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: 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?

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.

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?

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.

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!)

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

Testing 8 pin and 10 pin F125

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.

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.

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.

I'm going to go back over the documents Ainars posted and also look at the tracks more carefully as I'm not convinced my analysis above is spot on.
However I have taken my F125 apart and am pretty sure the pin/track/wire assignment is as follows:

4-arc section (easy): pins 7, 8, 9, 10 from inside to outside.
The incoming wires correspond exactly, going straight across to the tracks: Blue, Red, Green, Brown.

5-arc section (messy): pins 1, 2, 5, 3, 4 from inside to outside.
The incoming wires in the order they are in the terminal block are:
1 Black, 4 Yellow, 3 Pink, 5 White, 2 Grey.

So looking at the pictures of mine and MJs F125s, remembering the track order by pin is 1,2,5,3,4 and trying to guesstimate the movements of the contact arm, I reckon:

Track A, pin 1 as noted before is a common supply voltage, presumably +12v.
Track B, pin 2 is indeed PND (with gaps for R and 4/S etc.).
Track C, pin 5 is indeed D, 4/S or 3 with a single contact segment for that.
Track D, pin 3 is indeed R, N or 4/S with gaps for P, D, 3 and 2.
Track E, pin 4 is indeed N, D, 4/S or 2 with gaps for P, R and 3.

There's clearly not a precise correlation between the track sections for the different gears, so I wonder if that's a way of signalling to the TCU that the shifter is moving from one position to another, whatever it does with that information?

For clarification I've labelled this internal picture of the F125 tracks:

http://forum.a8parts.co.uk/attachmen...0&d=1599560393

So it can tell which way the shifter is moving. I thought that was the case but its cool to see it in the drawing +++

Well technically of course it can't see which direction, but it can potentially see "interim" positions and so infer direction.
That's assuming the human hand is making "normal" movements and it remembers what the previous position was.
Remember my gear(shifter/selector) position lines are only estimates, I didn't get that information from an F125 switch connected to the cable. ;)

But given the tracks very obviously aren't exactly aligned/delineated around precise locations of each stick position there has to be some logic to it..... doesn't there? :Confused:

In particular I note that pin 3 will get a signal as the shifter moves out of Park but well before it arrives in Reverse, though that is of course a longer than average travel.
Whereas pin 2 loses the signal only when the shifter is fairly precisely centred in Reverse. I suspect some subtlety of that is related to your problem.
It actually looks like, from a TCU view, it goes P, half-N, R, N, D etc.

Also, in the non-TCU (presumed) group the starter relay gets a very lax allowance on Park but minimal slack on N.
There is even possibly an interim position where you could be in Reverse (or at least have the reversing lights on) and operate the starter given the size of the contacts?

I've also only just realised that the four "relay" contacts run in grooves so can't really go adrift whereas the five TCU ones are not constrained and a slight distortion could have odd results, though they are mounted in pairs.

So I decided to try and work out the sequence from Park through to 2nd, pins 2,3,4 and 5 using the bits(values) as in post #9:
0000 (0) = Out of range before Park and below 2nd.
1000 (8) = P
1100 (12) = between R and .....
0100 (4) = R
1100 (12) = between R and .....
1110 (14) = N
1010 (10) = between N and D .....
1011 (11) = D
1111 (15) = between D and 4th/S .....
0111 (7) = 4th/S
0011 (3) = between 3rd and .....
0001 (1) = 3rd
0011 (3) = between 3rd and .....
0010 (2) = 2nd

Unused codes:
0101(5), 0110(6), 1001(9), 1101(13).

So when leaving R it doesn't know if it's heading for P or N, but does that matter?
And if leaving 3rd (on a non-S version) it doesn't know if you're heading for 2nd or 4th.
But it does distinguish moving between N and D from between D and 4/S.
And in all cases it can know the stick is moving, though not always which way.

As an observation, I reckon MJs suspect F125 pictured in her Workshop thread, may well have been giving a 1100 code (between R and... N or P) when R was selected (normally 0100).
This could have been due to shorting of the track for pin 2 to its contacts, while still having the reversing lights on. See the first picture in post 267 http://forum.a8parts.co.uk/showpost....&postcount=267

And then compare the relevant areas of MJs F125 and my old one and also the contacts on MJs arm, where the "spot" on one of the #2 contacts (left) suggests arcing:

I took the S8 out for a little run today to my younger son's house and back. Only about 6 miles each way but enough to get warmed up even though 40mph is the fastest I can legally do on that route.
But I did do a little test on the "between" positions by moving the shifter very slowly and carefully through the available range (PRNDS) while watching the cluster display.
Not a flicker at any interim position and the display just switched as you'd expect.

So from that can we surmise that mine was getting a completely invalid code from the switch since it displayed nothing? PRNDS was displayed with nothing highlighted with the shifter in R, and would only show R on the display when pushed to just the right spot.

That's my best guess now, and one reason why I did the test today. :cool:
My surmise above was probably wrong:But remind me how it behaved. I recall you said the reversing light was on (not a surprise, separate circuit) but had it selected Reverse gear, or not until indicated on the display and the TCU commanded the valve body?

It wouldn't select R unless the shifter was held in between P and R in an extremely precise position (within a mm or two). I think the necessary sprung contact was only making a connection right on the edge of the base contact, so any movement either way and it failed and dropped out of gear. R on the cluster was coincident with it being in gear, which makes sense as the TCU will send it a CAN message.

Having cleaned it up and put it back in, I was checking the shifter alignment yesterday using VCDS. Between P & R and R & N the TCU shows 'Z1' and between N and D it shows 'Z2' so those intermediate contacts definitely tell it something different. Presumably its so it can prepare for what is coming next - it apparently can't tell direction, since it can be put in to Z2 from D, and then put straight back in D, but I guess it uses the Z positions to prepare the gearbox for what may come next.

I didn't get the binary code for Z, but that can probably be worked out from the contacts, or observing VCDS again.

Excellent research. +++ I think that's consistent with what I worked out and documented in post #16 above. ;) Z1 = 1100, Z2 = 1010.
There should be another code for between D and 4/S. And a 4th code for 4-3 and 3-2 if have that available.

So we do now know that those intermediate positions exist and are recognised by the TCU, though not if it does anything (or what) as a result.
This could be the basis of another useful diagnostics test on the F125, moving the shifter slowly and monitoring the TCU.
Where exactly in VCDS do you find those codes?

I couldn't get it to show anything for that - it just went straight from one to the other.

Its in measuring blocks. I can't remember which one but not very far in. Scroll through and you'll see it.

Automatic Transmission (2), Measuring Block 4 and it also shows the binary 4-bit codes on the F125, exactly as I had worked out in fact. ;)
There's a higher block (11?) that shows some of that data (and Tiptronic IIRC) but block 4 is best. :cool:

My car is of course exactly the same, only Z1 and Z2, no code between D and S.

BUT a really interesting observation! When VCDS was showing Z1 or Z2 the DIS was no longer highlighting a selector position.
It was still showing PRNDS but no red highlight until the stick moved slightly and the F125 showed a standard position.
MJ, is this what yours was doing when you struggled to get reverse or was PRNDS missing?

Edit, I think you did have PRNDS displayed, so that could indeed have been a Z1 or Z2, not a totally invalid code, just "in transit":
And in fact my initial guess may well have been correct, with a Z1 code being read:

Yep thats exactly it. It must have been seeing Z1, so not throwing any errors. Only in that one specific position did it get R.

I wonder what Craigs TCU was seeing when he had his gearbox problems, maybe also Z1?
Bearing in mind it was his problem/question that caused this thread to be initiated. ;)

Good to see he's got it going now, but I did struggle to understand what the initial problem was. It looked like it was something really serious but I think it was just that "P" wasn't illuminating on the DIS (though everything may have actually been working) which was down to a shifter cable adjustment. :Confused:

This highlights a couple of things, firstly that the two sides of the F125 really are electrically/electronically independent and work very differently and secondly that mechanical devices can work incorrectly but be sending valid signals (at the wrong time) so often no errors are logged by the IT kit.

Just to throw in an update to this thread after studying the insides of an F125 a bit more. And my recent problems described in this thread: https://forum.a8parts.co.uk/showthread.php?t=17345
I now think that the (narrow) code between D and S/4 is definitely 1111.

As with Z1 (1100) and Z2 (1010) this code (1111) removes the highlighting and VCDS shows it as gear stick position Z3.

I don't have a PRND432 shifter to test but it looks like the 4-3 code is 0011 and 3-2 is also 0011.
So we have Z3 for code 1111 in addition to Z1 and Z2 so maybe 0011 will be called Z4?

I can confirm that today, while testing my F125 I did indeed briefly (and more than once) see Z4 as the gear in VCDS with 0011 as the code.

So we have recognisable interim codes of Z1 (1100) Z2 (1010) Z3 (1111) and Z4 (0011) displayed by VCDS. What the TCU does about them who knows, but it does definitely remove any highlighting on the DIS when they are detected.

But either the TCU or engine ECU do something!
Today, as noted elsewhere, my F125 was flickering between N (1110) and Z3 (1111) while stationary in Neutral with the engine running. Every time it switched to Z3 the revs increased to over 1000 and dropped immediately it went back to N.

As I was parked on level ground I released both footbrake and handbrake for a while during this. There was no movement so I think the gearbox stayed firmly in neutral but the engine was definitely preparing for something, presumably assuming it was between D and 4?