1983 ur-Q Ignition Control Unit

[scott.john.mockry@bangate1.TEK.COM]

I recently was trouble-shooting a friends 83 ur-Q that had the Ignition 
Control Unit
cutting out at random times. Here is some info that may help.

There is a small transistor inside the ECU that drives the Ignition Control 
Unit (ICU) 
internal transistor (which then turns  the Ignition coil on and off) This 
small transistor
inside the ECU receives its emitter ground from the emitter ground 
connection inside
 the ICU mounted under the dash. In other words if this return path ground 
wire
 between the ECU and the Ignition Control Unit goes open circuit or 
intermittent
the Ignition Control Unit won't be receiving any on and off signal from the 
ECU because the ECU internal transistor emitter is no longer grounded and 
the transistor base-emitter junction is no longer being forward biased. 
I found this out by probing around inside the ECU with an oscilloscope 
and by checking the factory wiring diagram. 

I don't have the ECU pin number handy that provides the ground 
connection between the ECU and  the ICU but I can check at home 
this evening. You may want to check the wiring  between the ECU and ICU. 

After screwing around with the car for some time, I later asked Ned Ritchie 
about
this problem and found out that the later ur-Q ECU's (035 905 383 F) were 
modified 
by the factory to  install a jumper internally to provide a local internal 
ground for 
this ECU transistor that drives the ICU. The later ur-Q wiring harness 
"may" have
 removed this return ground lead from the ICU to the ECU and that may 
explain 
why an earlier ECU 035 905 383 B does not work in the later ur-Q. If I 
remember 
correctly the replacement ECU from Audi 035 905 383 H? is designed to work 
with all of the ur-Q's from 82 -85.

You may also have a bad solder joint inside the ECU as this is what I found 
to be the problem on my friends ur-Q with the Ignition cut out problem. I 
looked
at all the joints under a micro-scope and noticed they all had some minor 
cracking.
I resoldered many of the joints and this did  the trick and the problem 
went
away.

Several others have reported finding broken or cracked circuit board traces 
inside
the ur-Q ECU's as well. Some of these cracked circuit runs were found 
underneath the IC's!
These older ECU's seem to suffer from solder joint fatigue over time
from the thermal cycling and normal joint degradation. Other circuit design 
gurus may
 want to comment on the problems occurring with solder joints over long 
periods of time.

The 86-88 5000TQ MAC11 ECU's have a similar wiring setup with the internal 
ECU
transistor driving the ignition coil power transistor module (mounted on 
the firewall in
the engine compartment)  The later 200TQ with the MAC14 ECU provides 
an internal ECU ground for this small transistor inside the ECU.  I found 
this
out when I attempted to run a MAC11 ECU in my later 89 200TQ that uses
a MAC14 ECU. The car would not start until I provided a ground return path 
for the
internal transistor inside the MAC11 ECU. I made up an adapter harness that 
allows the
 earlier MAC11 ECU to be tested when connected to my MAC14 wiring harness.
The 1986 New Model Introduction booklet from Audi shows the details on the
connections between MAC11 ECU and the external ignition coil transistor 
module.

Steve Eiche, Mike Hopton, and Dave Lawson have  repaired these
MAC-02 ECU's  and they provided some great places to look inside
for problems  when I was trying to sort our my friends ur-Q!

Hope all this stuff helps!
Scott M.