P E H

Are you talking about from P to R, or from R into D or any foward gear?
-Joe

wagonpilot · #11 07-21-2004, 12:23 PM

I guess I should have clairified it. ALL automatic transmission cars will do it, but on most american and newer cars you'll never see it because, as others have painfully pointed out) you have to be in park to get the key out. So, it is rare that you'll be in the engine out, drive selected configuration.

Seriously though, it has NOTHING to do with vacuum. When you are stopped, think of your automatic transmission as a automatic clutch slipper. By that I mean, when you are stopped, it's as if you were in a manual trans car and were slightly slipping the clutch. Not quite enough to make the car move, just enough to keep it from rolling backwards. Actually, you'll find many unknowing people actually perform this technique to hold their manual transmission equipped car still on a hill while waiting for a stoplight. Those same people wonder why their clutches only last 30,000 miles.

Hope this helped.

WP

Jim H · #12 07-23-2004, 12:23 AM

The torque converter is a fluid coupling between the engine and the transmission. It transmits torque through the action of the fluid, set in motion by the impeller, reacting against the turbine.

There is NO positive engagement, so there will always be some slip.

(We can safely ignore "lock up" torque converters, which lock at a given combination of speed and engine load.)

The fact that the car rolls when the engine is stopped should not be a surprise, since you can also stop the car with the engine running. In fact, you do this each time you make a stop and leave it in D.

That's also why all cars have a Parking Brake.

Best Regards,
Jim

P.E.Haiges · #13 07-23-2004, 01:38 AM

Benzboy,

Cars that you have to step on brake to shift out of park is what I'm talking about.

I don't like where reverse is either. It should be on notch back from L like older cars were. Much easier to rock out of snow there than having to go thru nuetral.

P E H

kmaysob · #14 07-23-2004, 03:16 AM

The other thing I do is turn the key and wait for the glow plug to turn off.........on GAS CARS!. I've created a monster! [/B][/QUOTE]

i do the smae thing in my moms car.

william rogers · #15 07-23-2004, 03:27 AM

wish I could put a good old GM turbo 400 in my 126's they shift smoothly and will take a beating and then some.as I have posted before Rolls used them for years as they could not build or find a better transmission. A good percentage of the post here are tranny problems ..........

William Rogers.......