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-   -   F125 switch versions (http://forum.a8parts.co.uk/showthread.php?t=15830)

HPsauce 16th August 2020 09:43 AM

F125 switch versions
As highlighted in Craig's recent thread there are (at least) 2 versions of the F125 switch. An earlier one with a round 8-pin connector and a later one with a more rectangular 10-pin connector, though I think with only 9 pins installed.
Internally they are built very differently but supposedly do exactly the same thing. So why two versions?

As MJ recently said:

Originally Posted by MikkiJayne (Post 159267)
There is no change in logic between the two types of F125 and the TCU doesn't care which switch it has.

I'm interested to know (largely an academic question) what the extra wire is for and whether there is actually a sensible mapping between the two so that either type of F125 can be rewired to the alternative plug.

My first guess would be that there's a common shared earth/return wire in the earlier version that is separated in the later one, possibly for compatibility to other vehicles in the VAG stable.
The internal view of the later "tracked" version is fairly simple to understand with a bit of thought and observation, the earlier one would require more dismantling and analysis to comprehend.

Unless someone has already published how it works in some sort of circuit/logic diagram?

MikkiJayne 16th August 2020 10:14 AM

My rationale for there being no change in logic is that you can swap the TCUs around and the F125 makes no difference. Both my and Mike's Murples are round-plug PRND432 cars but we both have the 2002 PRNDS TCU in them. The cluster is a little miffed as it doesn't know what 'S' is, but the TCU is fully functional.

If I get a chance later I'll see if I can find the wiring diagrams for them. Technically it ought to be possible to make an adapter to use a rectangular plug F125 in a round plug car, but since the round plug F125 is far better built its simpler to just repair it.

HPsauce 16th August 2020 11:01 AM

So for full compatibility with Sport mode you need Shifter, TCU and Cluster all consistent, but not the F125, it's just reporting a position that the software interprets?

MikkiJayne 16th August 2020 11:46 AM

Yeah basically. Only the aluminium shifter top too - the actual mechanism is exactly the same. The only positions the gearbox knows about are PRND. 432 and S are simply software - the cable stops moving after D, so for S mode you just need the top to stop the stick moving to a position the TCU doesn't understand.

Its usable without the cluster, and since that's too hard to code properly we haven't bothered that. From memory, the display just goes out in S.

HPsauce 16th August 2020 12:04 PM

But something is going on down in the F125 switch beyond PRND, basically the 5-contact arcs on the later version.
Is that gearbox internals moving the mechanism under software command from the TCU?

And as for Craigs car not displaying "P" that is weird, but could it actually be a broken F125?
(Like mine won't let me start unless I bypass the relay, though I've never actually looked at the display!)

HPsauce 16th August 2020 12:27 PM


Originally Posted by HPsauce (Post 159278)
later one with a more rectangular 10-pin connector, though I think with only 9 pins installed.

Definitely just 9 pins, corresponding to the internal wiring. I just looked at my spare/old one.

ainarssems 16th August 2020 01:01 PM

2 Attachment(s)
Testing 8 pin and 10 pin F125

HPsauce 16th August 2020 03:33 PM

Very useful Ainars. +++ And on studying them a fairly simple correlation:
Basically contacts 1, 2, 3 ,4, 5 and 8 are the same between both types. Pin 6 however is unused on the 10-pin so it has 9.
6&7 on the 8-pin are 9&10 on the 10-pin (P/N signal which is faulty on my car).
1&8 is the reversing light circuit on the 8-pin, so shares pin 1 with other functions. It's separated out onto pins 7&8 on the 10-pin.

It looks like pin 1 has battery voltage on both types plus pin 7 on the 10-pin (to supply the reversing light).
So rewiring shouldn't be too difficult. Unless there are separate fuses on the two power supplies in the later loom.

Additionally I note that:
S is equivalent to 4th gear. (As you'd probably expect)
Pin 2 says "Car in P, N or D".
Pin 3 says "Car in R, N or 4/S".
Pin 4 says "Car in N, D, 4/S or 2".
Pin 5 says "Car in D, 4/S or 3".
I assume this is some sort of 4-bit binary coding picked up by the TCU. I'll work out what it is shortly, unless a younger IT brain beats me to it!

Basically I think pins 2 to 5 tell the TCU what gear has been selected. Pin 1 is presumably power.
The other section of pins control the starter relay and power the reversing light. Both pretty directly, no ECUs involved.

HPsauce 16th August 2020 03:58 PM


Originally Posted by HPsauce (Post 159298)
I assume this is some sort of 4-bit binary coding picked up by the TCU.

And if we allocate 4 bits to the pins, we get pin 2 = 8, pin 3 = 4, pin 4 = 2 and pin 5 = 1.
So the signals are:
Gear Bits Decimal
P = 1000 = 8
R = 0100 = 4
N = 1110 = 14
D = 1011 = 11
4/S = 0111 = 7
3 = 0001 = 1
2 = 0010 = 2

Though Audi might do it in the reverse bit order, with the values then becoming decimal 1, 2, 7, 13, 14, 8 and 4.

HPsauce 4th September 2020 03:57 PM

Prompted by the comments in MJs workshop thread I was looking at the arcs inside my old F125 and trying to work out which was which re the pin numbers. But I couldn't. :Confused:
The patterns of contact areas in the arcs don't seem to correspond to the logic/bits I worked out above. Some do very well but not all.
Maybe I misunderstood the documents that Ainars posted, or there's a mismatch/error in them. Me misunderstanding seems more likely, I'll have a rethink....
There's also a possibility that there are some "transitional" codes that are designed to signal to the TCU that there is movement between one position and another. Nonetheless there are still segments that don't appear to correspond.

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