How to check if an A8 has servotronic, plus, where is the relay?

Julian Thompson · #1 22nd February 2023, 04:54 PM

A8 4.2 tdi lwb 2008 - I assumed it would have servotronic. Checked the relays under the passengers feet and the ones behind the fuse box up under the dashboard. No sign of relay 605 anywhere. On vcds I just get “module not responding” - seems to me it doesn’t have servotronic, but I thought they all did after the .3 revision in 08?

Thanks in advance.

steamship · #2 22nd February 2023, 05:15 PM

Do you still have the build sticker in the spare wheel well? If so, check for QZ0 (QZ* is power steering related) and 1N3 (1N* is also power steering related).

EDIT: According to the '06 brochure, all A8s had Servotronic.

Julian Thompson · #3 22nd February 2023, 05:46 PM

Fantastic thanks I will take a look this evening and report back. Assuming it does have it where the heck is the relay?

steamship · #4 22nd February 2023, 06:40 PM

Quote:

Originally Posted by Julian Thompson

Fantastic thanks I will take a look this evening and report back. Assuming it does have it where the heck is the relay?

From what I can find, there doesn't appear to be a relay. Check the following PDF from page 22. The N119 valve is controlled from module J520 (vehicle electrical system), which takes input from module J104 (ABS).

51-SSP 285 Running Gear in the Audi A8.pdfhttp://forum.a8parts.co.uk/attachmen...1&d=1420368035

Julian Thompson · #5 22nd February 2023, 08:20 PM

Here is the sticker. Can’t find either code.

Oddly when I rang the dealer to buy the servotronic relay he gave me part number 4D0907307 - and a quick search shows that on the Audi A6 of similar year some people don’t like the feeling of the extra assistance so have either removed the relay or rigged up a switch to knock it out when they want.

I have another D3 that’s a 2006 car which I’m almost sure doesn’t have servotronic and the steering on the 2008 feels the same weight wise, hence me thinking the relay could be worth changing.

Julian Thompson · #6 22nd February 2023, 08:28 PM

Looking at that really cool document you linked to I think that they’re handling the components a level closer to the action than the relays and fuses, hence no mention of them. I expect it will need the relay though to actually power the system once logic decisions are made by the processors.

MikkiJayne · #7 22nd February 2023, 10:19 PM

The servotronic valve is controlled by a 6V PWM signal so its entirely feasible to control it with the body module instead of having a dedicated unit used in earlier models.

There is a noticeable difference between broken / inactive servotronic and standard steering. I pull the control module in my D2 S8 for track use as it reduces assistance and adds feel.

steamship · #8 23rd February 2023, 11:46 AM

Quote:

Originally Posted by Julian Thompson

Here is the sticker. Can’t find either code.

Assuming that the brochure is correct and all A8s came with Servotronic, maybe that's why it's not listed on the build sticker - I never thought about that.

Quote:

Originally Posted by Julian Thompson

Oddly when I rang the dealer to buy the servotronic relay he gave me part number 4D0907307

Strange that they gave you that part number as it's for the PF D2.

Julian Thompson · #9 23rd February 2023, 06:01 PM

Ahh fantastic info thanks. Yes, it is odd but then I am not having a lot of luck with my local dealer in many respects so it doesn’t surprise me. To be honest they’re really only interested in oil changes and sending pictures of tyre tread depths!

Assuming now the car doesn’t have the relay and isn’t producing any errors I’m going to declare it fit for service in that department.

Because of the demanding access to the PAS reservoir in the 4.2 tdi it was clear that the car had never had any attention at all to fluid changes - the fluid was like treacle and smelled burned to death. Steering was heavy and inconsistent, hence me ordering the relay as a precaution. I sucked the fluid out of it and refilled it several times, running the pump from lock to lock several cycles each time - gradually the steering improved and the colour of the fluid lightened and became clear. I think I went through 2 litres of CHF to get to that point. The steering feels perfect now (I can’t properly test it as the sunroof and headlining are out at the moment) so I think we have a result.

Thanks so much for the thoughts and input.

Julian Thompson · #10 23rd February 2023, 07:38 PM

After a little more digging I found this table, attached, and the pic of inside one of these “relays” - it’s clearly the control module on the earlier cars, as it contains a lot of stuff which presumably later was moved to the valve on the rack.

Apparently the later servotronic module i have had the “boost” reduced in response to complaints from customers about too light steering, hence the earlier one was superceded.

That’s all on pre 2003 cars though, as per the table.

What I still can’t understand is why vcds can’t find the steering assist module on my car. Maybe I’m looking at the wrong module in vcds. I’ll have another look.