Odd one...

[HPsauce]

For info here's a few pictures to help anyone getting the same problem - S8 related but RS6 is probably the same.
Short bypass wire I made:

Relay socket, the wire joins the STEEL sockets which sit at 90 degrees to each other (thick red/black wires) not the parallel brass/bronze/copper ones:

The removed starter relay, it's a VAG204 on an S8:

VAG204.JPG (55.25 KiB) Viewed 607 times

[srichards]

The replacement F125 seem to be so bad it makes you wonder whether it wouldn't be better just making one with a few wires and some sticky backed plastic. Can't be less robust!

[Shoppinit]

They're laughably simple when you open them up. It's outrageous that Audi charges so much for them.

[HPsauce]

What is laughable is the Victorian "steam-punk" technology inside them, and the fact that they are located in a ridiculously vulnerable position beneath the vehicle.
As many of you will know there are two different sections, neither of which really need to be located there, or use the antique signalling method employed:

First section: Tells if the gearbox selector is in Neutral or Park (triggers a relay to allow starting) or Reverse (mainly lights but also Satnav).
Simple analogue stuff, high current for the reversing lights as they are directly fed!

Second section: Tells the Gearbox ECU what gear has been selected AND (not a lot of people know this) interim positions as the selector is moved. Binary signals basically.
So the ECU is in fact aware which way the stick is moving before it gets to the new position and the "codes" associated with that (Z1, Z2, Z3 and Z4) can actually be read with VCDS.
What the ECU does with that information I don't know, but people have speculated in the past that the ECU "knows" which way the stick is moving and takes preparatory action.

All of this could be done in so many other ways and more reliably. The relatively high currents through the Starting and Reversing functions in particular mean the sliding contacts burn out easily.
The sort of technology used in house alarms for example (sealed switches triggered by magnets) would probably be more reliable. The sliding metal contacts used are almost designed to fail.

This is the inside with the two-section contacts swing arm removed. The numbers correspond to the standard numbering of the associated 10-pin plug (earlier versions were 9-pin round).

[philS2]

That's great info guys, cheers.
I'm now going to put a jumper cable in my glovebox.
Shocking to hear how bad the OEM F125 is.
I nearly bought a chinesium one a while back.
But then gave myself a mental slap.
Might unslap meself.

[HPsauce]

Just buy several Chinesium ones, I've heard ZERO reports of them being less reliable than OEM and D2s eat them for breakfast!

[HPsauce]

Just out of interest I thought I'd share the evidence of my most recent F125 failure - earlier this year.

That is the contact arm leg that moves over the gearbox segment - the lower section in my earlier picture of the innards.
It basically delivers whatever voltage is on the innermost track #1 to a combination of the other four depending on position for the ECU to interpret.
As you can see the next to outer contact pair (track labelled #3) are badly corroded/contaminated, though I was able to clean them up completely.

That particular circuit should become "live" when selecting Reverse, Neutral or Sport (=4) but not Drive; how it got in that condition I have no idea.
I found that the car would often go into "limp mode" as soon as I moved out of Park. Stopping then restarting in Neutral sometimes allowed me to have a normal journey.

What it does show is that problems aren't confined to the other (Start/Reverse) segment.

[Steve2003rs6]

This is a great thread and very informative. I bought a new original F125 switch a few years back in anticipation of when it fails.
Haven’t had the need to use it yet.

Still on the original F125 switch.
Reckon I have tempted fate now though.
Covered 142 K miles.