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Tail Light Grounding - Unstable Lights

Question:	"My rear backup, brake, and turn lights get all funky, lighting up partially, totally, or not at all. What's the deal?"
Answer:	"Something's funked in your wiring. Let's diagnose."

Level of Difficulty:

Medium

Time:

30 minutes

Original article @ Huw's Site

Testimony: This works quite well. I had my doubts when Huw yanked out the light and subjected it to massive drilling, but the lights have never been brighter and my teeth have never been whiter. I would recommend this fix. While you're at it, check the wires on the driver's side of the trunk, solder, heat shrink and cover with plastic tubing, any broken wires. This is very important.

Directly from Huw's site:

A common trait among older Audis (funny to watch, a pain to suffer) is
the "Christmas Tree" tail light syndrome. The symptoms are random seeming
functions of the various lights in the rear clusters. You signal left, and both
brake lights flash as well as the left signal. At a much diminished brightness.
You turn on the lights and the whole darn cluster glows dimly.

This is caused by the "minimalist" approach Audi seems to have taken
with some of their wiring specifications. The entire tail light system, both
sides, is grounded through one small brown wire. Eventually this wire ages,
perhaps corrodes a bit, it's connections become poorer, and it just won't handle
the current required any more. Due to another interesting feature of much Audi
wiring, that devices when "off" frequently have their hot side grounded, the
voltage present at the desired lamp takes another path of lesser resistance:
through the other lamp(s) "backwards" to their grounded hot side.

This may seem odd, if you think of the tail light assembly as being
"grounded", but it isn't any more. It has a crummy old wire presenting 10 or 20
or 50 ohms to ground. So the active lamp and the brown wire (it's more of a
resistor now) act as a voltage divider and the whole ground portion of the tail
light cluster(s) sits at some voltage between 0 and 12.

The solution is to add a pair of adequate grounds to the clusters.
These assemblies are like giant scale PC boards. There are metal traces running
all over a big piece of plastic, leading from the terminal strip to the various
lamps. You first remove the wiring harness from them, usually by squeezing a
couple of snap locks on each side. (At least these don't seem to break, but I
wouldn't do it below 50 degrees F anyway) Then you remove the "circuit board",
again usually by squeezing a couple of big tabs to release it into the trunk
area.

Identify the ground track. On my 5k I had to drill a hole through the
ground track and attach a ring terminal to a bolt through this hole. On the
coupe there is a convenient 1/4" male tab on each one - as if they knew this
needed to be done! You lead a short 14 or 16 GA wire from here to a nearby
location on the trunk metal. Try to pick somewhere that stays dry and will keep
your new wire safe from physical abuse. Use an existing bolt to hold a ring
terminal if you can, if you must, drill a small hole and use a small sheet metal
screw to hold it. Work neatly, solder the wire to your terminals, and use heat
shrink tubing to keep them clean. Use dielectric gel on the contact points.

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Page last updated: April 28, 2003