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Audi Turbo - OBD II Diagnostics


On-Board-Diagnostics for turbo Audis is not new, my 86 5000CS Turbo has a computer 
diagnostic system that flashes a light on the dash in a certain sequence dependent on which sub 
system or trouble code is being indicated. What is new is the OBD-II requirements for advanced
monitoring of vehicles emissions.

>From the General Motors OBD-II training course:

OBD-II (On-Board-Diagnostics generation Two) is a Federal requirement on all 1996 vehicles.
The OBD-II requires that the on-board computer monitor and actively perform diagnostic tests on
vehicle emission systems. The Federal Test Procedure (FTP) sets maximum allowable emission
standards. A Malfunction Indicator Lamp (MIL) must illuminate if a system or component either
fails or deteriorates to a point where the vehicle emissions could rise above 1 1/2 times the 
FTP standards. 

A standard 16 terminal data link connector is required on all vehicles. All vehicles will have a
limited set of "standard" trouble codes that are the same across all manufacturers. In addition
each manufacturer may have their own set of enhanced trouble codes specific to their vehicles.

Most manufacturers work with a specific unique test equipment vendors to produce a Scan tool 
to read the serial  data, retrieve stored trouble codes, clear codes and in some cases 
exercise/test electrical components in the system. For years many vehicles have had the ability
to  display  trouble codes through a flashing light or from the vehicles  numeric display panel.

These Scan Tools have been required for several years to trouble-shoot many manufacturers 
vehicles ( Ford, GM, Chrysler, Nissan, Toyota, Audi etc).

The new OBD-II systems have Oxygen sensors upstream and downstream of the catalytic 
convertor to monitor catalyst efficiency as the Cat gets old. 

Engine misfire detection is done using sophisticated algorithms that use the crankshaft sensor to
monitor changes in crankshaft speed as a result of one or more cylinders misfiring. If the system
detects a misfire, it will log misfires in registers assigned to each cylinder on the engine.
It can momentarily hold off  engine starting at the next start sequence and determine if a cylinder
has low compression which may be the cause of  the misfire.

The system also includes algorithms to detect crankshaft speed variations caused by very 
bumpy dirt roads (manual transmission only and automatics with torque convertor lockup). 

System tests on the EGR, canister purge (fuel tank vapors), Oxygen sensor etc are done as the
vehicle is driven.

The emissions test centers can connect to the vehicle and pull out any stored trouble codes in
the computer. 

Of course disconnecting the battery for a while will usually erase any stored codes and any
stored vehicle information about shift points, driver habits etc.

It is certainly feasible to have the car transmit information to a remote location whenever the
vehicles emissions go out of spec. Some food for thought.

Scott M.

>I went on a test drive today in a new Audi S6 wagon with a tech/driver from
>Audi. Here is the inside scoop on the lack of turbos in Audis for 1996:
>As of 1996 all cars must have OBDC. (On board Diagnostic Computers) Mercedes
>came out with one a year early and it has been nothing but brain damage for
>the dealers and mechanics. A "shade tree mechanic" will need at least 4
>seperate hand held computers to find out why the lights are flashing on his
>dash.  Audi has not worked out the extra kinks of the OBDC system with