F125 issue or maybe not?

[airtite]

OK so did a few test drives and car is running well, no leaks and apart from a ticking noise from the exhaust (think its got to do with new gaskets I put on) the car is running really well. Yesterday doing the school run while parking and going from reverse to drive and back to reverse the PRNDS lights came on and had to limp home.

Scanned the car and got the F125 implausible signal.... whats irritating me is I havent had a single issue with my gearbox until I did the engine pull and we all know that changing that switch is a mission.

Today checked gearbox oil and had to fill probably 500ml and what I am noticing on my subsequent test drive this morning is when moving the gear lever between P and D the dash will show R, N but not always D so I went on about a 20km test drive through town and a bit of freeway and had it in S the whole time and the car was fine... at a couple of stops I tried to go to D again and even tip tronic but I had to play with the gear lever quite a bit for the D to light on the dash.

Could it be the adjustment of the selector cable on the gearbox itself? and how do I go about adjusting it, get under the car with it in D and adjust it until the D comes on on the dash?

[Shoppinit]

Sounds like F125 but since you’ve had the box off then it could be cable adjustment. If it was the cable you’d expect it to be a consistent offset though.

I’ve been having this issue at the moment and suspect the F125 but replacing the brake light switch has really helped. No codes for incoherent signal from BLS?

[Shoppinit]

Are you sure you connected up the F125 securely?

[airtite]

Sounds like F125 but since you’ve had the box off then it could be cable adjustment. If it was the cable you’d expect it to be a consistent offset though.

I’ve been having this issue at the moment and suspect the F125 but replacing the brake light switch has really helped. No codes for incoherent signal from BLS?

I ahvent seen any codes for the BLS

[airtite]

Are you sure you connected up the F125 securely?

I never removed it... I am going to do a clean up of the connectors for the gearbox and f125 and see, car is punishing me at the moment I would have thought the engine pull would have fixed most issues but it seems rto have created new ones..

[Shoppinit]

Test it with the Vagcom. You need to make sure both circuits change state at the same time when you press the pedal.

[Shoppinit]

Just checked mine again and the F125 is behaving itself after replacing the brake light switch. Either that or me working the lever back and forth has cleaned the tracks on the F125.

[airtite]

Test it with the Vagcom. You need to make sure both circuits change state at the same time when you press the pedal.

which measuring blocks?

[Shoppinit]

Dunno. You’ll have to dig around.

Fwiw, mine went into fault without leaving codes for the brake light switch. Fault hasn’t come back since changing the BLS. Could be coincidence but it would fault out almost immediately before.

[HPsauce]

Some comments on the above, based on experience, admittedly with S8's but they're very similar.
1. I've never seen any correlation between the brake light switch and F125 errors. The F125 doesn't see brakes.
2. The two circuits in the brake light switch are designed to operate at a very slight interval. The ECU one first as you press the pedal.
3. The only high-current circuit in the F125 is for the reversing lights and it's not monitored to give errors so won't throw an error.
4. I think it's Engine measuring block 66. 1000 then 1010 as you press gently then 1011. Be very gentle and don't run the engine so there's no servo assistance.
5. The F125 isn't self-cleaning so if there's damage/debris in there it will probably recur and get worse.
6. A poorly adjusted brake light switch can trigger the ECU circuit but not the lights, as if you are gently touching the pedal, but you wouldn't know. It's not an error so nothing is logged, but the engine power is reduced by the ECU. This can give some very weird, and potentially dangerous, driving characteristics.

Note that as the reversing light circuit is the only high-current one inside the F125 so if you open one up you will probably see scorching of that track or contacts or worse. All the other circuits are for ECU sensing apart from the starter one which needs enough current to trip a relay (not a lot).

[Shoppinit]

My F125 was very recent and had hardly been used *but* the car had been stored in damp conditions which I suspect has caused some mould or oxidation to get on the tracks in the F125. In this case, I believe the F125 could "self clean" by wiping the tracks clean with repeated use. Like I said, the BLS could be coincidence. The F125 doesn't see brakes, but the TCM does, and the TCM sees also sees the F125 output. I imagine that the TCM would expect to see brake switch signal present when changing from P to anything else. Whether or not it shits itself when it doesn't would be interesting to find out. I might disconnect the BLS to see what happens.

I remember now that I checked my BLS from the ABS module. Just came across a photo. Group 002. I'm not sure whether both circuits are hardwired to the ECU or ABS modules or whether in one or both cases the signal is being transmitted from one to the other over CAN.

IF there is a discrepancy between what the modules are seeing or if one of the circuits isn't working properly then you can get a DTC about incoherent brake switch signals. I had this one time, but I can't find it now.