[V8] on a mystery hunt

[dsaad at icehouse.net]

First off, a "ground problem" is a pretty generic term that usually means
some sort of connection problem.  It could be a corroded connector or
ground point, frayed wire, or any of about 10 zillion other problems.  The
trick is how to troubleshoot.

The sender is a variable resistor.  These are easy to test - but exposing
the wires is going to be hard.  The best place to test is from the
connector as it enters the instrument cluster.  Measure once with the tank
empty, and observe it while filing the tank.  If you can get the sender
out (not that hard at all on a V8) you don't have to mess with actual gas
levels in the tank - just move the float arm.  You also get to inspect the
float - which can also go bad.  I have fixed many a sender where the
contact arm wore off.  In each case I was able to solder something back on
to fix it - often better than new.  Each sender is different though and
YMMV.

Given your two closely related problems, I would pull the instrument panel
and give all the connectors a good reseat as a first step.  Hopefully that
is not too big a deal.

Dave


Then I would try a dash diagnostic and in particular I would look for a
voltage reference being out of whack.  That is possibly a common reference
for both gauges.



> Oh yee Audi fools and experts!  Here is a puzzment.
>
> For a couple of years we have enjoyed guessing at the fuel remaining
> within
> the tank of my wife's 100CS Avant.  I have adopted the procedure of
> filling
> the tank every three hundred miles, and then carefully checking the amount
> of fuel added against the mileage on the trip meter.  The car gets a very
> consistant 21-24 miles per gallon and has for 140,000 miles.
>
> The fuel gauge registers nearly full on filling and then at about ten
> gallons consumed is on the empty peg and by the time around six gallons or
> so remains the "tank empty" ikon is lighted on the warning panel.
>
> The logical conclusion (one reached in consultation with thou almighty
> Audiguessers last year) is the tank sending unit has failed.  That has
> been
> the opinion of my wrench also.
>
> BUT, at the same time that this has been going on, the coolant temperature
> gauge is registering zero also.  For a long time it has registered very
> low,
> but now doesn't move off the peg.
>
> We have had all the builbs in the instrument cluster replaced recently.
> But
> I am wondering if this might be a ground problem in the instrument cluster
> itself...just old age corroding the ground points for the fuel gauge and
> the
> temperature gauge.  No other instrument issues.
>
> Comments?
>
> I am reluctant to throw a fuel sender at the car until and unless I am
> sure
> that will cure the problem.  Is there a test procedure short of opening
> the
> top of the fuel tank?
>
> TIA
>
> Roger
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