gbmarc

I had been avoiding this job for a long time but finally forced myself to start.
The S8 was not getting any low speed steering assist and looking through the access hole I could see the wires to the servo solenoid were badly corroded. A test from the fuse box indicated there was no continuity to the solenoid.

I followed the procedure on the AudiPages which was very helpful.
Here's what I found:
The Audipages mentions 3 allen headed bolts holding the rack in, with the one farthest from the rack head being the most difficult and having to access it from the top.
On my car (2002 S8) however, it was relatively easy to extract from the RH wheel well. It was also a 12 sided 13mm and not an allen head. However, the middle bolt was very difficult. I began stripping its head with my 2 feet of extensions and swivel joints accessing from below. I had a moment of panic and "I'm never gonna work on cars again" thoughts. In the end I accessed this one also from the LH wheel well and used a pry bar to jam the socket into the bolt head really hard. I even turned the drive while still applying upward force to the underside of the socket.
One other thing that might have helped. The rack is really heavy, so I re-tightened the other two bolts first, before getting to this third difficult bolt, to avoid having all the rack weight, dragging down on this single bolt.
Also the large rubber grommet that seals the rack from the interior passenger compartment was very hard to pull off. I used a pry bar (once again from the wheel well access hole) to slowly move it off, but it took a while.
Once the rack was out, I could see the wires to the server were horribly corroded and one was broken clean off. (See attached photos).
Does anyone know how to test this solenoid?
I was going to send it 12v and see if I heard or felt anything, but I don't even know if 12v is what it needs.
No one seems to sell them, and the difference between getting a rebuilt rack with it on or off is another $400!!
The last problem I have is how to re-attach a wire to my old solenoid, when the wire has broken off right where it enters a black plastic block on the side of the solenoid. I'm hoping the plastic block will come off, because there's no other way I'm going to be able to splice it otherwise.
Like the AudiPages author, I found the tiny allen headed bolts attaching the servo to the rack, just stripped out, and I had to cut my own screwdriver slot in the bold heads just like he did.

gbmarc

Servotronic variable assist solenoid.
There seems to be very little information on this $400 device. With mine so corroded and the wires disintegrating I tried melting open the black plastic where they disappear into the metal casing. I managed to solder a new wire on and then gave it 12v. There was a small spark and a faint buzz.
Here's what I found out and maybe someone can shed some more light:
The part is ZF 7039 442
There are similar units that look identical except for the electrical connections for BMW
The BMW part is ZF 7039 442 102 with a BMW # of 32411134770
It looks like it's supposed to get only 6V and 0.8A according to a label I saw.
There's also a Lemforder 1063401 part that looks the same.
The prices of these solenoids run from $450 from the dealer to $89 for a used one from Latvia but they're hard to find.

I unscrewed the end off mine (the protrusion with the O-ring that sticks into the rack) and that reveals a little tiny lever sticking out of the metal box that clicks down when I give the unit power.
I guess it's blocking or opening some tiny hole but it's really hard to see how it works.

What I'd really like to do is go down to my local parts store and give them $10.35 for a new one. But I guess I'm just dreaming.

gbmarc

I'll wrap this story up: I decided against putting my original servotronic solenoid back in, because I simply wasn't sure the connection I had made would last. Instead I put a new BMW servotronic solenoid in because it was half the price of the Audi one. It looks identical. I think it's from mid-90's cars. The only difference is that it has a clip-on connector. It has two prongs just like a BMW fuel injector, so I had to rewire the Audi to have this plug.
On reinstallation I found it impossible to place the middle rack bolt into it's hole and get it threaded. It's so inaccessible. After a lot of frustration, I found the solution - I removed the rack and pre-inserted the bolt into the hole. (Mine stayed in place because it was held in by the solenoid wire.) I then bolted the other two bolts in, and this middle one threaded straight in by accessing from below with a long extension and swivel joint. One other poor access point is tightening the banjo bolts for the hoses. I started them by hand but if you have a flex socket (19mm and 22mm) you can finish them up from below quite quickly.
Fortunately, after I had everything together, and went to test, the power steering at low speed was finally fixed.

the_duke

Thanks for coming back and posting the final result! I'm sure this will help people in the future.

Y888

I have seen recently zf rebuilt rack on fcp euro site for 660ish $, i think it includes servotronic module but not sure.
It can be also ordered from european parts sites for similar price.
Some people go with Jorgen for 380$.

McBrain283

Thanks for posting gbmarc. Very helpful info as I currently have a similar issue.