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  #1  
Old 06-10-2000, 05:26 PM
netdw
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I have a 92 SL. While stop on a slight or steep incline, I can then let off the brake and the car stays without going back. My question, is that the transmission braking it, or like some Suburaru, the brakes auto engage. or is it?? And does it do harm to my car. thanks guys.
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  #2  
Old 06-10-2000, 08:25 PM
akry's Avatar
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Join Date: May 1999
Location: Burnaby, BC, Canada
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By stop the car on hill, and let go of the brake pedal, you are putting stress on both engine and, especially, the tranny. There is no auto-engage brake(My GF has a Subaru Impreza 2.5RS, and it slides backward when let go the brake on hills), and NEVER use such means as your red-light short stops....

Andy Kuo

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  #3  
Old 06-10-2000, 10:30 PM
roas
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I am not the most knowledgeable when it comes to auto transmissions, but this is your transmission keeping you from moving backwards. I believe the tranny at a stop in not actually disengaged in the sense that a manual would be. The engine and tranny are working on pushing the car forward, even at a complete stop, kind of like a boat prop at a very low speed. This is why you have to apply brake to all Auto trans cars at a stop or else the car will move forward. Floor it and the transmission fluid will "Grip" and drive the car forward.

Maybe someone with a more thourogh understanding can elaborate?

Ross

[This message has been edited by roas (edited 06-11-2000).]
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  #4  
Old 06-10-2000, 11:36 PM
akry's Avatar
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Roas,

You are right about the tranny. And never do it on hills. Imaging it, you stop on a uphill, at a red light, leave the tranny in D and let of the brake. The engine is idling, and pushing tranny, push the car to stay on the hill......if you can imagine the inside of the tranny......not good...

Andy Kuo

------------------

  • 1992 Mercedes-Benz 400SE
  • Pearl Grey/Black Leather

ICQ#26950002
Mercedes Owners ICQ ActiveList ID#61730549
Mercedes S-Class Page
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  #5  
Old 06-11-2000, 02:03 AM
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An automatic tranny in gear is "in gear". If the wheels aren't turning then nothing in the trans is turning.

The fluid part of the power flow comes from the torque convertor. The torque convertor has a pair of turbines (fans) inside. One is turned by the motor and the other turns the transmission input shaft. The coupling is similar to placing one electric fan in front of another and spinning the second one off the air of the first. The torque convertor is of course working with a much more solid fluid than air and is helped by the action of a stator mounted on a one way clutch, but the power flow becomes rigid at some speed (stall speed). When this occurs the connection is almost totally solid. This occurs at over 1500rpms on MBs.

On drag racing automatics they remove some of the fins from the driven fan to move the stall speed to over 4000rpm. The point is to get the rpms up for power before the car leaves the line. I only mention this to add understanding of the coupling effort.

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  #6  
Old 06-11-2000, 02:48 AM
Larry Delor's Avatar
What, Me Worry?
 
Join Date: Jun 1999
Location: Sarasota, Fl.
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netdw
My 83 300D is the same way...one day wilst waiting for a traffic light to turn the proper color on one of the steepest inclines for miles around, I let my foot off of the brake pedal very slowly, only to discover the car would not budge a bit. My 300 did not idle abnormally high to explain this behavior. Any other domestic car would have rolled backwards like a polititian trying to please the corporate sponsor of the month.
Naturally I wondered also as to wether or not the trans had some sort of hillholder or antirollback-while-in-drive feature.

-Larry
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  #7  
Old 06-11-2000, 05:13 PM
netdw
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So according to StevenBFL above, what does that translate to the braking on hills?? Good, neutral or bad?? My RPM do not rev nor have any noise from the tranny or engine, acts just like if I had my brakes on.
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  #8  
Old 06-11-2000, 06:17 PM
amgkid
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i think it's just the transmission holding it up the hill, just like as if you had it on a hill and in park without the emergency brake, which isn't good either.



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  #9  
Old 06-11-2000, 06:45 PM
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Many transmissions won't allow the gears to back up. If you are rolling backwards and through the thing in forward on many trannies the thing locks up. The one way clutch just stops it BAM!! Hondas are like this. I don't think any MBs do this.

If you are on a big enough hill and the engine speed is low enough the tranny will back-up. It is easier to see by doing the manuver above; the placement in drive while still moving backwards.

One thing to remember is that if you go in reverse while in drive the relative speed (one fan versus the other)goes up quickly do to the high (numerically)gear ratio of reverse.

I would say that there would be no more load in doing this than reving the motor (while in gear) to say 1000rpm while holding one's foot on the brake.

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26 years MB technician
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  #10  
Old 06-11-2000, 06:48 PM
roas
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Ladies and Gentlemen,

The answer is...... Don't do it. Simple, it causes unwanted stress in the transmission and engine.

One other point the is possibly even more important, is that you do not want to put a lot of stress on your engine at a low rpm. This would be, chugging up a hill in a high gear, or pulling a large load up a hill in a high gear (low rpm). If you "bog" the engine down over time, I have heard you can bend rods (not sure if someone else can confirm this), over heat the engine, as well as over-tax your lubrication system? This of course is far from ever occuring at a stop light while on a hill, but just one more thing to consider for engine and transmission longevity.

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  #11  
Old 06-12-2000, 02:06 AM
tracy_leb
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Interesting. FWIW, My (former) 300 TE 4-matic would not roll backwards at idle on even the steepest streets in downtown Seattle. It would never bog or have any problem accelerating away from a stoplight. In contrast, my ML320 will roll backwards on even the slightest grade, and it too has no problem accelerating away.
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