Odd one...

[TechNick]

Would welcome some ideas on how to resolve this one...
Electrical issue of some description.

Battery and alternator are relatively new and both appear to be functioning fine (based upon the voltage guage).

Car fires up fine if sat overnight or left for 45min+ since last use, and also fires up fine straight after reconnecting the battery (if had been disconnected for some time whilst in storage).

Issue arises if you try and start it after a few minutes from stopping it (e.g. a petrol station or popping in to a Shop).

You turn on the ignition, everything lights up and makes the noises it should, volt meter rises to just over 12V, but nothing... starter motor doesn't even try to turn or click.

Through trial and error, seems if I leave the car parked up for 45min or more, it then fires up fine.

VCDS seemed to show the immobiliser was being turned off correctly with the key, but acts like it's immobilised.

I even tried putting a fresh battery in my spare key and reprogramming that back up successfully, but won't start up with that key either, during that 45min dead zone.

I did have a probably related issue several months back. Starter motor would work, though, but just buzzed - like it had jammed/wasn't getting enough juice.
I replaced the related relay(s) behind the knee bolster area of the dash and it seemed fine after that.

I tried pulling and swapping those same relays when this latest issue arose, but didn't seem to help.
Disconnecting the battery for a few mins didn't seem to help either.

Only workaround seems to be to leave it 45min or more.

Presuming some sort of capacative charge remaining where it shouldn't be, which dissipates over time, but don't really know where to start to look to rectify it.

Any suggestions?

[TOPMAN]

I’ve had a similar problem to this in the past. The starter was old and suffering from heat soak. Had the starter rebuilt and it’s been fine since. Manually apply power to the solenoid and see if she works, if not then you’ve probably found your issue


Sent from my iPhone using Tapatalk

[Shoppinit]

Sounds like the crank position sensor. Quite common failure and symptoms like yours.

Doesn’t give any fault codes. Cranks but just doesn’t fire.

[TechNick]

I’ve had a similar problem to this in the past. The starter was old and suffering from heat soak. Had the starter rebuilt and it’s been fine since. Manually apply power to the solenoid and see if she works, if not then you’ve probably found your issue


Sent from my iPhone using Tapatalk

Thanks. Hoping it isn't the starter, as it's not very easy to get to on these, from what I understand (not that much is, of course!)

[TechNick]

Sounds like the crank position sensor. Quite common failure and symptoms like yours.

Doesn’t give any fault codes. Cranks but just doesn’t fire.

Thanks Ben.
It isn't cranking at all, during that dead zone period, though - turn the key and nada

[Shoppinit]

Oh. My bad. That’s a completely different problem then.

Immobiliser won’t stop it cranking.

Check the gearbox with Vagcom to see if it thinks the selector isn’t in P.

Check the 200A fuse in the scuttle.


Get a LED or multimeter on the starter to see
If it’s getting a signal to the solenoid.

Check the brake light switch. Belt and braces.

/my gut says starter motor, but that’s possibly because I’ve got them on my mind because I’ve a similar issue with the M3.

[TechNick]

Thanks. Few things to try!

[philS2]

Did you solve this?

[srichards]

That sounds similar to the issue I had. Think it was the F125 in the end as that is involved in whether it will start or not. Changed starter motor and it was exactly the same. Changed the F125 and the issue went away as far as I know as I sold it a few months later.

You'd turn the key and nothing. No cranking. Don't think you even heard any relay noises.

[Shoppinit]

Yeah, I had the same. Suddenly, the car just wouldn't turn over for no apparent reason and I was stranded at the testing centre. RS6 showing typical impeccable timing there. I was able to "hotwire" the starter by taking 12V from the battery and applying it directly to the connector that goes to the starter motor solenoid (by borrowing a jumper cable and a bit of wire). When I got it home I traced the electrics all the way from the ignition switch and it was the F125 that was dead. No faults stored.

/IIRC the earth from the starter relay goes via the F125 and that's what had failed inside the F125

//There are a couple of identical relays to the starter (horn, maybe?) that you can swap positions to eliminate. I did all that before having to hotwire the car.

[HPsauce]

I had the exact same problem on my D2 S8, caused by an internal failure in the F125. It's dead easy to get round if you know how and I took my time getting the F125 replaced as a result.
The starter relay activation circuit does indeed go via the F125 but you just take out the relay and put a connector between the relevant terminals! I used a short piece of thick wire with spade terminals crimped on the end. I now carry that in the car.
Obviously you "can" then activate the starter accidentally when in gear (the F125 limits you to P and N) but I never managed to do that!

I'd link to the relevant discussion on the A8parts forum but it's currently down.

[TechNick]

Did you solve this?

Yeah, sorry, with all the months that subsequently passed, I forgot to update this thread (especially as each 'fix' seemed to resolve it... for a while... but actually didn't)!!

In short, same as everyone else, it seems!, it eventually turned out to be the F125.
I'd originally dismissed it as it hadn't been that long since it was last replaced (new OEM unit), but it seems they have a much shorter half-life than even the alternator!

On the plus side, I now have a spare, perfectly fine, starter motor, and a collection of relays and other parts that were all falsely accused!

P.S. Reference the typical impeccable timing... the first time it happened, I was at a petrol station, on New Years Eve, and they were about to shut up and lock up for a day or two...!!!!

[Shoppinit]

The OEM F125 is expensive junk. I now have 3 aliexpress spares sitting on the shelf (and one installed).

[HPsauce]

I too have cheap Chinese spares on hand, and the one on the car is also Chinese....
BUT, with the D2 S8 there is a problem in that the copies have a slightly shorter cable than OEM making it a PITA/impossible to fit. This may not be a problem on the RS6 though.
However my local VAG independent managed to get it installed somehow, next time I'm in there I might ask what the trick was.

[Shoppinit]

Not a problem on the RS6. It was a straight swap.