Re: Fuel gauge adjustment

[Igor Kessel <four_rings@worldnet.att.net>]

Brian Montgomery wrote:
> 
> HI Igor,
> My 1993 S4 fuel gauge does not read totally accurate with a full tank of
> gas. Looks more like 4/5 ths ...
> I went back and looked at the discussion on this adjustment several weeks
> back and have a question on your adjustment. I just got my set of Bentley's
> so I found the procedure for testing the fuel gauge but do not see a
> procedure for adjusting. You mentioned a 4mm allen to adjust with.
> 
> Where and what are you adjusting?

Brian,
this is a recurring question so I decided to post the reply to both
lists. I hope you don't mind.


What I wrote before pertained to the 200 only, where you can both test
and adjust the gauge.

Not so with the C4 Audis. I just checked my Bentley. It indeed states
the checking procedure only.
Vol 3, Page W 90-54:
Connect VAG 1301 instead of the sender.
550 [ohms] - "empty"
48 [ohms] - "full"
if not - replace the gauge. [gotta love this "replace" advice in later
cars. As if we are too ignorant to perform even a simplest electronic
repair].

Fortunately I have a spare cluster [metric, was originally installed on
my S6] on the shelf. Just like the 200 cluster it has a through hole for
the Allen wrench right below the fuel gauge, but unlike the 200, this
hole leads to nowhere. Yes, you're correct, the gauge is not adjustable.

However:
Most automotive instruments with electric senders are ampermetres <G>
and their senders are simple ballast resistors <R.bal>. The gauge also
has a trimmer resistor <R.adj> for fine tuning. I.e. it's an elementary
series circuit: <+>-<G>-<R.bal>-<R.adj>-<->.

The VAG 1301 is a fancy speak for a variable resistor. I've built my own
out of a Radio Shock box, three rotary 12x1 switches and 30 precision
matched resistors. I drew, printed and glued to the box  rotary dials
for Ohms x 1, Ohms x 10, Ohms x 100. This box allows me to discretely
dial any resistance value in integers from 1 ohm through 1k.
Indispensable for troubleshooting of any electrical gauge in the car:
oil, temp, fuel etc. Just substitute the respective sender with the said
box and observe the gauge reading.

Now, in light of the above, you might want to take the instrument
cluster apart, trace the tracks for the fuel gauge and find the
described above adjustment resistor in series with the fuel gauge which
might be either a fixed resistor or a trimmer pot for fine adjustment. I
do not believe for a moment that this adjustment resistor is not present
on the PCB even though it might be a fixed value, not a pot. Otherwise
how does VDO adjust the Audi instrument clusters on their production
test set-up? The gauges should have some variances to be accounted for.

If the trimmer pot is present - adjust it to spec per the Bentley values
above.

If the trimmer pot is not present, measure (read if visible) the value
of this resistor.
Say it is 50 Ohms (I am guessing). Go to Radio Shock and buy a trimmer
pot for about 100 Ohm. I.e. it can be adjusted to values both below and
above of the said fixed  adjustment resistor.
This pot has to be at least 1w of power dissipation. Replace the
adjustment resistor with the pot and adjust the gauge per the Bentley
values above.

A word of caution:
The above advice has been given based strictly on theory. I have never
attempted to do this in my car. If you have a vague knowledge of
electricity and fry your PCB you're on your own.


-- 
Igor Kessel
Two turbo quattros.