Non Starter - Troubleshooting Help Required!

[Si Blythe]

Sorry for the poor English and grammar guys, my iPhone seems to also want to play up!

[prj]

I had a qualified auto electrician diagnos the car for (I wouldn't bin an S2 on the off chance it maybe this maybe that) the relay doesn't energize when cranking

If he was qualified he would tell you what is wrong with the car.
Diagnosis involves establishing a fault, just connecting vag-com to the car is not diagnosis.

I have changed the crank sensor for a known good one too. The flywheel hasn't lost it's sensor as first thought, I have also tried a different ECU.

It's the other sensor then. Engine speed sensor or ECU power.

I will change the cam sensor and probably the POS's too.

Why?
Absolutely no reason to do this. If your fuel pump is not energized when cranking then 100% the problem is not the hall sensor or the POS.
It is the fuel pump relay or the ECU not getting the correct signal from G28, engine speed sensor or the ECU is not getting power.

I have triple checked all of the connectors and I may take the dash back out just in case it is a connection somewhere in there. Other than that what can I do? I have also checked every fuse wire etc etc.

You can take the Motronic connector and verify that all important leads are 0 resistance and not short to ground, with ignition on, you should verify that all 12V connections to the ECU are correct and all grounds are good.

In fact, if your "electrician" was anywhere close to qualified, he would have done that.
This job takes an hour or so, and all you need is a digital multimeter, which costs 10-15 quid for the cheapest one that will do the job.

Are there really not any mechanics with half a brain in your area? This fault probably would take a few hours to solve by someone who has experience with this and knows how the system works.

In short:
You still have no diagnosis for your car. Changing random parts will only be expensive and will not make it work. You first need to diagnose the fault, you still have not done this.

[Si Blythe]

The diagnosis was the crank or cam sensor - that was the code that was thrown up.

[prj]

That code is always thrown, even in normal operation.
Anyone who knows anything about Audi's of that vintage knows this.

[UrS4boy]

The diagnosis was the crank or cam sensor - that was the code that was thrown up.

You need to tell us exactly what codes were "thrown up" (which, by the way, in English means "to vomit").

When codes are "pulled" or "retrieved", you will always get an engine speed sensor (G28) failure code - IF the engine isn't running when the codes are pulled. A G28 code could be real but typically it is a false positive.

A G4 crank position sensor code could be real.

A G40 cam shaft (Hall sender) code could be real.

The FP relay will not be energized by the ECU without a good G28 reading (>= 13 RPM).

The injectors and coils will not be triggered by the ECU unless both the G4 and G40 are working properly.

You need to tell us exactly what codes were retrieved if we have any hope of helping you.

[GT500]

Has it an Immobiliser fitted?

[Si Blythe]

No after market alarm is fitted, I had the Clifford Concept 50 removed.

I have remembered an intermittent problem it has, when I start the car sometimes the engine just turns over quite slowly without actually firing up. If you then switch off and try again it fires up. Is it possible this is the root of the problem?

[prj]

If it cranks and turns over fine but does not start and does not energize the fuel pump relay, then I already told you what the problem is and what to check...
And for the final time - reading codes from a car is not diagnosis. Diagnosis is saying what is wrong with it and why it does not start, which you still don't know.

There are very few things to check. I already explained everything.
If you really don't have anyone who can do some simple checks with a multimeter, then go on and scrap it, then purchase some brand new car, so that the warranty takes care of everything

[Si Blythe]

Mate what's with the hostility? I come on here for advice - I have been given several different options, I haven't rubbished or discounted yours - I am not an auto electrician. I am a qualified aircraft mechanic though! so relax, all advice is greatfully received.

[ericd]

If it cranks and turns over fine but does not start and does not energize the fuel pump relay, then I already told you what the problem is and what to check...
And for the final time - reading codes from a car is not diagnosis. Diagnosis is saying what is wrong with it and why it does not start, which you still don't know.

There are very few things to check. I already explained everything.
If you really don't have anyone who can do some simple checks with a multimeter, then go on and scrap it, then purchase some brand new car, so that the warranty takes care of everything

I'm liking your approach to helping people are you a teacher ?,or a kepner tregoe professional ?, why not keep posting your advice, but remove the belligerent attitude and the air of infallibility, your advice like everybody's is not always correct , that's why you and the rest of us are here to try and help others and be helped...

[S2central.net]

A no-start situation like this shouldnt be complex to fix by any decent auto spark - as long as he has some basic equipment and the schematic.

ABY engine schematics are on S2central.net

The equipment required is just a multimeter and an oscilloscope - no professional auto spark should be without those two crucial items.

Plugging in a code reader to the ECU tells us very little - you get some meaningful info from ECU when engine is running but almost nothing when it is not. For example....... when I wire ECUs up on a bench with just about every sensor disconnected - the only faults you see reported are for G28 missing and TPS disconnected.... they 'mask' all the other conditions.

The auto spark should test the presence for 12V and 0V at the ECU plug - sad boys like me know this from memory on the S2/RS2 - I wouldnt expect a general auto spark to know - but thats what the schematic is for.

permanent 12V at #18
switched 12V at #27 (key on)

0V at pins 10, 14, 19 & 24

12V output from ECU #37 when key is on.... easiest place to check is hot side of fuel injectors
5V output from ECU #12 when key is on... easiest to check at CMP/G40 connector

With good power and earth in place - the ECU will not energise the fuel pump relay until engine speed exceeds ~13rpm.
The ECU closes a path to 0V at pin #03 when this occurs - this is what activates the relay... with the lid off the relay you can see this happening on cold cranking.

If the relay doesnt activate then either -

1 - relay is dead
2 - ECU is not activating it because ECU is faulty
3 - ECU is not activating because weak / missising signal from G28 (rpm sensor)
4 - wiring fault to/from ECU and relay or ECU and G28

The ONLY way to examine G28 properly is with an oscilloscope - I posted waveforms on here in the past but it looks like a sawtooth waveform with pkpk amplitude of 5-15V and a frequency derived by 135 flywheel teeth per crank revolution...

WIth the 'scope - verify you get a good G28 signal between pins #49 and #48 on the ECU connector while cold cranking.

[Bowlerwell]

Great constructive diagnostic write-up Paul!

[Si Blythe]

Guys as you can appreciate finding this "simple" fix for my car is quite frustrating! I would just like to say that I do appreciate you taking the time to post and try to help me.

I promise I won't break the S2 - she will live on!

[prj]

Mate what's with the hostility? I come on here for advice - I have been given several different options, I haven't rubbished or discounted yours - I am not an auto electrician. I am a qualified aircraft mechanic though! so relax, all advice is greatfully received.

I'm liking your approach to helping people are you a teacher ?,or a kepner tregoe professional ?, why not keep posting your advice, but remove the belligerent attitude and the air of infallibility, your advice like everybody's is not always correct , that's why you and the rest of us are here to try and help others and be helped...

The reason for my remarks is this:

ABY, I will price up a new cam sensor and failing that it will probably be heading to the big car park in the sky!

Perhaps I like these old cars too much.
Anyway, getting that off the list...

As Paul said, anyone decent with this stuff should be able to diagnose it.
It is common not to only these cars specifically, but many older Motronic injection systems.
And I was not giving advice, I was actually diagnosing the fault based on the information given.
Paul further elaborated on that and showed how to test all these things.

If all the symptoms are exactly as you describe, then those things I said and which Paul spelled out once again, are causing the no-start condition.

If you can not find anyone who is qualified enough to do this, then do it yourself.
If you really need, we can post up which pins to check for what and walk you through it.

Paul wrote you the four things to check, are you able to check them on your own? Or do you need further steps on how to check each one of them?
Because one of those is what is wrong with your car, if the information you gave us is indeed correct.

[Jonnybravo]

Good right up there Paul, I have already loaned Simon a working ecu and crank and I presume speed sensor ( the 2 that rear off the flywheel ) which didn't sort it, it certainly needs further investigation and the info on have posted is easy enough to check for.