90q instrument illumination- Please Help!

Huw Powell human747 at attbi.com
Mon Dec 16 13:57:08 EST 2002

> The wire that I left dangling was a gray/blue wire that I
> believe is the 'standard' color for dash illumination.

That is correct, and I tend to agree with your fear of how the problem
erupted - that stray wire shorting on a grounded metal case.

> I _think_ the whole
> problem began when it grounded out on something- the back of the radio, the
> climate control head, something like that.  I will check the grounds on the
> back of all of the switches in the general vicinity, because I did do some
> tugging to get the white antenna lead to the radio.  I think the circuit is
> ok though because the license plate lights and glove compartment lights
> still work.  Just no dash lights!  Argh.

I'll try to explain what I remember of this circuit to half-help you
trouble shoot it.  First you have the headlight switch, which turns on
the various running lights and the dash lights.

The dash light circuit goes first to the rheostat on the cluster, which
adjusts for brightness.  In the old days, this "dimmed" signal was fed
straight to all the bulbs, but starting with the type 44's and type 89's
(yours), there were two many bulbs for the rheostat to handle, so Audi
started using a tall relay type module to buffer it.  The adjusted
voltage from the rheostat goes to this relay thing, which then has (more
than one, I think) and output that is electronically set to match the
rheostat.  These outputs then drive the various bulbs.

I am not sure whether the rheostat drives any bulbs directly.  OK, so I
looked it up, and it does.  The instrument cluster bulbs are run
directly off it, and the bulbs all over the dash and in the switches run
off the booster thing.

So, barring any lost grounds or melted wiring, it may be that this
controller thing was damaged, but I think a more likely suspect is a
charred rheostat or damaged circuit trace to it.

There are pictures on my web site showing removal and disintegration of
your instrument cluster, btw:


> Does anyone out there have access to the schematics for an '89 90q (circuit
> for fuse 14) that they could copy for me?  Or, better yet, sell me a set?  I
> think I need to trace the circuit to get to the bottom of this.

Well, you can, and should, try to buy a copy of the shop manual
published by Robert Bentley ("the Bentley").  It will provide you with
many hours of entertaining reading and greasy handed frustration.

Huw Powell



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