Re: Re-engineering the F125, a philosophical discussion
Posted: Tue Jul 28, 2026 3:52 pm
So on to the 5-contact side of the F125, which I believe is solely aimed to provide electronic (probably +12v) signals to the TCU.
This is my analysed picture of the internals, the numbers correspond to the numbers on the wiring connector and the labelled lines show where the contacts from the arms touch for the various shifter positions: Ignoring the 4-contact (upper in this picture) side my thoughts on the lower 5-contact side are as follows:
The Black wire (1) supplies a voltage that is then signalled back to the TCU via a combination of voltages (or not) on 2, 3, 4 and 5.
The combinations were all documented by MJ and me (and matched to VCDS readings/codes) in the other (currently down) forum.
There are also interim positions (not marked on the picture) as mentioned earlier of Z1, Z2, Z3 and Z4 that can be detected, show in VCDS and presumably assist the TCU in knowing which way the shifter is heading.
Z1 (1100) appears both between P and R and R and N. Z2(1010) between N and D, Z3(1111) between D and S/4 and finally Z4(0011) between 4 and 3 also 3 and 2. I personally have definitely seen Z1 to Z3 (I have PRNDS) but IIRC MJ has seen Z4.
The contaminated contact in my previous post is contact #3 (2nd bit of code) which if it was non-conducting might make N(1110) look like Z2(1010). But it would make R(0100) into 0000 which is invalid, so explains why I could typically start the car in N and drive off forwards, but starting in P would often activate limp mode when moving the stick through R. The 4 bits of the code as I've stated them are from contacts 2, 3, 4, 5 in that order. P is 1000 R "should" be 0100 and N 1110. D is 1011 and S/4 is 0111. For completeness 3 is 0001 and 2 is 0010.
Given all the above, if you have the patience VCDS and a bit of logic, gently moving the shifter and noting codes would probably enable diagnosis down to which specific contact was failing and then assess how urgent a repair was. Though if there was an actual loose component or debris that might not work. And anyway sooner or later the F125 would probably have to come off and be opened up to confirm.
This is my analysed picture of the internals, the numbers correspond to the numbers on the wiring connector and the labelled lines show where the contacts from the arms touch for the various shifter positions: Ignoring the 4-contact (upper in this picture) side my thoughts on the lower 5-contact side are as follows:
The Black wire (1) supplies a voltage that is then signalled back to the TCU via a combination of voltages (or not) on 2, 3, 4 and 5.
The combinations were all documented by MJ and me (and matched to VCDS readings/codes) in the other (currently down) forum.
There are also interim positions (not marked on the picture) as mentioned earlier of Z1, Z2, Z3 and Z4 that can be detected, show in VCDS and presumably assist the TCU in knowing which way the shifter is heading.
Z1 (1100) appears both between P and R and R and N. Z2(1010) between N and D, Z3(1111) between D and S/4 and finally Z4(0011) between 4 and 3 also 3 and 2. I personally have definitely seen Z1 to Z3 (I have PRNDS) but IIRC MJ has seen Z4.
The contaminated contact in my previous post is contact #3 (2nd bit of code) which if it was non-conducting might make N(1110) look like Z2(1010). But it would make R(0100) into 0000 which is invalid, so explains why I could typically start the car in N and drive off forwards, but starting in P would often activate limp mode when moving the stick through R. The 4 bits of the code as I've stated them are from contacts 2, 3, 4, 5 in that order. P is 1000 R "should" be 0100 and N 1110. D is 1011 and S/4 is 0111. For completeness 3 is 0001 and 2 is 0010.
Given all the above, if you have the patience VCDS and a bit of logic, gently moving the shifter and noting codes would probably enable diagnosis down to which specific contact was failing and then assess how urgent a repair was. Though if there was an actual loose component or debris that might not work. And anyway sooner or later the F125 would probably have to come off and be opened up to confirm.