I am printing some TPU models lately, with third bowden tube going at the back and into AMS HT via TPU output.
It is working fine as long as I leave some filament loose on the spool.
The printer is able to pull the filament through the bowden, but is not able to rotate the spool, the friction on the rolls is too much.
Maybe there is a way to measure the tension on the filament, and than rotate the spool to keep the filament loose?
Printing with TPU via TPU output and just rotate the spool.
Is it the friction of the rollers or the PTFE protuding too much inside the cover, this has to be no more than 4mm to work. What TPU are you using, I have no issue with TPU 90A
Well, it is 85A, so a bit on the flexi side.
I know, according to the manual, it should go from the top without bowden, but it is working for me, right? only I have to unspun the amount for the next print each time. Just now it it for the first two layers for some feet, and then switch to PLA, so it is not a big problem (60 cm or so, fits loosely in the AMS). And maybe I will have to feed it from the top for a bigger print anyway.
But, nevertheless, I see possible improvement here 
You’re pushing your luck with 85A, may be when bambu releases its star shaped PTFE tube with reducef friction it might work.
As it is, I am not exactly printing TPU model, but adding a couple of layers either on bed, or on top of the model, to add some friction.
Since I am using both nozzles, I need at least one AMS.
And both (Pro and HT) usually stand ot top of the printer, and it is real pain to reconfigure everything just to print 90 cm of TPU.
If the extruder can somehow measure the drag (maybe?), it should be feasible to rotate the spool from time to time and keep up with the print.
Probably can be useful in different scenarios too.
And it is working, kind of, apart from spool rotation.
I have straight path outside of the printer, no problem pulling filament through bowden during loading, but as soon as the tension is on the spool, it is a no-go.