X5 3.0 Steptronic Question

[withidl]

Originally Posted by Steved

withidl, good explanation of the mechanics of the torque converter I've just been having a similar discussion with rgnbull on a private message (about his 4.8is), I should point him in the direction of this thread to read your explanation also.

The ZF 6-speed gearbox in the 4.8is works to a similar 'mechanical' principle as your 5-speed auto but the software is now more sophisticated. I have the BMW TIS journal for the 6-speed ZF Auto which I have posted a few abridged sections on X5 World before. Essentially the torque converter works hydraulically by the same principle as you described, but the mechanical lockup clutch responds to several factors:

- Engine load (as per your 5-speed)
- Vehicle road speed (but I'm not sure if it uses the same 60-62mph trigger)
- Transmission oil temperature (only when above 35 degrees C)
- Selected software shift program (there are 13 available shift programs including four programs: XE (extreme economy), E (economy), S (sport) and XS (extreme sport).

All of these factors affect the way in which the engine is mechanically connected to the input shaft of the transmission via the lockup clutch.

To ask my own question to the open forum, why does the engine temperature rise when driven in the steptronic MS mode? Any thoughts? In every 4.8is that I've driven the engine runs at around 90 degrees when in drive but if driven for 10 or 20 miles in MS mode the temperature rises to around 100-110 degrees. Perhaps the torque converter is more engaged in 'multiplying' the torque (when in MS mode) allowing the engine to spin faster than the transmission and therefore heating up both?

Steved, regarding engine temperature being higher in MS mode, I agree with you that in MS mode the torque converter will more readily unlock and stay unlocked (mine does) because the programming is anticipating that you are driving the twisties and will be demanding more immediate power. With the torque converter unlocked the engine will be making more rpm's AND like you say will therefore be generating more heat since more fuel is being combusted.

But there is an additional factor to consider, and that is when the torque converter is unlocked there is dramatically more fluid friction = heat being generated within the torque converter itself which MUST be dissipated.
The heat is dissipated through the transmission pumping it's oil to the engine radiator which increases the load on the radiator and therefore increases coolant temperature.

Incidently, at cold start-up the transmission's pumping of it's oil to the radiator functions in reverse in that the transmission draws heat from the radiator coolant being heated by the engine so as to come up to operating temperature more quickly.