back-up switch wiring diagram

Kent McLean kentmclean at
Tue Jun 9 09:11:07 PDT 2009

Huw Powell wrote:
> Kent McLean wrote:
>> I'm trying to decipher the Greek (sorry, Pantelos) that is the Bentley V8 wiring
>> diagram.
>> 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.

Regarding making the back-up lights work on my '90 V8 converted to a 5-speed
manual, the saga continues (how can 2 wires be such a pain?)...

First, a general question regarding the Bentley wiring diagram. If the
bar at the bottom of the page represents ground (does it?), then what
is the bar at the top of the page? Power?

What I've learned so far, working backwards:

The back-up lights are wired to a relay, which in turn connects to "318"
and "x1".

x1 goes to a blue wire in the AT Multi-Function Switch (MFS), which I know
that when I put 12V to it, it will turn on the back-up lights.

318 goes to A20, a wiring connection in the dashboard harness.  I assume this
gives power to the lights through the relay. Where in the dash I don't know.

The back-up light switch itself has a br/bk wire that lead to the ATCU.
I now see that the ATCU is not the same as the AT MFS.

Given that some wires seem to be buried in the dash, and in the tradition
of DPOs, I think it will be easier to run new wires from the switch to the
relay (via the blue wire in the AT MFS) rather than trace the wires
into the dash, so I'll refocus my attention there.

In an Ah-Ha! moment, I decided to look for the back-up light switch by
removing the right-front wheel. Low and behold, I think I found it.
 From what I gather, I think it appears in the red circle.
(Ignore the grunge from what I believe is a power steering rack leak. :( )

If that goo-covered thing is the back-up light switch, then I just have
to run 2 wires from there, up into the engine bay to the AT MFS harness
where I can pick up switched 12V and the blue wire that triggers the relay.

My plan for another day is to clean that switch, confirm it closes when
the transmission is put into reverse, then run new wires from there to
the appropriate feeds in the AT MFS harness.

Kent McLean
1999 A4 Avant, V6 Tiptronic
1991 200 TQA #3, with mods
1990 V8 w/5-speed and other mods
gone: '91 200 TQA x2, '94 100 S Avant, '89 200 TQ "Bad Puppy"

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