Electrical power supply problems
 

You may have seen my starter motor replacement thread which came about after some fault finding when my 8s electrics died in Exeter.

Just to give a few thoughts on the cure I eventually found with lots of help. This is the well abridged version! :)

Symptoms:

Car failed to start after a journey break. Jumped by a passing motorist.
Vagcom scan showed voltage really low - first thought alternator but no charging light.

Drove on monitoring voltage with vagcom and it steadily dropped. At 9.8V the abs light came on and at 9.3V went into limp mode. Recovered by RAC.

At home voltage at the battery was 9.5V so onto a charger for 48 hours.
Didn't do squat. Consultation and the interweb suggest that alternator fubar and battery having been taken so low also fubar!
Order replacement alternator from A8Prts here next day. Fitted, but car won't start. Move battery to connection point under chassis rail at the front, still won't start. take battery to Halffrauds for check. gets the thumbs up! So after more discussions, possible duff starter.

Change starter. Still won't start. FFS, get new battery. Pop it in boot. Still wont start. Stick second battery on connection box at front and engine slowly coughs into life and runs smoothly. Hmmmmmmmm

Next morning try to fire car normally, nada!
Second battery at the front again and slowly cranks and fires.

Something not right here is a statement of the blindingly obvious so start to take some voltage readings. Started at the rear 12.8V worked my way to the front and at the alternator earthed to its frame 13.8V!!!!! So the alternators kicking them out. Earthed to the engine = 13.8V.
Earthed to the chassis rail = 12 .9V!!!!!!!!!!!!!!!!!!!!! :eek2:

Back to basics here and what I should have done first, start to disconnect and clean all contacts and terminals. When I disconnected the earthing strap at the chassis rail for the engine, gave it some emery lurve and refitted it, it all came good.

Conclusion:

A bad earth at the engine earthing strap probably caused the battery to run the car. Running it so low on an aged battery may have killed it. Alternator is probably toast as well because without load, or too high a load it goes west.

Check all connections and contact first is the moral of this story.

Glad you got to the bottom of it mate- both earth and live connections at the front OS are regular corrosion points on the earlier cars. After all your work recently it just shows how the simple things can cause a real pain!

Another item to check & clean.
 

Thanks again for your valuable inputs and sharing. It's another item to check & clean while I do my 4.2 L donor engine swap. With the engine bay open...this should be a zip to check & clean.

It's on my long laundry list...it's going to be a huge time & $Money saver...not including a trouble...incident...events avoided in the future while enjoying my pleasurable Audi A8 ride(s)! :cool:

I now have this problem hoping it's a simple clean up and not the soliniod or motor, the skin has just grown back over my knuckles after fighting with the Land Rover at the week end