So it wasn't the starter motor...

[HPsauce]

The F125 basically only allows you to start in P or N, and it's a mechanical device subject to wear, debris, breakage etc. so can be very variable.
The jumper wire in place of the relay just means you can operate the starter irrespective of the gear selector position, though that's pretty unlikely for most people to do.

I've found that even my fairly new F125 doesn't really like me starting off from cold by reversing (limp mode occasionally) so I now always reverse into my drive so I can drive straight out.

I think the C5 and D2 have a great deal of "stuff" in common, certainly in this area.

[steve2003rs6]

Hpsauce. Is your f125 switch a genuine switch.

I only ask as I reverse out of my drive every morning without any problem in all weathers.

[HPsauce]

Absolutely 100% genuine. If you've ever looked at the inside of one you'll see why they can just randomly misbehave!

[steve2003rs6]

I can remember a wheeler dealers program.
A similar problem with a track and arm.
I cannot remember the make of car involved.
Some guy in a garage did a mod to convert it from mechanical to electrical, to eliminate the tracking wear issue.
Pity someone hasn’t done a similar mod for the f125 switch

[HPsauce]

Several of us on the A8 forum have done a fair bit of analysis of the F125 and it has two totally separate sections and functions, all driven by the position of the gear stick as measured at the gear box.
1. Purely mechanical "slider" switches that trigger (a) In "R" the reversing light circuit and (b) In "P" or "N" the starter relay.
2. A collection of "slider" switches that effectively send a code to the TCU as to which gear is selected, but also (possibly) some interim positions as the stick is moved. More complex.

It could probably all be modelled by a collection of alarm-style sealed reed switches and magnets in a pattern on a moving disc.
The Reverse and Start functions would probably then need a relay each, the gear selection just a direct signal feed to the TCU.

The Reverse circuit in particular takes a significant current as it directly drives the reversing lights, and also provides signals (e.g. to the RNS) that Reverse has been selected.

Attached picture shows the signal tracks inside; a pair of arms with contacts swing over these. The numbers correspond to the external connector plug labelling.
On the lower segment contact 1 provides +12v (I think) fed to the others as appropriate to position. These are small pointed contacts (hence the lines you can see) as they don't carry significant current.
On the upper section each circuit has two tracks and a pair of big flat connected contacts on the arm.

[steve2003rs6]

Very informative mate. That is a lot more complicated than the issue I watched on wheeler dealers.
Many thanks hpsauce.

[HPsauce]

It's quite logical when you sit down and work out what it does.
The TCU definitely gets signals (as confirmed by monitoring with VCDS) as the gear stick transitions from one position to another, they are interpreted as "Z-codes". What we don't know is what, if anything, it does with them. In theory it can predict what gear selection you're heading for and take anticipatory/preparatory action.
But it only really applies when you're overriding by dropping out of drive. And even that only applies to earlier PRND432 variants, not the later PRNDS versions.
The F125 is the same, but the TCU software interprets the signal from it differently and with PRNDS you never get to the 3 or 2 positions anyway.
(The Tiptronic + and - are signalled directly to the TCU by a different route wherever they originate)

[HPsauce]

Another important point that I and several others have realised is that, now we know how the F125 works and how it can fail it's nothing like as scary!
Yes it's a PITA WHEN it goes wrong (and it will!) but many issues that have been interpreted as a dead gearbox just are not. And in many cases there's a "quick fix" until the F125 is repaired (yes it can be!) or replaced.

[Shoppinit]

Yeah. First time that PNRDS comes up you need new underpants.

[HPsauce]

Indeed, I think that there should be a HHGTTG style message along with the limp mode warning - DON'T PANIC!

[Shoppinit]

So long and thanks for all the fuel.

[srichards]

I do wonder if the whole F125 function could be replicated by some plug in module with firmware on it instead of having the rat's nest wiring?

I'd definitely add the Don't panic And possibly an automatic underpants dispenser. A proper gearbox malfunction then dispenses a fresh set of under pants out of the glovebox...

[Shoppinit]

It wouldn't be so bad if it wasn't such a faff to replace. And so ridiculously overpriced for the OE part.

[srichards]

It's 2 hours to do I think as that is what is on the change invoice from 2017 when mine was last done. The Metzger one I've ordered was £140 ish so not as bad as the OE. Think I paid £250 for it last time. No idea whether that was OE. I'm assuming it was or it was a recommended one from TPS or similar.

[Shoppinit]

Where did you get the Metzger one? Have you got a link?