Ignition switch/barrel and starter motor

[31satara]

My brothers A4 needs some help this time! 1997 A4 avant Tdi AFN (not a/c)
Phone call along the lines of: wipers and indicators not working, some electrics ok. Driving home (10 miles) funny rattling noise from engine but pulled fine. Stopped outside house, just started to turn key to switch off and rattling stopped - engine running fine. Turned off, tried starting but just got a whirring sound..............
I'm thinking a faulty ignition barrel or switch. I suspect the former is more likely - and I assume these will be expensive requiring a new key also? Any ideas?
My idea is that the starter motor was trying to start whilst the engine was running (due to play in the barrel not disconnecting properly) and therefore caused the rattling noise with the solenoid trying to engage it. Thus eventually causing something to break inside. The whirring noise is the starter turning but not being engaged with the engine. If tried with the headlights on they go dim.
Is this a likely scenario, or does anyone have something better for me? Thanks.

[Zenerdiode]

Headlights don't dim on VW/Audis when starting - they are extinguished by the X contact in the ignition switch. Terminal assignment as follows:

30 - supply to switch
S - energised with key in barrel
15 - energised with ignition on and start
50 - energised in start position only
X - energised in ignition on only (so when in start position de-energised)

X has a number too, I just can't remember it...
Headlights, blower fan and heated rear window are the three big players connected to the X contact, for load reduction when starting.

What you're describing sounds like the ignition switch/barrel. Either the switch itself fragments internally, or the piece of metal from the barrel that turns the switch breaks, or both, result in the various conacts above making and breaking when they shouldn't and give your symptoms.

New barrel: 107 905 855 CB (quote chassis number to keep original key)
To remove barrel, place key in and turn slightly, then push a thin metal skewer in the little hole in the reader coild surrounding the chrome keyhole, this pushes a tab to release the barrel - pull out with key.

New switch: 4B0 905 849 - notorious for failing internally, (they use this switch on the Porsche Boxster and 911 and have left loads of people stranded) bit of a b*gger to change as to get at it you need to remove the lower column cover. Sounds easy? To remove the lower cover you need to remove the steering wheel. To remove the steering wheel you need to remove the Airbag.

If you have the expertise/guts to remove the airbag, remember to place it PADDED SIDE UP and keep it in the boot out of harms way. If you place it padded side down and it deploys with static, it becomes a 250mph projectile.

Correct airbag proceedure is battery off for 20 minutes, remove and place in boot as described above. Reconnect, switch ignition on and close door, then reconnect battery. This ensures you are out of the vehicle if it were to deploy on power up.

[audimad]

Fualty switch is my bet, its quite common to go, and the switch alone can be replaced which is good.

And yes, starter is an unfortunate casualty of the dead switch, i think the switch is around £30 from the dealer iirc

[31satara]

Thanks for the comprehensive replies - very helpful. I'll start by replacing the starter (a used one hopefully - there are Audi breakers in Nottingham and Worksop - both about 20 miles from here). Then try replacing the switch - in this case I guess it's best to get the proper new VAG part from the dealers.
I've read through the procedures in 'Haynes' and in theory I should be able to do it! Any tips for the starter motor removal?