90q instrument illumination- Please Help!

DePenning, Charles [FRCO/MTN] Charles.DePenning at EmersonProcess.com
Mon Dec 16 14:25:05 EST 2002

Thanks Huw.  I did check the rheostat on the cluster- I'm not sure what the
resistance range should be, but it did move gradually from very low
resistance to around 25 ohms, so I concluded that it was ok.

I love how this cluster comes out as compared to the 83 5kt and the 84 4kq-
electronic speedos rule!  I looked pretty closely at the disassembled
cluster, looking for popped traces or other char marks.  Didn't see
anything.  I have the Volume 1 of the Bentley set for this car, but no
schematic book, so I had limited abilities to check the voltages supplied to
the cluster through those three multi-pin connectors.  I did check pin 1 of
the yellow connector, which is one of the voltage supplies to the cluster
lights, and the voltage followed the headlight switch.  It was unclear from
the book that I have if there are any other supplies and what their state
must be relative to the headlight switch position, so I quit there.

I am definately getting goofy results- _some_ of the dash lights are on,
like the lights in the fog light switches, ABS switch, etc, but the climate
control head lights go out when I turn on the headlights!  Muy confusio.

I did take a look at some of the interior lighting schematic, and I was
looking pretty closely at the "instrument panel light booster".  If that was
fried, it looks like the cluster lighting would be out too, since on the
schematic terminal 1 of the booster unit goes "to Console Guage Lights via
K1".  My question- where is this module?  Maybe if I can find it I can look
at the outputs.

I would love to find the second volume of the Bentley for this car, if
anyone has it available in paper form.  That would be Bentley #6 in my
collection, stretching back to the 77-79 Rabbit/Sirocco!  That older book is
the most valuable one in my library- more words, fewer audi shop
instructions.  It even explains how to fabricate special shop tools if you
don't have the recommended factory tools.  I like that!



-----Original Message-----
From: Huw Powell [mailto:human747 at attbi.com]
Sent: Monday, December 16, 2002 12:57 PM
To: DePenning, Charles [FRCO/MTN]
Cc: 'quattro at audifans.com'
Subject: Re: 90q instrument illumination- Please Help!

> 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,
> climate control head, something like that.  I will check the grounds on
> 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
> 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
> for fuse 14) that they could copy for me?  Or, better yet, sell me a set?
> 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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