Lampman

Hi All,
 
Slightly delayed update - I've been busy over the weekend very carefully drilling out the remains of sheared screws (1 in the ECU case and 2 in the top of the intercooler, for the Boost Sensor) and then (successfully) reinstating an original thread in the ECU case.  On top of that, I wanted to be absolutely sure of the success of latest developments before declaring (I didn't want to go counting my chickens too soon!).
 
In a word, it appears to be sorted!.  With reference to my last post, my gut feeling about the potential RFI source created by the water ingress related crud on the edge of the PCB and ECU case contact points clearly had some mileage in it. I carefully cleaned up all the deposits on the PCB, and reinstated a clean contact surface on the edge of the ECU lid to enable it to make good contact with the bonding strip on the PCB.  After examining the PCB under a monocle to confirm there were no obvious dry joints, I then put the ECU back together and tried it back in the car....
 
With battery reconnected, first stage was just with Ignition on......No Traction Control light, even after a good minute or so.  Mmm, promising!
 
So I then plugged the fault code reader in (without cranking, just with ignition on),.  Faults still logged since last power-up were erased...then codes were re-read.  No error codes stored.  This was even more promising (fingers were tightly crossed by this point, with mood starting to err towards a mild 'chufty'!)!
 
Then I went for a start-up, with vacuum gauge connected to the OUT port on the Boost Solenoid Valve (N75).  Hey presto, bags of vacuum!  Switched off and restarted a number of times, just to make sure it wasn't a fluke.  Bags of vacuum every time, varying as it should with revs - WINNER!
 
Switched off and then reconnected all pipework, vacuum hoses, et al, to get it into a state as close as possible to enable normal running (car was generally still in a state of disarray due to previous fault-finding related dismantling).  Revs were raised/lowered as appropriate with the aid of my resident helper, while I watched for Turbo vane actuator action, which behaved impeccably. No misfiring/shaking above 2000 RPM, and Turbo was even producing its usual light whistle when releasing the revs.  All good so far...
 
So after then spending the weekend removing the intercooler, sorting its sheared Boost Sensor screws, then refitting it and generally getting everything finally back together late last night, the car was used for work today and has driven perfectly there and back.    
 
Back to the cause of the fault itself, I am working on the following theory: The (missing) signal required from the ECU to drive the N75 Boost Solenoid Valve is a PWM signal.  If that PWM signal was being interfered with by a sufficiently 'noisy' RFI source (definition of 'noise' being any unwanted signal) being generated within the ECU itself, this would account for the issue I was having.  The ECU case provides screening from RFI sources, and it clearly does this by electrically bonding with the PCB itself.  As the bonding between the case and the PCB was not 'clean', it must have been creating an internal RFI source within the ECU itself.  My background is in RF comms, so I am familiar with RFI issues that can be caused by nothing more than poor bonding.
 
And for those wondering how I re-sealed the ECU, the original beads of adhesive sealant were left in place, then with lid fitted but not fully tightened down, the ECU was gently heated with a hair drier to soften the adhesive/sealant and put it into a soft 'tacky' state.  While still warmed up, the lid was then screwed firmly down to spread the adhesive/sealant in sympathy to achieve a sound bond/seal. Job done.
 
So thanks for all the hints and tips along the way.  This one was certainly not a run-of-the-mill fault, but I've certainly learnt a lot about this system in the course of getting to the cause of it.  Every cloud, and all that...