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Gearbox has started to ping up the solid red gear indicator in the cluster , just plugged in and retrieved this code

18155 - transmission solenoid power relay
P1747 35-00 open or short circuit to ground

No engine codes or abs codes

Cant seem to find much info regarding whether it's in the mechatronic or in a relay in the car somewhere so I can start checking power/ground etc.

Wondering if anyone has had this before?

If that power relay has any input via the F125 switch and that hasn't been replaced in a few years I'd probably start with that.

There's a fusebox under the bonnet for gubbins as well.

Couple of wiring diagrams in the Bentley manual that might help. Looks like there are a lot of earth points to check.

Is the fault always present or does it occur randomly? If you reset it, does it go away for a while?

This fault code is to do with the power supply to the solenoids so I'd check that the TCU is getting the right voltage first since they get their power from there. Measuring block 003 will tell you the supply voltage. If that looks OK, I would check the 16pin connector to the valve body for corrosion / damage.

If that all looks OK then I'd swap out the TCU to see if it's that. Not easy if you don't have a spare.

My feeling is that it's not the F125 but it's such an insidious little <beep> that you can never be sure.

Thanks for the replys gents , it happens all the time now , it was sporadic the first few days , will get the carpet up and check the TCU first then move to checking wiring.WIll report back if I find anything.

Also battery and alternator. Any power issues and they're worth looking at to rule out.

Wouldn't be surprised if you find water ingress to the TCU under the carpet. My money is on that or corrosion on the 16 pin connector.

Audi and their positioning of delicate electronics in the dumbest of places...

As if by magic ,passenger well ,bone dry ,drivers side foot well ,soaking wet ,water poured out of plastic box that TCM sits in , think I was just in time ,I can see where a power cable has been arching to enclosure which I think was throwing the short to ground code.

Inside of TCM is dry .just some green corrosion on connector pins but internals look fine
Will clean everything ,dry out foot well ,refit and test and will report back

What do I win?

If it were me, I would wash the lot with isopropanol to get the water out and let it dry thoroughly. And clean those contacts as well as possible with a tooth brush and isopropanol. Same for the mating connector.

Obvs the leak needs to be sorted.

I'm seeing a lot of corrosion on the PCB. You *definitely * want to clean that off and hope that it hasn't weakened the solder joints.

I'd be inclined to get it professionally cleaned and checked. You don't want it doing weird stuff with the gearbox.

Yeah, I'd be feeling glad that I had a spare TCU at this point.

Update

Cleaned the TCU PCB and the connector pins with my graphite pencil

Even depinned any connectors from the plug that had corrosion and cleaned them Refitted the TCU nad went for a drive

Gearbox was faultless , before this reverse would be very temprementle and would work 1 in 5 times , all gears were selected no problem and the gear box performed flawlessly
Block 3 was showing 13v Testig some other voltages while on the drive However after about 20mins the Gear display turned red and the same code appeared , power to solenoid relay

Now block 3 showing 0v So I need to find whatever block 3 is referncing which it looks like its supplied by Terminl 15 under the dash on the drivers side ,i have found Terminal 30 but not 15 yet , I have found another wiring diagram this afternoon so I now something else to look for , need to look at possibly pin 2 on the TCU which may be the feed from Terminal 15? Does anyone know what supply Measuring Block 3 is referencing??

Any help is allways appreciated!!

What you've highlighted is the output to activate the gear lever locking solenoid. The power for that comes from S231 via T17m, pin 15. The TCU then allows the current to go to ground to activate the solenoid. I think you want to be looking at the one next to that, coming from T17m pin 17. That goes to pins 54 and 55 on the TCU. I suspect this is what is being measured in group 003.

You definitely want to check the T17m connector. It's probably had a bath, too.

I'd definitely clean the 16pin connector to the valve body, too. And the one to the F125 for good measure.

It was the T17m connector I was trying to identify , do I just follow the TCU lom to under the dash?

im getting back on it tonight ,I have seen the two black wires going to pins 55+54 , they are joined together just before the TCU conector with another 2 black wires with a metal crimp with a rubber cover , I may break the joint and use another good 12v fused temp supply to supply pins 54+55 to see what Block 3 reports back. will report back

pwack wrote: ↑

Thu Jul 03, 2025 2:36 pm

It was the T17m connector I was trying to identify , do I just follow the TCU lom to under the dash?

Yeah, I think it goes up the A-pillar, so remove that bit of trim and you should see a bunch of connectors. It's a blue one.

pwack wrote: ↑

Thu Jul 03, 2025 2:36 pm

I have seen the two black wires going to pins 55+54 , they are joined together just before the TCU conector with another 2 black wires with a metal crimp with a rubber cover

Start here. Wouldn't surprise me if they are "joined" in the Audi, under-carpet special style, i.e. twisted together and held with chewing gum and all corroded to nothing.

One of those wires goes to the F125 .