80 / 90 instrument cluster repair (bouncing speedo, dash lights out)

[mswanson at sonitrol.net]

One for the archives...

I've been living with a problem my 90 has been exhibiting for quite some
time just because I've been busy and haven't had a chance to get around to
it.  The speedometer bounces regularly, usually by about 15-20 miles per
hour +- the actual speed.  That in and of itself wasn't enough to make me
get off my rear and fix the problem.. why would I want to know how fast
I'm going anyway? :)


The next problem that cropped up was a bit more annoying..  The right half
of my instrument cluster went dark one day (meaning that the tach and
coolant gauge stayed lit, but the speedometer and fuel gauge lights went
out).  For a while I could get it to come back by giving the dash a good
whack.  Eventually this stopped doing the trick...

I knew it wasn't bulbs just because they all pretty much went out at once
and I had already replaced almost all of those bulbs less than a year ago.
 I suspected that the two issues (bouncing speedo and dead dash lights)
were related.


So we had a beautiful day in CT yesterday and I decided to take the
opportunity to dissasemble the IC.  After some investigative work with my
trusty DMM I discovered that the power for the instrument cluster lights
travels from the left to the right side of the IC via one of the left
sides instrument cluster bulbs.  Oddly enough, that one wasn't lit...  I
was reading a strange increase in resistance from a nearby solder joint to
that connection.  Silly me forgot to disconnect the bulb from the socket
while measuring as when I tried it again I got a completely open circuit
(I was reading the resistance across the bulb the first time).  Bingo.

Pictures here:


http://www.mswanson.com/images/?mode=album&album=Audi%2F90q%2Finstrument_cluster_repair

hopefully that won't wrap.. here's a shorter link if it does for the lazy:

http://tinyurl.com/3cozw


So basically all I had to do was scratch away some of the insulation layer
on the circuit board to see where the break had occured:  right where the
bus line went from a small diameter to the larger pad near the bulb
connection.  I just added some solder to the connection and covered the
area with some electrical tape and presto!  I'm back to being able to see
my dash at night :)

As far as the speedo goes I suspect that a weak connection had occured
between the gauge and the circuit board... They are attached via some long
pins that connect to assemblies... certainly a possible failure point.

One other troubling thing I came across was a discoloration in the board
near the trimpot for the dimmer on the dash lights.  There doesn't appear
to be any problems with the device as the dimmer works as one would
expect.  I'll just have to keep my eye on it if any problems come up with
it.

Just adding some BTDT for the archives

-Marc-
87 4ktq (engineless, awaiting 10vt to 20vt swap)
88 90q (with working dash lights, for a change)