Clunk when slowing down

[AWo]

This is a very basic summary, ZF 8HP has a well packaged compact design and would have one of the most complex gear flows to explain. Looking at an 8HP gear sequencing changing from from 3rd to 2nd the transmission is going from one brake and two clutches in 3rd to 2 brakes and one clutch in second, using euro transmission terminology. US terms a brake as a holding clutch holding generally the planetary ring gear but may hold another planetary element to the transmission case whereas the clutch holds two rotating elements together. During a coast to stop the transmission line pressure (main pressure) will be near the lowest pressure to allow for a softer slower shift. A sudden throttle up during the shift will be detected quickly by the software with the hydraulic system being slower to react, the electronics will also detect this with input and output speed differences not meeting the algorithm and likely ramp up the engagement oil supply to the oncoming clutch to engage quicker and firmer to reduce slip. As you come to a stop, assuming a rolling stop, it's more so the lockup disengaging, this is also assuming the Grenadier TCM is programmed more towards a commercial vehicle then a passenger car which would drop or slip the lock up earlier for comfort. TC lock up can be controlled in any forward gear and has three states, engaged, disengaged and controlled slip. First gear only releases when stopped in gear foot on brake by keeping one clutch engaged, releasing one clutch and control slipping the third clutch required to deliver drive, this is to reduce fuel use, NIC neutral idle control.
It will be interesting to see if some of the new Cummins 6.7 gas engine and ZF Powerline 8 speed (big version of the Grenadier 8HP) in the new RAM trucks will share similar shift quality issues.

Images of ZF 8HP 45 for reference. The basic construction and operation remain the same over the models and generations with changes in ratios and component sizes.
View attachment 7899585View attachment 7899594

If someone can't follow this, here's a good general overview. It's German, use the translation if necessary.

View: https://www.youtube.com/watch?v=6wpzrbl7C5M


AWo

 

[AWo]

Ahhh, the same in English:

View: https://www.youtube.com/watch?v=u_y1S8C0Hmc


AWo

 

[Tom109]

I wouldn't call myself an expert in the 8sp field but have had considerable past field service and overhaul experience with ZF and Allison heavy commercial vehicle automatic transmissions.

I really appreciate the shared knowledge. That what makes this place great.

 

[255/85]

So the ZF units run four planetary gear sets to get 8 forward gears vs the 3 for 6 shown in the video @AWo posted. Pretty clever really.

They really didn’t test this truck out much to see if all their ideas were practical in application. If my truck is facing down hill when I start it and pull out, it won’t upshift. Drifting up to the 20mph speed limit has the engine howling. I have to step on the gas pointing down a steep Pittsburgh street, and then jump on the brake. Stooopid. I can push a button on my diesel f350 and have it act similar, but it’s driver activated for the situation. Wtf good does that do me in the Ineos?

Being accustomed to manual transmissions I actually like this feature. Yes, there is a manual mode for the transmission but why bother for daily driving? Yes, I live on a hill.

There are shops that reprogram the shifting in bmw’s for the “sporty” type owners. I wonder if they can make this act more like a car, or if a TCU or whatever controls this tranny, and be swapped in from a regular old x5. I drive in traffic jams every day, and shifting to N all the time is a bigger pain than my h55f.

I've been wondering for a while whether the mechatronic unit that controls these transmissions could be swapped from one trans to another - like from an 8HP51 into an 8HP76.

 

[NQ94]

Ahhh, the same in English:

View: https://www.youtube.com/watch?v=u_y1S8C0Hmc


AWo

The new 8 to 10 speeds are now using 3 clutches for the gears plus a torque converter lock up, more complex gear set combinations and concentric drive drums, a big change from what we were taught back in the day when a six-speed using basic on/off solenoids mixed with old style hydraulics was the newest design.

 

[NQ94]

I've been wondering for a while whether the mechatronic unit that controls these transmissions could be swapped from one trans to another - like from an 8HP51 into an 8HP76.

There has been some mention of interchangeability and people are trying it. The information has come from people tuning and doing conversions with older 8HP70 and 8HP45, some builders saying the valving is slightly different and can work, the TCUs are mentioned to be the same, but OEM software may be an issue, also part numbers for the unit can change within the same series of transmissions. People are also using stand-alone controllers with less sophisticated software then ZFs software for their conversions. There is still a lot of trial and error and testing happening with non-OEM changes to the transmission.