Torque converter lock up?

[Cooky]

I've searched the forum for this subject, but what I found was inconclusive. My particular question relates to a C250TD (1998 model).

Should the 5 speed auto transmission lock up the torque converter in 5th gear at a particular speed? Mine shows 75 mph at 2500 RPM, but if I back off the throttle the revs fall back a little; this suggests that there is no lock up - but is as it should be?

I have a particular reason for asking; I tow a caravan and my speed limit is 60mph (about 2000 RPM). If there is no lock up of the torque converter then I will get a fair amount of torque converter slip under these conditions and I may be better off towing with the gear selector in 4th; that way I get about 2500 RPM at 60 mph and the whole thing seems more flexible with less slip.

Does any knowledgeable or experienced Forum member have any views on this subject?

Thanks in advance, if you can help.

 

[wingco]

Sorry I cant help with your question, but what do you mean by torque converter lock up?

 

[Cooky]

Torque converter lock up is where (at a pre-determined road speed in top gear) the torque converter input and output are mechanically locked so that there is no slip in the torque converter. This gives the benefit of improved fuel consumption and engine braking, down as far as the pre-determined speed when the converter unlocks and reverts to its normal mode.

Lock up can usually be perceived by a slight drop in revs when it activates, and it could be confused with a gearchange - except that above this speed engine revs are locked to road speed, as they would be with a manual transmission.

 

[wingco]

Thanks thats very interesting. I would like to know the answer.

 

[OlafMaxwell]

I use mine for towing too but only a small camping trailer although I am thinking of upgrading to a large caravan to leave in France so this is ofminterest to me.

I have looked in the manual and there is no reference to a mechanical lock up. I was very familiar with that and the benefits but there is no reference of any kind to it. A look in a Haynes manual may clarify it for you, it's the only thing I can think of other than asking one of the auto transmission specialists. Try some in Mercedes Enthusiast.

 

[Cooky]

Thanks Olaf - I couldn't find anything in the manual either.

I need to get the transmission fluid changed anyway, so I'll find an auto box specialist to do it and ask him about lock up. When I find the answer I'll post it.

Cheers and beers

 

[dieselman]

The T/C lock up clutch is controlled using gearbox oil pressure through a proportioning valve.

The faster the car goes and with the lower the accellerator depression the more the clutch locks.

i.e. High speed, low load = locked
Low speed, high load = unlocked
It is proportional between the two extremes.

 

[Cooky]

Thanks Dieselman - that's very useful information. It tallies with what I have observed, i.e. no definite lock up point (such as I had with a four cylinder Vauxhall automatic) but a sense that there is some "firming up".

TTFN

 

[oigle]

Further to dieselman's reply, the torque converter never really totally locks - there will always be some slip so revs will drop a touch even at high speed on backoff.
Re towing, correct gear usage at cruising speeds is governed by the size of the caravan and how easily the car tows it. A good way of determining which gear to use is to cruise your vehicle at a given speed on a long straight piece of highway in 5th. Holding the throttle steady, change into 4th and see if the speed increases or decreases - choose the one that gives you the highest speed. This will also give you the best economy usually. Of course, in undulating country, you will probably find that the lower gear performs better.
Hope that helps. I tow a 1600kg van with an ML270.

Oigle