The A8 Wont start / Gearbox electronics Investigation

[ChrisG]

First post although I have been lurking here and I thought I would share some experience I have just had to go through with my A8 and I hope that if someone else ends up in the same boat as me, this will be useful information.

My 2002 A8 Quattro Sport 2.8 has always been something of a problem child, and considering how technical cars are these days its very easy to be blinded.

The problems started about a year after I bought it. Dropped my daughter off at school, came back to the car, and it wouldn’t start. AA was called out and they fixed it, however a week later, same thing happened so I took it down to a garage. First instinct was the starter motor was broken.
Turned out, I was wrong – the garage found the terminals on the starter motor were corroded. Cleaned up, and the car worked again.

A year later, the alternator played up – turned out the alternator was broken, so that got rebuilt, however even with that fixed, the car seemed to be loosing charge, and every so often I would have to take the battery out, charge it and put it back in. The garage had no explanation for this.

Then to compound all of these issues, the gearbox started acting up. It was refusing to go in to reverse. You would engage it, there was a big “clonk” followed by the Gearbox symbols on the dash board turning red. You would have to turn the car off, back on again, and usually it would work correctly.

Earlier this year though things started getting worse. Firstly the battery went flat again – had to take the battery out and recharge it.
Then the Reverse gear started getting quite narky – It started not going in to reverse at all.
And finally, in March, the starter motor packed in again.
Trip the garage again for that – they checked the terminals, and although they said they didn’t find anything too out of the ordinary, the car worked again, and oddly from March on the car behaved itself. Kind of. The Reverse gear did keep occasionally acting up.

In August the car got its MOT and much to my surprise, went through first time. I was quite stoked by this as normally the car throws up some very expensive issues.
Well, my happiness didn’t last. Second week of August, the cars starter started acting up, and in the end, it refused to work at all. Garage collected it, checked everything they could and the diagnosis was the starter motor was fried so they replaced it. At which point the car worked.

For all of two weeks, before the gearbox went ballistic. Reverse gear would not engage at all and I was having to park it forward. Oddly the box in Drive worked perfectly, there were no noises or grumbling or odd things going on. It was just dead in Reverse.
It was odd that I then remembered, a year before, Tiptronic had stopped working. You tried to engage it, and the car remained in Auto.
First reaction – Gearbox is gone and I have to admit I was on the verge of ordering a replacement box. But, thanks to some good advice, I was urged to have it plugged in and checked on the diagnostic machine.
Took it to the garage, they plugged it in, ran it through. Phoned them up and the reaction was what I expected – Gearbox is gone.
So, went down to pick it up, and got a bit of a surprise. The car was working again. Reverse was engaging correctly, there were no issues. Asked them what they did and the only thing they did was during the diagnostics, they had cleared the memory on the gearbox.
Well, cheap fix, but it didn’t last. The car 5 days later went back in to locking itself out on Reverse again.
The Error Code from the box was F125 related, which is an electronic control box on the side of the gearbox that takes mechanical input from the gear selector and tells the box what to do.

After lots of calls, conversation etc, they replaced the F125 unit, and I am pleased to say at this point, everything on the gearbox is working fully as expected. No further issues.

So first tip. If the car is refusing to engage reverse, but seems to be working in all other respects, check the cars computer – if its coming up with F125 Range Selector and giving errors such as improbable signal, there is a strong chance its this unit. And it is cheaper than a gearbox.

Now at this point, I think I have got everything sorted. As always, wrong.
A week later, I went to pick my daughter up from school, and much to my surprise and annoyance, the car wouldn’t start. Same issue as before.

During the F125 replacement, they had an issue with the car not starting, so back at the garage, they removed the connectors, checked them, made sure of everything and checked the lot over. It worked again, but it only took it a few days to stop working again.

By this point though I was adding things up and seeing a pattern.
The car had worked all summer without a fault.
Then in the second week of August, it started acting up. Right about the time we had lots of rain.
Then, the first time I went to get my daughter, it was raining.
Then, when it wouldn’t start the last time it was first thing in a morning, it was cold, and it was really damp and misty.
Physically you could hear a metallic click which meant it was trying to do something, but then wouldn’t.
This problem seemed to be electrical.
I then had my garage check the obvious culprits – Starter motor relay, or the ignition barrel.
Sadly both items came back as working perfectly but after doing some research, the garage found there was an error on the computer saying that the car thought it was in 4th gear. After talking to some people, they were leaning towards some electronics physically inside the gearbox that was locking out the starter because it thought it was in gear.
Needless to say, I went through the roof, and wouldn’t accept that.
After researching on here, I found two threads about virtually identical issues to mine, and ultimately after trawling through the whole internet, and talking to a few A8 Experts (who I have to say thank you to – in particular the owners of this forum), they all said the Gearbox error was a red herring.
The problem was most likely corroded positive main power connections.
Or in other words, the starter was not getting enough juice to start which made sense.

As is typical with my problem child of a car, they stripped everything off and eventually located these terminals, and found they were actually clean as a whistle and still copper greased up from the factory so we were back to square one, but thankfully, this exercise had sparked off an idea.
They hooked a booster pack to the front of the car, directly to the positive, and on to the primary earthing cable. Car started first time. Take it off again, nothing – wouldn’t start.

At this point, they tried to removed the primary Earthing point bolt, which snapped. The connection was furred up terribly, and the steel bolt was rotten and so was the aluminium it was going in to.
They managed to drill the bolt out, tapped it out, helicoiled it, and replaced it with a new bolt, as well as lots of copper grease.
Car started up fine. So, for 3 years, two of the cars problems were actually down to one bolt which had gone rotten. Why two?
Well, the starter motor was not getting a good earthing connection, and when it got wet or damp, it lost the connection even more, resulting in the car not starting.
And, the Alternator not charging the battery was exactly the same – without a solid earth, the car was unable to put enough juice in to the battery to keep it fully charged.

So, second tip.
Your car isn’t starting – first check the earthing point – it may save you a lot of heart ache and money. Be careful though as if it snaps, you are going to be going through a lot of drill bits. You also cant change the earth point as the car being Aluminium means Audi had to beef up where the earth was – other areas simply don’t have enough meat. It may be best having a garage do this, as if it snaps, they will need to drill it out.
Still not starting after that – then you need to check the main power terminals at the front of the car – they are both under the drivers side inner wing. Make sure they arnt corroded.
After that you need to check the starter motor relay which I have been told have a tendancy to fail. And Finally you may want to check the ignition barrel which may have damage.

Its worth asking this simple question – how do I know its was the earthing point that had failed. The car has done its mysterio thing before where a little tinkering has cured it for a time.
Well, when I picked the car up and every time subsequently, its done something it never did before.
Normally when you engaged the starter motor it was slow as in
“ruuuuuubar, ruuuuuuubar, ruuuuuubar”
Now the starter motor sounds like its about to take off as in “tigi tigi tigi”.
The simple fact the starter motor is going like a bat out of hell has convinced me. That, and the fact that this week has seen heavy rain, cold misty morning etc, and every single time, its never missed a beat.


I am going to sum this up with a simple fact.
The blown alternator, may not have actually been broken. The starter motor may also have not been broken, but the signs from the car were that these parts were gone.
If you add up the costs involved, I have probably sunk over £1000 in to the trying to fix these items and solve the issues, for it to be a simple as a 30p bolt.
We rely far too much on modern cars computers to tell us whats wrong, and garages do too.
They forget to do what should really be second nature – check the simple things first.

I hope this has been helpful.

[David's8]

Glad all came good in the end. Your problem(s) was like a lot of issues - when finally solved it all seems clear and logical in hindsight but is totally mystifying at the time. The issue of main earthing point is one that a few on here regularly point out as a potential source of some rather unlikely symptoms.

[tonupkid]

Great write up, great result and should your four wheeled child again become stroppy on the school run. You have a choice, replace one of the children.

On a serious note, it's good that you've have so lucidly shared this solution. I'm betting there's more than a few 8's been prematurely scrapped, because of that bolt.

[HPsauce]

Great info. Where is the bolt?

[notorious]

Absolutely great info.
Location and picture of the bolt please.
Will fix mine preventively.

[The_Laird]

Yep, I'd like to know too.

BTW, welcome to the forum and, if you've been lurking you'll know, pictures are a must!

[johnny_quattro]

Originally Posted by HPsauce

Great info. Where is the bolt?

I think it may be shown in one of the two photos that I took when my old barge was having its gearbox done last year. The clearer photo of the back of the engine (nearside on the left of the photo, offside on the right as you look at it) also shows quite nicely two of the big fat drains either side of the flywheel/gearbox housing. I gave them a good squeeze at the time, due to the lack of access when it was all put back together.

There is a big old earthing strap which is brown in colour coming from somewhere on the engine/gearbox housing which leads to this black plastic box on the chassis rail - you can just about see it on the far right hand side of this photo. The other photo which is slightly out of focus (apologies for that) shows the back face of the OSF wheel and chassis rail power block/box.

I hope that this information is of some help. Please feel free to correct me if I'm wrong in any of my assumptions.

[johnny_quattro]

Originally Posted by ChrisG

Physically you could hear a metallic click which meant it was trying to do something, but then wouldn’t.
This problem seemed to be electrical.

That was exactly my experience a couple of years ago. It resulted in me getting the Audi placed on a lowloader and taken back home. A new starter motor was fitted to no avail, and I'd replaced the battery with a Bosch Silver jobby. It turned out to be a similar fault to your problem and was discovered by the garage after a bit of troubleshooting. The connections inside the box were all gunked up, so they were cleaned up and put back. Everything worked perfectly. The garage let me have the new starter motor for free as it was too much trouble to take it all out again - result!

[Norretal]

Originally Posted by tonupkid

On a serious note, it's good that you've have so lucidly shared this solution. I'm betting there's more than a few 8's been prematurely scrapped, because of that bolt.

+1 on the above statement

Great thought processes and very well written up, and welcome to the forum from me too

[oldnick]

yes , please find out the location , preventative change sounds a good move !

strangely enough had a similar problem on my old landy ....earthing on the body turned out to be the problem ; got lucky though ...came across an ex army mech who told me that the first thing the army do when they get a new one is change the earth to the block ! never had another problem !