Re: Motronic question

[Scott Mockry <scottmo@teleport.com>]

Huw and DeWitt, 

The Motronic ECU as used on the Audi 20V Turbo engines (and likely most
other non-Audi engines), uses the O2 Sensor signal to vary the fuel
injector pulse width to tweak the mixture. It uses the O2 sensor voltage
transition across the 0.450 Volt mid point to fine tune the mixture for
best emissions when in closed loop to approximate the 14.7 to 1 A/F ratio.

If you use a 2 channel Oscilloscope, with one channel you can view the O2
sensor signal slowly oscillating up/down, (1-2 times/sec) and on the other
channel, watch the fuel injector pulse width change slightly in response to
the changing O2 sensor voltage signal.

The 20V Motronic has the ability to adapt and change the standard control
value (midpoint setting) with modified values due to changes in operating
conditions. (engine wear, small vacuum leaks, minor fuel pressure changes,
fuel injector performance etc.).

If the required control value exceeds the capability of the ECU to adjust
the mixture, the ECU will trip a fault code indicating the set point is out
of range due to some problem. (fuel pressure too high or low, low engine
vacuum, possibly due to some mechanical problem etc.) 

You need the VAG1551/2 factory scan tool, or equivalent CarComp/VDS Pro
software package to read the actual stored adaptive mixture set point
inside the ECU. The ECU uses a number from 0-255 to indicate this mixture
set point.

The 1991 200TQ 20V actually came with a CO% adjustment screw on the MAF
(Mass Air Flow) meter, but there is very little documentation on this
adjustment, and the Bentley does not discuss making this adjustment. 

Normally, there should be no need to adjust this, as a change in mixture is
more likely due to another problem (vacuum leak, fuel pressure change) etc,
that should be repaired instead of playing with the mixture adjustment. As
mentioned, the adaptive ability of the ECU can handle minor tweeks.

There are some factory Audi Service Training booklets that give some
additional insight into Motronic system used in the 1991 200TQ 20V
Engine/ECU, the V6 ECU and the 1992> Audi S4 Motronic system.

WSP-521-209-00  The New 20 Valve Turbo Engine for the Audi 200 Quattro (4/90) 

     WSP-521-213-00  The New Audi V6 Engine with MPI Engine Management
System (8/91) 
 
     WSP-521-218-00  Motronic Engine Management Sys. for the Audi S4 & 4.2
l V8 (3/92) 

Dyment Publications 1-800-544-8021 in Ohio who supplies all the factory
based service literature to Audi, sells these books for about ~$15 each. If
you call Dyment, you should indicate that you are an independent repair
shop, and use a Visa/MC to order these.

These books give a great description of the Motronic system and its
components, and gives details on the default operation if these components
should fail. (Limp home modes)

A listing and description of other Audi Service Training booklets, can be
found at

http://www.sjmautotechnik.com/techlit.html

HTH
Scott Mockry
SJM Auto-Technik