Bentley manual vs Audi Training Booklets

["Scott Mo." <scottmo@teleport.com>]

I think the Bentley manual is a required item if you plan on doing any
extensive work or trouble-shooting on your Audi. Unfortunately like many
repair manuals, it doesn't always explain how a system works in the
first place. 

The Bentley may give you a laundry list of test procedures when trouble
shooting the ECU/engine electronics.  Check for fault codes, measure
resistance here, measure voltage there, measure continuity between
another set of wires,etc.  but....... if you don't have a good basic
understanding of how the whole system works. i.e.
engine/ECU/sensors/fuel injection components etc. and how all the
separate components work with the ECU "together", you can be misled by
the test procedures and often go down the wrong path during the
trouble-shooting adventure. 

This is where the Audi Service training booklets can fill the gap of
knowledge in my opinion and improve your trouble-shooting ability.
Knowing how the system works allows the trained mind to focus and find
the problem much quicker  than reading every page and test procedure in
the Bentley manual. 

An understanding of basic electrical/electronic theory doesn't hurt
either, often times the Bentley will give a trouble-shooting procedure
involving a resistance measurement, indicating it should be between
10-35 ohms, but what if it is 8 ohms? or maybe 38 ohms? If you know what
the circuit is doing, what it is trying to accomplish, and have some
basic electronic theory in hand, you can avoid replacing a perfectly
good part costing big $$$. 

There is also the issue of the meter or DMM you are using to make the
voltage or resistance measurement. Inexpensive meters don't always have
very good resolution and accuracy which can foul up the best
trouble-shooting process if you are measuring a very small resistance or
voltage. Understanding the internal reistance or impedance of the meter
you are connecting to the circuit under test are very important as well,
in some cases a basic incandescent test light may be more appropriate
than that expensive DMM and vice-versa. The early analog meters usually
had much lower resistance/impedance and may load down a sensitive
electronic circuit under test, when compared to the 10Mega ohm input
resistance of most modern Digital Multi-meters 

When checking the main battery cable, 0.1 ohms may not look like much
resistance but with 30-100 amps flowing through it, this can cause some
serious voltage drop problems. The inexpensive meter may show 0.1 ohms,
even though the real resistance is less than 0.01 ohms. In the lab, 4
point contact resistance measurement techniques and instruments are used
when making low resistance measurements to avoid errors.

Even with the Bentley and Audi training books, you can still be unaware
of system quirks. Like the fact that the cold start valve on the 1986-90
5000/200TQ actually operates during initial acceleration when the engine
is cold. It can also operate at times when the engine coolant temp is
very hot from running high boost levels and when the engine may be close
to overheating. This information came from an SAE Technical paper
#860103 on the 1986 5000TQ Engine design. Even the factory trained Audi
technicians I spoke with at my local dealer were unaware of this cold
start operation. 

If you connect a test light across the cold start valve when the temps
are low in the winter time, you can see this brief cold start valve
operation.

Enough ranting from me.....back to the salt mines....
-- 
Scott Mo.
1989 200TQ
1988 5000TQ
1966 VW Beetle
http://www.teleport.com/~scottmo