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auto box questions

greg123

Member
Joined
Feb 10, 2003
Messages
46
Location
Nottingham UK
Car
12, daily driver a hotted up Octavia 1.9tdi (custom injectors, ARB, poly bushes, cupra wheels etc)
I have the E class bible and manual but they have very little info on the auto box. According to the bible the torque converter has a 98% efficiency as full working speed (I guess approx 3k revs) but no mention of a lock up clutch. In the Ford, GM, ZF (BMW/Peugeot/Range Rover) they all have a clutch in the torque converter to lock the flywheel to the gearbox eliminating the torque converter once the car is rolling in 3rd/4th gear. This has great advantages in power and economy and lowers the revs of the engine. I can't believe with the price of the car that MB didn't have a similar system, do you all notice if your cars on the open road are 'locked' to the rear wheels ie take your foot off the throttle and the revs don't alter? Anyone know the tech specs of these boxes and if they have a converter clutch or similar system? Also any differences between the 300/24v box and the later 320 engine's box - I hear it starts in first, maybe it also has a converter lock up?
 
I don't know too much about that model, but I know that the SLK32 and C32 have a lock up converter in every gear apart from first, so I would imagine the 'standard' cars would lock up in fourth and fith.
 
Greg123,

Further to my other posting on the subject.

I've sat behind a GM autobox for the last 8 years (in a Senator).

The torque converter lock-up only happens when there is next-to-no speed difference in revs between flywheel and output shaft.
It is purely an economy device.

The purpose of the torque converter is to transmit torque as efficiently as it's design constraints allow. To get maximum transfer of power, you need maximum rate-of-change between the in and the out, so locking the converter in this instance would be counterproductive. This is why the box software unlocks it when it does.

If you don't do huge numbers of motorway miles, the economy improvement over a non-locking converter won't amount to much.

Agree about the E-class Bible, not a lot of in-depth information there..........
 
Paul,

Originally posted by PaulG
The torque converter lock-up only happens when there is next-to-no speed difference in revs between flywheel and output shaft.
You're right, the lock-up is getting the last 2% - assuming Stu is right with the 98% efficiency of the converter.
The purpose of the torque converter is to transmit torque as efficiently as it's design constraints allow. To get maximum transfer of power, you need maximum rate-of-change between the in and the out, so locking the converter in this instance would be counterproductive.
The purpose is to transmit torque and revolution.
To get the car moving, the converter converts revolutions into torque (engine is running, trans not). To do that, the converter is slipping.
Once the car is running and the revolution of the engine and the trans is (nearly) the same, the converter should transfer revolution and torque with minimum slip and minimum power loss.
This is why the box software unlocks it when it does.
Sorry, there is no software in a 722.3 or 722.4 transmission. It is hydraulic only.
Agree about the E-class Bible, not a lot of in-depth information there..........
How much did you pay in the UK for the Bible?
I met Stu in an american mailing list and he should have some knowledge of the W124 (and other Mercedes types).
Anyone want's to sell his Bible to me? :D

bis denn,
Christian

1989 300TE
 
lock up

I hear you! That said, saying it makes little difference is pretty much the same as saying driving down the road in a manual while slipping the clutch in 5th, and pulling off in 3rd while slipping the cluch is a good thing. That too gives more torque than when the clutch is let out (why we slip the clutch to wheel spin off, and need to slip it to pull off in 2nd or there wouldn't be enough torque at the low revs) but obviously the main reason why manual boxes are quicker and more ecnonmic than auto (old) boxes is that they lock up the box to the engine with 100% efficiency. If you need more revs you don't slip the clutch (converter) more, you drop into a lower gear where you get those same revs but with 100% efficiency. I'm not too worried about smoothness, I want power and economy which is why I go off on one too much about locking converters!!! My BMW 525e 2.7 can manage 40mpg, that's on an auto. Of course the engine is in a torque tuned state but even so it's still efficient!
 
Greg123,

This is turning onto one of those Saloon-bar discussions after about four pints..............

It's not quite true to say that slipping the clutch on a manual is the same as slipping the torque converter in an auto. The torque converter actually gives torque multiplication (up to 2:1 in the 124 transmission), something that the manual can't do. This is why for any given situation an auto is FASTER than an equivalent manual, not slower. The exception being at wrung-right-out top speed where a small amount of power is lost to slip, and at the other end where the auto boggs-down the engine for a second or so until it reaches it's power band.

This last point is why manual cars always appear faster 0-60 than autos, because the testers dump the clutch at maximum power, so there's no bogging-down. NO owner would accelerate his own car like that on a regular basis (alright, not many would) ;)

There's no doubt that pumping losses account for the higher fuel consumption of an auto over a manual, but the gap is closing all the time......
 
Electronically Controlled Automatic Transmission

The optional electronically controlled five-speed automatic transmission independently adapts the shift characteristics to the current driving situation and the driver's own wishes. When cornering, or on uphill and downhill inclines, the transmission's electronic unit alters the shift points automatically to prevent sudden gear changes which could otherwise hamper handling. Continuous comparison of data from various sensors with stored target values allows the transmission to adapt to the current situation promptly. Furthermore, the electronic unit monitors the movements of the accelerator pedal to work out how quickly and up to what speed the driver would like to accelerate. The torque converter lock-up also activates at low speed, thereby helping to reduce fuel consumption. This operates with a continuous computer-controlled slip, which de-couples the link between the engine and transmission just enough to eliminate any irksome vibrations. The winter program can be activated by pressing a button on the centre console. This makes pulling away on icy or snow-covered roads far easier. The transmission's winter program also comprises a second reverse gear with a longer ratio.

The above courtessy of the MBspy website on their glossary page.

Hope there is something in there of use.

S.
 
lockup

I think the main point that illustrates it all is, for example, when in our Range Rover at 55mph and the converter locks up, WITHOUT moving the throttle you instantly get a push, the revs drop and the vehicle climbs to about 65mph, if less throttle is then used (and less RPM than before lockup at 55mph) I can cruise at 60, on the same throttle it goes to about 70 for the same engine rpm too. That illustrates just how torque converters are LESS efficient than a clutch. The only advantage is they allow, as stated by other posts, torque multiplication to occur by letting the engine speed up and generate more torque and then transmitting most of this torque. It is never as efficient as using a much lower gear to multiply the torque and having a locked transmission though, which is why racing cars and indeed modern autos are in reality automatically shifting manual boxes, often there are no converters to slip and if there are they are just used for purposes of moving off, once rolling the appropiate gear can be locked up to the trans. Anyhow this doesn't really help me get the W124 locking I guess! Thanks for the tech on the merc guys, was needed (bible hopeless).
 
There is no lock up clutch on the 4 speed boxes, this feature was only introduced for the 5 speed 722.6 box.
Even on the 5 speed you wont know it's there due to it engaging proportionally to vehicle speed and inversly proportionally to engine load.
It has three modes, disengaged, slipping, locked up. To obtain full lock up the vehicle must be passing the Star Ship Enterprise with no pressure on the accelerator pedal.
 

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