Hello World,

problem description in short: 1996 Audi A4 w AEB engine doesn't start after full swap of harness and ECU to newer generation.
symptoms: ECU does not prime fuel pump when ignition on / ECU does not communicate to VCDS on diagnose bus

Now here's the full story:

in 2015 i bought my 1996 A4 with a stock ECU, year 1996 uses ECU type M3.2 (five connectors)
-ECU and immo box communicated on the diagnose bus (I have old VCDS logs available)
-physical immo box in place
-at that time someone already had immo box wires on pins 7+8 (K-line and W-line) bridged / I wasn't aware of this fact until today

in 2017 i replaced the original ECU by a tuned M3.2 ECU w/ immo off
-still ECU and immo box communicated on the diagnose bus (I also have these logs available)

somewhen in 2018 I bought a 1997 donor car, year 1997 onwards comes with M3.8.2 (two connectors)
-this car did run and drive, so the M3.8.2 ECU + harness are proven to be in working condition
-the M3.8.2 is tuned (has a PLCC44 socket soldered on)
-owner also told me it had immo off - this was proven wrong last week

last week I decided to swap ECU and harness on my car from M3.2 to M3.8.2
-swap is super easy, because harness ends at the fire wall, where 4 connectors are located:
---yellow connector, named T10z
---red connector, named T6z
---brown connector, named T10_
---black connector, named T10m
the only difference in wiring harness (and on the connectors located at the firewall) is the second coolant sender on MY96 - so nothing to worry about.
swap was completed within 2 hours.

that's where the issue started:
right after harness + ECU swap I tried to start the car - it does crank but not fire up.
I noticed it does not prime the fuel pump at iginition on, which in the end means, the ECU does not trigger fuel pump relay J17.
I did not try to connect to the ECU with VCDS at that time.
My assumtion was, that something is wrong with immobilizer.

Thus I desoldered the 24c02 and found that immo was not deactivated; so I did immo off myself.
But the car still doesn't start - still no fuel pump priming at ignition on.

Today I also tried with disconnecting the immo box - no success.
That's when I found that someone made the wire bridge on immo connecto (pin 7+8).

In addition I wanted to check, if the ECU throws any DTCs - but VCDS is not able to connect to ECU, while all other ECUs (immo, brake system, instrument cluster, ...) connect AND do not show any DTCs at all.

I studied wiring plans of MY96 and MY97 this morning over and over again. I really don't think it's a wiring issue - even if the only connection to the diagnose bus is the wire running to the immo box (pin 9).

Ground on pin 2 of the big ECU connector is available, also on the housing of the ECU.

Any clues?
Can I connect to VCDS on the bench somehow? 3 wires should do the job, right? (power supply, ground, K-Line)
Or is it maybe related to the wire bridge on the immo box...?

Your help is much appreciated.
Michael

hello gremlin, thank you for your answer.
I wasn't sure if A4 B5 would start + stall after 3 seconds (I know Golf Mk4 would do so).
Anyway:

I connected the M3.8.2 on the bench, without success.
VCDS shows the identical message like in the car: "ecu doesn't answer"

yet what I noticed:
as soon as I powered up the ECU, there was irregular high frequency noise for 3 to 5 seconds...?
(tried 2 times)

is there a critical chip/component on the pcb, which is known to fail sometimes?
Or is there anything I could notice by visual inspection of the pcb?

As I wrote in the initial post: donor car was up and running with this ECU + harness, except for the fact that I did immo off after the first attempt of firing up didn't work.
symptoms are identical compared to "before immo off" so I don't think it relates to a failed immo off