G28 sensor issues.

[Stefen]

Flywheel is a solid one IIRC. Not sure of origins as it came with the engine.

[twoqu]

We think that when the hall sensor is exposed the V should drop to near zero when measured on Pin 2. So if you rotate the engine by hand it should be measurable.
However we are stuck in the dark ages. If you can get a reasonable scope for £40 it would be a useful addition to anyones toolbox IF we understand where to probe and what to expect signal wise.

[Stefen]

Scope graph from tonight:

[UrS4boy]

If the G28 signal is the yellow squiggle, the signal is too irratic and will cause running issues. Remove, clean and replace, then test again.

IMO

[Stefen]

Yellow is the G28. It wasn't good on that session, but has been better before and after.

Below is another graph. G4 (2 Blue) croc clip has dropped off, G40 (3 Purple) is still going, but I've added the green probe (4) which is connected to an injector. It leaps a bit, which suggests a voltage issue, but since it is triggered then the ECU is happy. I think I have also found a dodgy earth on the coil pack (ground for internal POS) which probably stopped the sparking.
Voltage is too low during firing ~8.5V so that also needs addressed. Getting close to sorting this now.

[twoqu]

There are some waveform on here for comparsion. Although its the 3B the basics are the same even though the hall sender is in the dizzy.

https://www.picoauto.com/library/case-studies/audi-s2-intermittent-and-progressive-failure-to-start

Not sure if the scale you are using is making it more difficult to read. We know the hall sender should have 5V reference for example. The blip from the crank sensor looks like it should be @3V etc.
Without either expert knowledge or something to compare with its difficult for the layperson to interpet.

[Stefen]

That's a good link to help understand what the various waveforms are supposed to look like. Thanks for that.
Success has been achieved! The dodgy ground for internal POS in the coil packs proved to be the last hurdle, after that she started sparking. The battery was getting lower and lower over the evening and was too low to start her, but after a charge I'm sure she'll fire up tomorrow.
I'd never used a scope before and it was interesting playing with it and learning a little about the signals etc. Good experience.
Thanks to everyone who contributed and spent time thinking about it, pasting links to help etc. It's a great forum.