need help with "limp mode" and F125 range switch replacement

[Puddin Tane]

I originally posted this to get help but nobody replied, so I'll just write it up myself for the next guy

The car:
2005 B6 A4 Quattro 1.8T Avant w/ 01V transmission (ZF 5HP19FLA) 155K miles

Two months ago it started occasionally jolting into "limp mode" (PRNDS display all red; only 4th gear available; very harsh slamming shifts). A few days ago this resolved as the car going into "limp mode" every time I shift into reverse. Shift into Park, turn engine off, restart, shift straight into D, then it shifted fine til the next time I used R. CEL came ON for the first time. Definitely seemed like a flaky electrical problem.

I don't have VAG-COM / VCDS but contemplating getting it; no way at present to pull TCU codes, HOWEVER: the symptoms just described are the definitive pattern for code P0706, i.e. end-of-life for the transmission "neutral safety switch," "range sensor," etc. (VAG part #01V-919-821B), so I'm just gonna run with that, as lots of folks do under my circumstances, since the Chinese clone switch is only 37 bucks. Intermotor NS-349 switches can be had for 150-ish; to justify paying that much or more I would need the code, but I can't and won't so I don't

Location of switch is on left side of tranny mid-way on pan and low down close to it, just aft of front axle.
multi-function range switch.jpg
switch is marked 01.400 in this diagram; ZF calls it a "position switch."

5hp19fla left side F125 placement.jpg
The switch is seen just to the right of the left front axle output flange.

What's not shown is how inaccessible this part is in the assembled car. it is literally INSIDE the left transmission mount, which has to be completely removed to gain access to the switch.

This YouTube playlist is the closest thing to a "procedure" I could find.

Procedure:

1. raise front of car enough to get under tranny area; support on ramps and/or jack stands
2. remove left front wheel
3. support transmission left side with a jack under the pan using a wooden board as a cushion
   NOTE: most of the socket-head fasteners in the next steps were factory installed with BLUE loctite, which is why they're such a-holes to break loose. Torque needed to free them is far higher than what should be used to put them back. Straight hex bits ONLY should be used for removal, since the force required may snap tips off any brand of ball-tips. Ball-tips are great, however, for putting them back.
4. six-cylinder cars ONLY: remove two heat shields from transmission left side: 1) a stamped, dimpled aluminum heat shield over the pyramidal left transmission mount (one piece, 3 bolts); 2) a die-cast half-cylinder heat shield over the inboard joint of the left drive axle (one piece, 3 bolts).
5. remove the large vertical hex bolt and nut (16mm hex, 15mm nut) securing left side transmission mount to its frame bushing, then raise the transmission slightly to relieve the load on mount and bushing
6. remove two 13mm hex bolts securing bushing to frame; remove bushing
7. remove three 8mm socket-head bolts securing pyramidal transmission mount to left side of transmission (this is where it gets FUN!); if working on a 1.8T 4-cylinder car, the top bolt can be attacked from ABOVE by reaching under the coolant reservoir, which is much easier-- albeit your arm needs to be four feet long; with all bolts removed, remove transmission mount; F125 switch and its electrical connector are now fully accessible
8. remove two T27 torx bolts securing the F125 switch to the tranny and pull the switch off the shaft
9. remove the 6mm socket-head bolt securing the electrical connector support bracket to left side of transmission; disengage F125 electrical D-connector; F125 switch is now free of car; Thoroughly clean TCU side of connector with electrical contact cleaner and inspect for damage, corrosion, etc.; migrate support bracket from old switch to same position on new switch
10. Place new F125 switch on shaft, making sure to index "flat" of shaft with that of switch
11. Reverse all above steps; apply new BLUE loctite to all fasteners and torque moderately (all thread into aluminum); I could not find torque specs

[cleoo]

I had these issues: Upon cold start, the motor would take a few seconds to start finally cranking (as if the starter was going bad) and then PRNDS would light up RED. Sometimes it would be kind to me and let me drive it without the PRNDS warning.

I had a used transmission installed and this is what would happen all the time.

I tried the trick LXMONTES06 suggested by putting the selector on NEUTRAL and starting from there. It worked and would skip the PRNDS issue.

I finally come across your post and look at some of the videos. I found this video (not listed) where he opened up the Multifunction Transmission Switch in a Passat (similar to the Audi A4) and cleaned it:

“Automated Gearbox in Limp Mode VW Passat” by CRANK
https://youtu.be/cpbLbsb03gU

I didn’t have the bucks to buy a new one and thought cleaning up the switch would work, so that’s what I did.

Once removed, I drilled out the 6 rivets with a 5/32 metal drill bit, with the switch clamped in a vise.

Opened the switch and saw that there was some dried gunk in the switch which coated the contacts.



I used some CRC QD Contact Cleaner it worked on the less funky areas. So I scrubbed and still had some issues. I also used a fiberglass pen to scrub away some of the gunk on the contacts and on the wiper contacts.

Put it back in together in the same position it was upon opening.



Got some Home Depot screws (Everbilt Round Head Combo screws and nuts #8-32 x 5/8” 8 pack part number 528 730) and screwed it down where I removed the rivets.



Installed it and now I don’t have any issue. Cheap, easy fix for me.



Maybe later I will buy a new one but for now this works.

Confirmed the operation through VCDS and it doesn’t delay showing the correct position like before.

[Puddin Tane]

I finally come across your post and look at some of the videos.

I'm real glad my post helped you out, and thanks for adding your own wealth.

For the next person who wants to rehab their F125 instead of replace it, I recommend an amazing contact cleaner & lube that car mechanics don't seem to know about called CAIG DeOxit
341-220_HR_0.default.jpg
100% concentration in this tube. Ludicrously expensive but this little tube is a lifetime supply. Also comes in a 5% dilution spray that's handy. It's legendary within the electronics, ham, and sound engineering communities. Makes any connector pin or wiping contact much more reliable and noise-free. I wet the female pins of every D-connector before snapping them together and would definitely use it when closing up an F125 switch, then work the switch back and forth.

[deft]

I have a 1998 Audi A6 C5 2.8 Quattro, 142kw ACK engine, ZF5HP19FLA transmission.
Now I have this problem. When I drive on cruise speed my car shifts smoothly thru all gears, BUT if am in a hurry, it goes 1-2-3-4 then, if I hold the gas pedal on 2000rpm or low, it shifts smoothly to 5th gear, BUT if I push the 4th gear above 2000rpm, it slips and goes into LIMP mode....until OFF & ON.
After driving like for about two years I decided to throw in another transmission and behold...exactly the same problem as the former.
I have tried another TCM on it....same issue.
VCDS suggested changing the Multifunction switch(F125), that I have not done.

What do you think about this?

[Cybersombosis]

Did you swap the f125 switch from the old trans to the new? If it’s a totally different switch then the odds of it being bad are slimmer but not zero. One thing you can try is to adjust the two bolts that hold the cable to the trans. That will cause the trans not to go in to the correct gear when shifting. Kind of an adjustment if you will. It’s simple enough, easy and free. Just have someone sitting in driver’s seat while you adjust. Try and match it up to the old marks for a start.

[Turbo510]

... if I push the 4th gear above 2000rpm, it slips and goes into LIMP mode....

Aside from you having two transmissions do this, the symptom sounds like clutch slip. You aren't getting an "incorrect ratio" code? Using VCDS you can also watch the gear selections with ignition on, engine off to see what the F125 is doing as you move the shift lever.