What disconnects the transmission when you are in park?

reuben

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I know very little about engines etc, but I'm trying to understand them a bit more.

So when I put the car in 'park' then I assume there is a clutch which is between the torque converter and the engine which disconnects - is this right?

Also, in the torque converter, is there also a clutch which engages at higher speed for greater efficiency?
 

television

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All that happens when you go into P or N is the oil supply the the converter is shut off with just a minimum amount of oil in the converter,hence the creep.
The 722.6 never completely locks up,,you can prove this yourself. just drive along say at 70mph and blip the throttle slightly, the RPM will change but the car speed does not
 
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reuben

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Got it, that makes sense. And that test also makes sense, thanks.

So the input side of the torque converter is turning when you are in P or N, and the driven output side is not turning? Then when you engage D or R the oil is pumped in and the torque increases on the driven output side?
 

television

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You are spot on sir :D:D
 
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reuben

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Ok, so next question is, what controls the amount of oil pumped into the torque converter? Variables which must affect it would presumably be the speed of the car, the postion of the brake pedal and of course the position of the gear stick?
 

rich.g.williams

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I think the oil pump is in the Torque Converter. Torque Converter needs engine RPM, low RPM no drive high rpm lots of power passed through torque converter.

In Park there is a "mechanical pin or pawl" that locks the transmission. So no output rotation possible.

In neutral if engine revving output of torque converter will be revving but the control of the transmission does not allow any gear selection so no drive out.

In any other gear as many as three brakes and three clutches in the "gearbox section" activate to put the transmission in appropriate gear and you get drive.

These clutches and brakes are controlled by valves and solenoids which in turn are controlled by the ETC computer.

There is no clutch between engine and torque converter or between torque converter and gears, its the lack of rpm.
 
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television

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I think the oil pump is in the Torque Converter. Torque Converter needs engine RPM, low RPM no drive high rpm lots of power passed through torque converter.

In Park there is a "mechanical pin or pawl" that locks the transmission. So no output rotation possible.

In neutral if engine revving output of torque converter will be revving but the control of the transmission does not allow any gear selection so no drive out.

In any other gear as many as three brakes and three clutches in the "gearbox section" activate to put the transmission in appropriate gear and you get drive.

These clutches and brakes are controlled by valves and solenoids which in turn are controlled by the ETC computer.

Only the output shaft is locked in P. The oil pump is separate
http://www.detali.ru/cat/oem_mb2.as...24716.633&CT=GA&cat=23V&SID=27&SGR=020&SGN=02
 
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reuben

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Ooh thats good reading, roadhog. Going through it now.
 

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Television has it right, in park its just like neutral, there is no oil pressure supplied to any of the many (13 in my 124) clutches leaving the output shaft completely uncoupled from the input.

The parking pawl is no such thing, it was a simple bit of steel that was thrown into a gear so as to lock it on very early power shift transmissions, it is in reality a dog clutch surounding the output shaft which jams the transmission in park, it lives right in the back of your transmission, this arrangement is much stronger than a pawl and considerably less prone to damaging the box if an attempt is made to engage it whilst in motion. Severe damage can occur to all the rest of driveline if an attempt is made so don't try it.

Please dont rely on it as a handbrake substitute, use both.

Sleep tight.
 
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reuben

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Bit confused now.

Malcom says that in P or N the oil in the torque converter is shut off, meaning the output to the gears doesnt turn at all, although the input from engine does. I took this to mean that if the car is stationary, then the output of the torque converter does not turn at all.

Wireman says that its the oil pressure to the clutches, which I assume are the ones in the gearbox (i.e. past the torque converter). This would suggest the output of the torque converter does turn when you are in P or N but the gears are not engaged?

Which is it?
 

television

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The torque converter is nothing more the a box containing two impellers, the box containing the first impeller is fixed to the engine fly wheel, so that goes round and round all the time, If you have another impeller facing the first one, and this impeller is attached to the the prop shaft, once you fill the torque converter box with fluid, the impeller that is fixed to the flywheel will force the fluid to go round with it, and as it does so it turns the impeller that is joined to the propshaft.

Some call this a fluid coupling,and it is. Once you cut the supply of fluid as it does when in idle, then there is not enough pressure in the converter box to transfer any fluid to the impeller on the prop shaft, therefore no drive.

When you are in D there is not enough pressure within the torque converter box at idle to turn the second impeller very much other than to make the car creep. As as you rev the engine then there is enough pressure to turn the second impeller on the propshaft.

The gear box section is some thing else and just means of having gears. The first auto boxes were only 2 speed, but fitted to the USA giant v8's.

Yes there is a true N but note that the fluid pump as in my first link is also on the drive shaft, so its output is proportional to the engine speed.

The parking pawl is 35 in the link
http://www.detali.ru/cat/oem_mb2.as...17717.418&CT=GA&cat=23V&SID=27&SGR=095&SGN=09
 

rich.g.williams

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malcolm, reuben,

the traditional torque converter (and the one in 722.6) works on the presence of oil (pressure it is under is not relevant) the presence of oil provides a hydraulic coupling between input and output.

more recent torque converters (and in 722.6) have the addition of a "lock up clutch" inside.

the lock up clutch does work on oil pressure and it can be activated externally by the lock up solenoid (pwm solenoid) via the valve body etc. the purpose of the lock up clutch is to join input and output shaft mechanically such as to remove wasted energy due to the hydraulic coupling.

the lock up clutch is secondary to the main elements that you need to study to understand how an automatic transmission such as 722.6 works.

PS I am hoping I have explained correctly!
 

television

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malcolm, reuben,

the traditional torque converter (and the one in 722.6) works on the presence of oil (pressure it is under is not relevant) the presence of oil provides a hydraulic coupling between input and output.

more recent torque converters (and in 722.6) have the addition of a "lock up clutch" inside.

the lock up clutch does work on oil pressure and it can be activated externally by the lock up solenoid (pwm solenoid) via the valve body etc. the purpose of the lock up clutch is to join input and output shaft mechanically such as to remove wasted energy due to the hydraulic coupling.

the lock up clutch is secondary to the main elements that you need to study to understand how an automatic transmission such as 722.6 works.

PS I am hoping I have explained correctly!
Yes you have thank you.

The 772 box never locks up completely, only momentary when changing. this is shown on cars with the glycol issue where the humming is caused by the faces of the clutches being raised by the glycol. they only make the humming when pulling lightly in top
 

rich.g.williams

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malcolm,

I am not questioning what you say, my knowledge of MB is very recent and limited in comparison.

Can I just say it this way, assuming you are talking about the lock up clutch only:-

The 772.6 box lock up clutch will close at high RPM (approx 2500 and up) but is open or slipping below this.

The presence of any glycol makes the lock up clutch grip more at much lower RPM - low enough to cause humming/vibration.

When changing gear the lock up clutch is probably disengaged there is no need to engage it, the gear change is accomplished by the other clutches and brakes B1 B2 B3 K1 K2 K3

The 722.6 is a superbly designed item of electronic, mechanical, hydraulic and electromechanical technology.
 
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television

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I have never given the workings much thought until today and most was as I see it

As in you last post, the humming from glycol is only there at an RPM of up to 2300 RPM so the raised clutch faces are touching the plate and it that that make the noise. If it was locked up it could not make any noise.

Here is a cut away of the 776.9

http://www.mercedestechstore.com/pdfs/286_722.9/286 Poster cutaway.pdf
 

xavierx

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The 722.6 never completely locks up,,you can prove this yourself. just drive along say at 70mph and blip the throttle slightly, the RPM will change but the car speed does not

I have already found this to be true with my E320CDi, and was wondering if you could explain why Mercedes gearboxes don't lockup? I was under the impression that lockup improves fuel efficiency.

I understand that the Mercedes torque converters do have a lockup clutch, so when is it used?

Thanks for your help


Edit : Ooops sorry didn't see the second page. OK so they do lockup and higher RPM, makes sense, but why not while cruising on the motorway at say 60-70mph which on a CDi is very low rpm? Surely that is where you would see a great benefit from fuel economy improvement?
 
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