[V8] back-up switch wiring diagram

[Huw Powell]

Kent McLean wrote:
> I'm trying to decipher the Greek (sorry, Pantelos) that is the Bentley V8 wiring
> diagram.
> 
> Back-up light switch, p. 45:
> <http://home.comcast.net/~kentmclean/audi/v8/backup_lights/45_backup_light_switch.jpg>
> 
> Back-up light relay, p. 47:
> <http://home.comcast.net/~kentmclean/audi/v8/backup_lights/47_backup_light_relay.jpg>
> 
> P. 45 nicely shows the back-up light switch. I can see a brown wire goes
> to ground (Note 102, in the transmission harness) and a brown/black wire
> goes to the Automatic Transmission Control Unit (ATCU). My guess is that
> the ground follows the other wire up to the ATCU.

solid brown is *always* ground.  striped brown is a switched ground. 
The switch works by switching a ground to the ATCU.

> P. 47 confuses me.  I can see the wiring from ground to the lights, then to
> the relay (black/violet).  From the relay, a black/blue wire goes to box "318",
> and a blue/red wire heads for the ATCU before veering left to (no-box) "x1".
> 
> Questions:
> Is 318 a connection box on another wiring diagram page?

Yes.  See the numbers at the bottom?  That's what those boxes refer to. 
  The "318" box is on track "416", so go back a few pages, find track 
"318", and there'll be a box marked "416" there, that is the 
continuation of that wire.  By the way, it's bk/bl, which is usually, as 
I recall, ignition power.

The relay is switched by power coming to that bl/r wire coming from, it 
looks like, the page before, at the place marked "x1" - almost certainly 
from the ATCU.

> Is x1 also a pointer to a link on another page?

I guess I answered that!

> I would expect those wires to end in the ATCU, so the back-up light switch
> can power the back-up light relay. I can power to the relay from a blue wire
> in the ATCU and cause the back-up lights to come on.

Keep in mind the switch is not "powering" anything - it's "grounding" a 
circuit.  Very common in VAG wiring.

> All I need to do is connect the back-up light switch to the relay wires I
> found in the ATCU. 

That won't work.  The switch gives you "ground" for "on" - so you need 
to power a relay with ignition on 12 v, and ground it via the switch.

If you take what you've got and disconnect the bl/r wire from the relay 
and connect that relay terminal to the existing bk/bl wire (after making 
sure that it is indeed ignition on 12v!), and then run the striped brown 
wire from the switch to where the brown wire is attached to the relay, 
you'll have it - with one small "issue" - the ATCU will no longer know 
when you are in reverse.  I don't know if that will affect anything.

I'd like to find wires in the existing harness, rather
> than cobble something together. (And I still haven't found the switch on
> the transmission; I'll have to jack it up to feel for it.)

If you take what I have "explained", you ought to be able to test 
voltage/grounds at each point along the way under four conditions (ign 
on/off, R switch on/off) to see where the fault lies.

Best of luck and I hope my explanations are more understandable than the 
Bentley!

PS, if you're still adrift, post page 46 as well ;)

-- 
Huw Powell

http://www.humanspeakers.com/

http://www.humanthoughts.org/