Dropping charge between alternator and battery. Car won't start.
 

Not sure where this thread should live so apologies if this is the wrong section to post in.

My car is not starting. Once jumped she drives like a dream, but stop her and she won’t start again. All power gone.

We have measured 14.5V coming from the alternator (it is a brand new alternator). At the battery end however it is getting 12V. So we are dropping current along the way.

Tomorrow we will trace the loom and see if there any faulty connections.

Has anyone experienced this? Is there anything we should be looking for/testing?

The only other thing I can add is that a warning light on the dash beeps and comes on. It’s yellow and looks like a lamp. Trying to find an example online but can’t seem to.

I assume you still have the original black Varta battery with the sticker on top of it stating the part number 4E0.
If yes, then it's about the time for it to die so I wouldn't be surprise if it's the battery giving up and getting a new Varta or silver Bosch S5 will then more likely solve the problem.
If that's not the case, then you will have to wait for forum Gurus to reply to that and help you investigate this issue.

If you would have a VCDS cable and post a scan then that would help.

Where can I get a VCDS cable?

A new battery has been put on. I will report back with make and model.

Battery charge rate is controlled by the battery ECU. If the ECU believes the battery is fully charged, it will not pass the alternator voltage to the battery. You could have a faulty battery ECU. I would scan the energy monitor ECU For faults first, and then suspect the battery.

+++
Here is the link to VCDS cable http://www.gendan.co.uk/product_VCHU.html

One more question: do we have to put an an actual Audi battery on the car and then code it to the computer?

No, any brand will do as long as the charactersitics match the original, which is usually 110Ah or 95Ah. You can use the VAG equavalent part number or the VARTA part number. Y
ou also need to change the serial number in order to reset the accumulated battery data back to that of a new battery.

How do I do this?

I used these instructions...
http://wiki.ross-tech.com/wiki/index...ry_Replacement

http://www.ross-tech.com/vcds/tour/r...n.html#battery

Thank you very much. Turns out the battery I have in the car isn't one of the choices in the drop down menu.

Can pick up an Exide tomorrow.

Not the best way to start A8 ownership, but at least it's made me interact and read through much of the forum - you guys rule! - and I'm no longer "glazing over" when it comes to thinking about the computer etc.