18155 - transmission solenoid power relay

4.2 V8 40v biturbo - 450 bhp
4.2 V8 40v biturbo - 480 bhp (plus)
Post Reply
pwack
1st Gear
Posts: 55
Joined: Wed Mar 29, 2023 9:18 am

18155 - transmission solenoid power relay

Post by pwack » Mon Jun 23, 2025 6:33 pm

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?

srichards
4th Gear
Posts: 780
Joined: Sun Oct 18, 2015 12:39 am

Re: 18155 - transmission solenoid power relay

Post by srichards » Tue Jun 24, 2025 7:45 am

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.
Attachments
IMG_0805.jpeg
IMG_0806.jpeg

User avatar
Shoppinit
Cruising
Posts: 20294
Joined: Wed Dec 31, 2008 6:24 pm

Re: 18155 - transmission solenoid power relay

Post by Shoppinit » Tue Jun 24, 2025 8:59 am

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.
Daytona RS6 C5 Avant. Viper'd, Billies, Waggers, MTM box brain, C6 stoppers, xcarlink, R8 coolant cap (woohoo)
///M3 E46 | XC90 (V8, natch) | Passat GTE | RR Classic V8 flapper
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair."

pwack
1st Gear
Posts: 55
Joined: Wed Mar 29, 2023 9:18 am

Re: 18155 - transmission solenoid power relay

Post by pwack » Tue Jun 24, 2025 9:29 am

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.

srichards
4th Gear
Posts: 780
Joined: Sun Oct 18, 2015 12:39 am

Re: 18155 - transmission solenoid power relay

Post by srichards » Tue Jun 24, 2025 9:46 am

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

User avatar
Shoppinit
Cruising
Posts: 20294
Joined: Wed Dec 31, 2008 6:24 pm

Re: 18155 - transmission solenoid power relay

Post by Shoppinit » Tue Jun 24, 2025 9:48 am

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... :audibash:
Daytona RS6 C5 Avant. Viper'd, Billies, Waggers, MTM box brain, C6 stoppers, xcarlink, R8 coolant cap (woohoo)
///M3 E46 | XC90 (V8, natch) | Passat GTE | RR Classic V8 flapper
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair."

pwack
1st Gear
Posts: 55
Joined: Wed Mar 29, 2023 9:18 am

Re: 18155 - transmission solenoid power relay

Post by pwack » Wed Jun 25, 2025 2:46 pm

20250625_143444.jpg
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.
20250625_144623.jpg

Inside of TCM is dry .just some green corrosion on connector pins but internals look fine
20250625_144408.jpg
20250625_144427.jpg
Will clean everything ,dry out foot well ,refit and test and will report back

User avatar
Shoppinit
Cruising
Posts: 20294
Joined: Wed Dec 31, 2008 6:24 pm

Re: 18155 - transmission solenoid power relay

Post by Shoppinit » Wed Jun 25, 2025 3:24 pm

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.
Daytona RS6 C5 Avant. Viper'd, Billies, Waggers, MTM box brain, C6 stoppers, xcarlink, R8 coolant cap (woohoo)
///M3 E46 | XC90 (V8, natch) | Passat GTE | RR Classic V8 flapper
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair."

User avatar
Shoppinit
Cruising
Posts: 20294
Joined: Wed Dec 31, 2008 6:24 pm

Re: 18155 - transmission solenoid power relay

Post by Shoppinit » Wed Jun 25, 2025 4:19 pm

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.
Image1.jpg
Daytona RS6 C5 Avant. Viper'd, Billies, Waggers, MTM box brain, C6 stoppers, xcarlink, R8 coolant cap (woohoo)
///M3 E46 | XC90 (V8, natch) | Passat GTE | RR Classic V8 flapper
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair."

srichards
4th Gear
Posts: 780
Joined: Sun Oct 18, 2015 12:39 am

Re: 18155 - transmission solenoid power relay

Post by srichards » Wed Jun 25, 2025 4:25 pm

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

User avatar
Shoppinit
Cruising
Posts: 20294
Joined: Wed Dec 31, 2008 6:24 pm

Re: 18155 - transmission solenoid power relay

Post by Shoppinit » Wed Jun 25, 2025 4:27 pm

Yeah, I'd be feeling glad that I had a spare TCU at this point.
Daytona RS6 C5 Avant. Viper'd, Billies, Waggers, MTM box brain, C6 stoppers, xcarlink, R8 coolant cap (woohoo)
///M3 E46 | XC90 (V8, natch) | Passat GTE | RR Classic V8 flapper
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair."

Post Reply

Return to “RS6 / RS6 plus (C5 Typ 4B) 2002-2004”

Who is online

Users browsing this forum: No registered users and 64 guests