Correcting trip computer fuel-consumption factor

[Phil Rose <pjrose@servtech.com>]

Does your Audi's trip computer show an innacurate value of "average MPG"?
If so (and if such things bug you), read on. The procedure I describe
applies to a '91 200q and other models in the '89-'91 100/200 series. I
don't know how general the setup is. Tony Lum tells me that the Bentley
manual describes a different (5 position) switch for doing this job in
earlier turbo and turbo quattro models.

In my car, a set of three very tiny switches behind the instrument panel is
used to correct the "fuel-consumption factor", which the board computer
(not ECU) uses to calculate the amount of fuel ingested by your Audi. The
Bentley description for doing this correction can be a bit confusing,
or...you simply may not have a Bentley manual. So here's _my_ description
of these switches and how to correct your board computer's average-MPG
output. It's really easier to do than to describe.

You can correct the computed "average MPG" by up to +/- 15% (in increments
of 5%) by merely changing the setting of one or more of the microswitches
located on the top edge of the instrument cluster (above the board
computer). This can be done with the tip of a ballpoint pen, a jeweler's
screwdriver, or even a small finishing nail. Of course you need to read up
on removing the instrument cluster, if you haven't done this previously.

The diagrams below indicate the effects of various switch combinations.
Note that the position of Switch #1 serves only to determine whether the
other two switches will increase (+) or decrease (-) the fuel consumption
factor. If your computer has been OVERestimating the average MPG, then you
must make a POSITIVE correction to the factor--(naturally, do the opposite
if you have a pessimistic computer). In short, there's an inverse
relationship between the change in fuel consumption factor and its effect
on the computed average-MPG.

The "X" denotes switch (handle?) positions. Sliding a switch toward the
instr. panel face turns switch ON). With the factory setting (all switches
OFF), this is what you'll see (schematically) looking down at the top edge,
with the instruments facing _towards_ you. I've shown the orientation you
have if you leave the instrument cables connected, which is opposite
(front/back) to the orientation in Bentley:

	(backside of instr. panel)
	_______________________________________________

	switch# 3  2  1

		X  X  X	 (OFF)
		_  _  _  (ON)

	_________________________________________________________________
	(face of instrument panel)

Switch #1 determines whether change is "+" or "-" (OFF and ON, respectively)
Switch #2 gives a 5% change (when ON)
Switch #3 gives a 10% change (when ON)


Next to each combination is shown the change in the fuel consumption factor:


		X  X  X	  0%   (factory setting)
		_  _  _


		X  _  X  +5%   (to cause 5% DEcrease in calculated MPG)
		_  X  _


		_  X  X  +10%
		X  _  _


		_  _  X  +15%
		X  X  _


		X  _  _  -5%
		_  X  X


		_  X  _  -10%
		X  _  X


		_  _  _  -15%
		X  X  X


To check the status of the correction (before or after the adjustment), you
can read it in the computer display by doing the following: press computer
reset button (bottom of stalk) for a couple of seconds and then turn on
ignition switch; release reset button; press the _upper_ function selector
(at the end of the stalk). The % correction is displayed (with a minus
sign, if appropriate.)

To bad we can't change our _actual_ fuel consumption so easily. :)

P.S. My numbering of the switches is done for ease of description and might
not correspond to an actual label. But the switch orientations themselves
are correct.

P.P.S. It's likely someone has described all this previously, but I figured
once more couldn't hurt. BTW, an experienced Audi mechanic got it backwards
when he tried to do this for me last year.



Phil Rose				Rochester, NY
'89 100 (For Sale, 111K, automatic)
'91 200q				mailto:pjrose@servtech.com