Re-engineering the F125, a philosophical discussion

[HPsauce]

So, we all know the F125 is a bag of worms, an electromechanical unit, totally critical to the vehicle working, placed in the most ridiculous location on the underside of the vehicle and responsible for many A8's being scrapped in the past with apparent "gearbox failure".

My brain has been saying for some time "There must be a better way", but what?
I "think" I (definitely) know how it works and (mostly) what it does, but I'm not really sure why it's exactly where it is?
Most of it seems to be signalling (to various places) what the gear shifter (in the cabin) is doing. But I don't know enough of the mechanical and electrical links from that to understand why the F125 is where it is and whether it (or parts of it) could be elsewhere or implemented differently.

I can see there's potentially a difference between what the shifter is set to (requiring) and what is ACTUALLY going on in the gearbox, but does it all need signalling from the side of the gearbox? How much is mechanical and how much controlled via ECU?

e.g. I assume there is a certain amount of mechanical linkage from the gear shifter to the shaft on the gearbox to enable/command specific functions, is this just P (R?) (N?) and D and do the ECUs know what's going on independently of the F125 or not?

Anyone interested in discussing this or am I on my own? It just seems to me that the F125 incorporates a lot of functions that just don't need to be there, especially on cars with the later PRNDS setup.

[MikkiJayne]

So the F125 is attached to the actual shift rod on the transmission, not just put there because a ZF engineer's wife once slept with a mechanic. The shift rod is connected to the valve body and at its most basic level it controls the fluid flow to the forwards or backwards bits of the valve body, or shuts them off, or locks the parking pawl in place. Those functions inside the transmission are purely hydro-mechanical. It will drive purely on that mechanical input without even having a TCU plugged in to it, although as most of us know, it will slam in to gears and only have 4th gear. Limp=mode is the TCU effectively just giving up and letting the valve body do its default things.

Does the F125 need to be down there? Not even slightly. From an electrical perspective, it would make no difference whatsoever if it were part of the shifter mechanism and inside the car. I think ZF designed the F125 as part of the transmission (since the 5HP was used in loads of other stuff including Jags, BMWs, Range Rovers) and left the manufacturers to do their own shifters, therefore the F125 was attached to something in ZFs control. The shifter is attached to the shift rod with a chunky bowden cable. Again, this was left to the manufacturers to implement, along with their chosen shifter design.

Could it be moved? Theoretically yes, the functionality could be achieved by something attached to the shifter pivot or potentially a link rod attached to the bottom of the shift lever which actuates a separate mechanism. The optimal solution for a switch would be an optical encoder, thus getting rid of the frangible metal contacts. That might need some electronics to interpret the signals, or the TCU might cope with an optosensor grounding its inputs. However, design and construction of such a thing in a reliable format is likely beyond the home gamer. There's virtually no space to play with so it would be an extreme packaging challenge. Not impossible, but not economically viable to do on a commercial basis.

I have considered an optical replacement for the existing F125, but again it is extremely tight. Alas I don't have the time or energy any more to devote the hundreds of hours such a thing is likely to need to perfect. We're pretty much at the mercy of the Chinesium clone switches now that ZF have forsaken us with original parts.