Battery failure? help!

[brasiliangringo]

IMO the 2 hall sensors are usually fairly robust or better put : simple by design (i doubt they are at fault) but they are cheap as chips 2nd hand from Ebay (same part fitted to most petrol engines of the era) which will rule them out and easy to fit. Then the CPS (get a new one) and that will eliminate crank and camshafts positions being at fault (assuming cam chain and belt timing good) are key to start up along with the obvious basics spark, fuel and air.

[HPsauce]

Biggest problem I've had with CPS's is them coming loose, though not on an Audi. Happened several times over the years on different (company) cars.

No idea where they are, but if accessible that's an easy check.

[David's8]

Originally Posted by brasiliangringo

If you cant start it a CPS might be at fault agreed, but they dont often show a code. I had a situation with a 2008 A8 TDI and when hot mainly it wouldnt start and new CPS sorted it - took much head scratching and injector testing etc. Cant recall where they are on the S8 but they are usually easy to replace and not expensive and whilst i hate throwing parts at cars to fix a problem without diagnosing 1st, sometimes its a more economical use of time / money and the only option.

I have downloaded the G40/163 sensor replacement procedure from the Service Manuals uploaded (by Delboy?) - thanks for that resource. They are at the camshaft ends - one at the front of Bank 1 and the other at the rear of bank 2. The latter is easy to get off but the one at the front requires bumper and fan removal . You are right - they come in at around £13 on ebay.
Anyway just replaced the fuel filter (needed doing anyway) and there was plenty of pressure behind it. Next - check the earthing connection but now need to dry off and change clothes as its chuckd it down with hail, rain and snow whilst i was doing tthe filter.

[brasiliangringo]

You should be able to work around / through the fan with the top engine timing cover removed with the right tools..

Just get a spark tester to hook up between coil and plug.

[David's8]

Cheers for that advice.

[ainarssems]

While crankshaft position sensors do fail it is very unlikely they will both fail at the same time and engine would run with 1 failed, they also register fault codes if they are faulty. I think it's more likely to be fuel delivery issue, can you hear fuel pump running for couple of seconds when you switch on ignition?

[Architex_mA8tey]

Originally Posted by ainarssems

While crankshaft position sensors do fail it is very unlikely they will both fail at the same time and engine would run with 1 failed, they also register fault codes if they are faulty. I think it's more likely to be fuel delivery issue, can you hear fuel pump running for couple of seconds when you switch on ignition?

I agree with Ainars - dirt in the fuel could have settled due to non use and then blocked everything or fubarred the pump. I would still try putting some more fuel in to make sure it's not the fuel pickup issue that seems to cause issues when the tank gets low

[David's8]

Originally Posted by Architex_mA8tey

I agree with Ainars - dirt in the fuel could have settled due to non use and then blocked everything or fubarred the pump. I would still try putting some more fuel in to make sure it's not the fuel pickup issue that seems to cause issues when the tank gets low

The sensors listed as possible causes under the fault codes are the Camshaft sensors G40 and G163 - but, like you, i fail to see how a period of non-use would cause them (or, indeed, a crankshaft sensor), to fail.

Today I replaced the fuel filter and there was quite some pressure in the system so the pump is doing something. Then I checked the chassis earthing strap and cleaned it up as suggested earlier. All done in the open on the drive and in rain, snow and hail - I had 2 changes of clothes and was frozen.
I think that the fuel issue is worth following further so will look to get some more fuel in the tank and check the pump and see if fuel delivery is occurring at the business end.
If only I had a garage....

[ainarssems]

Sorry did not realize you actually had fault codes for camshaft sensors. If there are fault codes then of course you need to investigate them first. It is very unlikely for both to fail at the same time but maybe 1 has been faulty for a while just was not noticed. Could be also wiring or ECU fault, possibly moisture got in and corroded contacts or caused short circuit in the plugs somewhere.

What are the actual fault codes, are they saying:
1.Implausible signal;
2.Short to earth;
3.Open circuit or short to positive;
4. Wrong allocation

I have attached pdf about testing camshaft position senders.

[David's8]

Originally Posted by ainarssems

Sorry did not realize you actually had fault codes for camshaft sensors. If there are fault codes then of course you need to investigate them first. It is very unlikely for both to fail at the same time but maybe 1 has been faulty for a while just was not noticed. Could be also wiring or ECU fault, possibly moisture got in and corroded contacts or caused short circuit in the plugs somewhere.

What are the actual fault codes, are they saying:
1.Implausible signal;
2.Short to earth;
3.Open circuit or short to positive;
4. Wrong allocation

I have attached pdf about testing camshaft position senders.

Thanks Ainars, The fault code from my first scan were for multiple mis-firing which, according to rosstech wiki include Camshaft Sensors amongst other potential causes such as fuel supply, coils etc . Unfortunately I have not been able to get the engine to run since to delve any deeper using VCDS so I cannot be certain that it is the sensors. I am therefore faced with trying to fault find everyone of the potential causes.....