[T6_measured] A Comparison of 4Motion traction modes

[mmi]

4Motion Adaptation channel - MAS06098-Acoustic measure wiring logic​

STANDARD / INCREASED TRACTION / REDUCED NOISES​

T6/T6.1 Haldex clutch has an adaptation channel which presumably changes clutch clamping behaviour. The channel can be tweaked with various tools.

Conditions for the measurements​

• Dry tarmac, fairly flat terrain, straight road (no steering input needed)
• The van was crawling with no throttle applied - gear engaged - at stable speed
• The accelerator pedal was floored and kept there untill max speed (road speed limit) - thus recording maximum "degree of blockage"
• Then the van was let to slow down back to crawling - thus recording minimum "degree of blockage"
• Gears 1-4 were measured on all modes, covering speeds up 100+ km/h

Principles of 4Motion​

Have been discussed here - 1st post

[T6_measured] 4motion Abnormality And Subsequent Failure

Reference documents: VW Self Study Programme SSP 561 –The T6 2016, which in chapter Power transmission (p.26) for details on 4Motion control logic refers to: VW Self Study Programme 414 – 4MOTION with 4WD coupling generation IV. The document can be read and downloaded at SSP 414 Spesifics...

www.t6forum.com

Measured parameters​

• Speed of front and rear wheels
• Speed difference between front and rear axles
• Haldex pump current, current PWM ratio, voltage
• Haldex clutch degree of blockage (based on the pump parameters above)
• Sampling rate approx. 5.5 Hz (for full data set)

Haldex clutch - vehicle speed and Haldex clutch degree of blockage as function of time - Gear 3 - STANDARD​

Initial spike most probably is because of flooring the accelerator pedal - timewise it's quite short - approx. 0.5 seconds.
Also in the measurements here the spike is not in "useful" engine rev region (better visualized in pictures below). The spike being at different speed brackets in different modes below is just "operator's error" - obviously didn't let the van crawl slow enough.

Haldex clutch - degree of blockage as function of vehicle speed - Gear 3 - STANDARD​

Initial spike most probably is because of flooring the accelerator pedal - timewise it's quite short - approx. 0.5 seconds.
Also here the spike is not in "useful" engine rev region.

Haldex clutch - degree of blockage as function of vehicle speed - Gear 3 - INCREASED TRACTION​

Haldex clutch - degree of blockage as function of vehicle speed - Gear 3 - REDUCED NOISES​

Apologies for titling as [T6_measured] although this one being a T6.1 - just wanted not to deviate from other "reports" in the series . On the other hand the engine on this one is the same CXFA series as experimented in thread [T6_measured] 4motion Abnormality And Subsequent Failure thus the results should be similar on both T6 and T6.1 with same engine.

 

[mmi]

 

[mmi]

Something very different - gear 4​

Haldex is disengaged between 60 - 70 km/h, or 1700 - 2000 RPM​

 

[mmi]

Reserved.

 

[mmi]

Reserved.

 

[n10n]

Haldex clutch - degree of blockage as function of vehicle speed - Gear 3 - INCREASED TRACTION​

View attachment 157453

Quite interesting this one has less blockage (coupling) in the mid rev range, less power to the rear wheels, kinda would assume the opposite. Shows something changes with the setting as compared to standard / reduced noise modes though.

I wonder if the traction is so good anyways in dry tarmac and with no cornering that haldex doesn’t really engage in any of the tests?

Thanks for taking the time to investigate this, your efforts are truly valuable and appreciated!

 

[mmi]

Quite interesting this one has less blockage (coupling) in the mid rev range, less power to the rear wheels, kinda would assume the opposite. Shows something changes with the setting as compared to standard / reduced noise modes though.

Yes, exactly my thoughts, too.

I wonder if the traction is so good anyways in dry tarmac and with no cornering that haldex doesn’t really engage in any of the tests?

I'd like to state that actually Haldex coupling is (pre-emptively) so tight that it doesn't allow any slippage between front and rear axles - thus no speed difference can be seen. Actually I wouldn't expect that this would be any different even under more slippery conditions.

As a preglimpse - The slippage can be seen e.g. on gear 5 which mysteriously releases (completely) the clutch within narrow speed range. See below 80-95 km/h, axle speed delta (grey curve) jumps up. So there definitely is enough torque to make front wheels "slip" even on dry tarmac - without Haldex clutch engagement.


Note - for better illustration "Axle speed deltas" are scaled (multipliers in the legend of the picture)

 

[n10n]

As a preglimpse - The slippage can be seen e.g. on gear 5 which mysteriously releases (completely) the clutch within narrow speed range. See below 80-95 km/h, axle speed delta (grey curve) jumps up. So there definitely is enough torque to make front wheels "slip" even on dry tarmac - without Haldex clutch engagement.

Hmm do I read this right... sampling rate of 5.5 Hz there's some 36 or so samples of "slippage", ie approximately 6.5s, while speed went from 80 to 95 or thereabouts. I assume the gray curves are change in speed of front and rear axles, right? Which one is which? I was just wondering if the higher curve could be actually negative speed change of the rear axle "dragging" while the haldex does not put any power into the rear. So while there's no power put to the rear the rear naturally tries to slow down due to friction etc.

I totally could be wrong and happy to accept that, but having the front's slip due to torque on dry tarmac at 5th and >80km/h does sound bit much even for such powerful vehicles

 

[mmi]

Hmm do I read this right... sampling rate of 5.5 Hz there's some 36 or so samples of "slippage", ie approximately 6.5s, while speed went from 80 to 95 or thereabouts.

Correct.

I assume the gray curves are change in speed of front and rear axles, right? Which one is which?

Correct - it's what Haldex has calculated from all 4 wheel speed data - speed difference between front and rear axles. Front wheels are spinning faster when engine torque is applied. Below a snippet from the actual data.

I was just wondering if the higher curve could be actually negative speed change of the rear axle "dragging" while the haldex does not put any power into the rear. So while there's no power put to the rear the rear naturally tries to slow down due to friction etc.

I'd say the speed difference is because of engine torque into front wheels and rear wheels are just free wheeling (below in and around 920-925 seconds, no Haldex engagement).

When no throttle (declining slope) and no Haldex engagement the axle speed delta is close to zero (within 935-950 seconds).



Note - for better illustration "degree of blockage" and "axle speed deltas" are scaled (multipliers in the legend of the picture)

 

[n10n]

Thanks, that makes much more sense except for that weird gap where haldex is giving up for a while.

I guess the two delta curves named ”axle speed delta” with tangential velocity unit (km/h), although the metric sort of refers to angular velocity, confused me - but its obvious now seeing the data table that it’s really just precalculated difference from wheel tangential speeds.

I'd say the speed difference is because of engine torque into front wheels and rear wheels are just free wheeling (below in and around 920-925 seconds,

Agreed.

Sorry no intention to question the data - I’m just inspired to see some concrete measurements and desire to understand them as well. Carry on!

 

[n10n]

Something very different - gear 4

Haldex is disengaged between 60 - 70 km/h, or 1700 - 2000 RPM​

Can’t help but to think this is a fuel consumption optimization. That rpm range hit’s the sweet spot of max torque with minimum rpm so it’s most economical. Usually 4th is minimum gear for longer stretches, 1-3 pretty much used just for acceleration.

If you were to drive at steady speed it sort of makes sense to disengage haldex to save fuel. Obviously full throttle is not a wish to drive at steady speed though so maybe it’s

a) software bug that ignores driver input at sweet spot

b) a way to get lower emissions score in some tests

c) field testing of long term savings / seeing if anybody finds out or cares

d) something else

 

[mmi]

Can’t help but to think this is a fuel consumption optimization. That rpm range hit’s the sweet spot of max torque with minimum rpm so it’s most economical. Usually 4th is minimum gear for longer stretches, 1-3 pretty much used just for acceleration.

If you were to drive at steady speed it sort of makes sense to disengage haldex to save fuel. Obviously full throttle is not a wish to drive at steady speed though so maybe it’s

a) software bug that ignores driver input at sweet spot

b) a way to get lower emissions score in some tests

c) field testing of long term savings / seeing if anybody finds out or cares

d) something else

Well, interesting thing is that this weird behaviour happens only with engines CXFA/CXHA (110kW/150PS).

The behaviour with CXEB engine (150/204PS) is different - no sweet spots - without Haldex engagement.



Actually, what I think - the most weird part on gear 4 is: the Haldex clutch won't disengage at all below speed 58 km/h - not even foot off the throttle.

The same on gear 5 below 77 km/h - Haldex fully engaged always
The same on 6th gear below 90 km/h - Haldex fully engaged always.

 

[n10n]

Well, interesting thing is that this weird behaviour happens only with engines CXFA/CXHA (110kW/150PS).

The behaviour with CXEB engine (150/204PS) is different - no sweet spots - without Haldex engagement.

Total speculation of course but to me that sounds exactly like a internal test version of the software slipped into production vehicles. Testing if such sweet spot would improve economy. Too costly to pull back after approved for production and hey, it still works…

Different engine variants are probably tuned, tested and approved at different times and maybe even by different teams so it’s totally plausible one has issues while another has not.

Actually, what I think - the most weird part on gear 4 is: the Haldex clutch won't disengage at all below speed 58 km/h - not even foot off the throttle.

The same on gear 5 below 77 km/h - Haldex fully engaged always
The same on 6th gear below 90 km/h - Haldex fully engaged always.

This could be a test version symptom as well, perhaps eg eliminating as many variables as possible from acceleration up to sweet spot for the purpose of reducing noice from test results.

Enough of tinfoil theories for the time being though. There’s zero evidence, just some symptoms that may or may not point to something like above