The P400 and P300 automatic transmissions both have the same gear ratios and towing limits, but different torque ratings -- 700 NM for the 8HP70 on the P400 and 450NM for the 8HP45 on the P300. I have slowly been increasing the towed weights on my 2020 P300 110 and have been experimenting with different driving techniques, a bit.

I have found when towing above 4500 lbs (or so) in Hi range that manually upshifting at RPMs a bit higher (500+ rpms) than what is programmed into the S or D modes results in what feels like a slight slip within the shift elements. It is not a frightful occurence, just a bit disconcerting. I have concluded I will not upshift manually when towing towards the high end of allowed specification.

When the transmission runs the show, in D or S, it is very happy in all driving regimes -- smooth and effective.

I have not encountered any issue when downshifting (as you might do when utilizing engine braking). I have disciplined myself to start in Lo range (per the manual) when towing at heavier weights, and find that works very well.

I am wondering if anyone else (with either a P300 or P400 transmission) has experienced something similar?

Enjoy !!

Two interesting and useful videos are here:

 

As always, @TrioLRowner - interesting/informative post. This is a lot like the "Rock Crawl" mode to me. Why the heck do I now have to do something else? I wish JLR would have just done all the steps for me (yeah first world problem). I don't tow a lot with my Defender and am nervous as heck that I'll forget to do the right "start". I wish you could just tell it that the load is heavy (or better - detect it!) and take care of it for me. I'm very nervous I'd screw it up.

 

Adding for completeness a video on the new "mechatronic" electronic valve body which is in our transmission. Really a new world to my older pre-electronic mindset:


Just to note that there is no mechanical cable to this transmission -- all is executed via electronics -- "shift by wire" technology.

Enjoy !!

 

So, I'm picking up an Airstream in September that should run about 7,500 lbs loaded. I know it's recommended that I start out in lo and then once getting to 25-30 mph, shift to the hi range. I guess I'll just have to give it a try, but I'm not optimistic that, particularly on a grade, I'll get all of the shifts between neutral and drive, and the range selections done smoothly and quickly enough as to not drop the transmission onto the pavement.

 

Poptopp:

Extended low speed maneuvering of the trailer while in LO range lets you use a gradual throttle and things happen at a controllable pace. it helps the precision of your driving markedly. And, of course, you are not loading up the transmission with heat, without sufficient torque being transmitted.

When shifting from LO range to HI while motoring, I avoid doing so on a upward incline. On a flat road, I lose only about 5 mphs double-checking the dash board as it confirms the change occurred as requested. I have learned not to rush it.

The transfer case shifting as described is silent -- and if the computer controlled shifter is not happy with something you have asked it to do (like shifting at too high a speed or shifting in the wrong direction -- from HI range to LO -- it will flash the appropriate warning on the dash and will not act in a manner to harm the vehicle).

It is a pretty smooth operation at this point -- it consistently works very well.

Enjoy !!