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

okay thank you.

while I don't really wanna mess with MPPS again (did it once on a ME7.1), this may be the best solution...
because 8D0 907 557 T ECUs are overpriced massively atm.

I am almost 100% sure that this is the main reason for the "dead" behavior of this ECU, which was "alive" before.
Poor electrical contact of the installed chip or a crack or oxidation in the soldering of the plcc socket.

Just FYI, there are full hardware compatible ECUs - 8D0907558E or 4B0907557A or 4B0907557B

alright, thank you guys.
I'll let you know once I made some progress.

Yeah just get a M3.8.3 or a M5.9.2 (they are really the same thing and compatible) if you want to mess with it, it's all so cheap now that it makes zero sense to do anything else.
Then you can just flash it, put an emulator into it or do whatever else.

I think there's M3.8.3 Stage 1 posted on here, anyway there's a big thread with info.

Forget current tune, get the car running on new ECU first. Flashing it can come after.