Audi 100- won't start.

[leggy]

Originally Posted by Flying Frank

...If anyone can sort this, it has to be you . We all normally bring our problems to you.....

I was guna say exactly the same thing Flying Frank

Originally Posted by Flying Frank

What was the alarm sounding in the dash, low oil buzzer ?.....

Wasnt there a thread posted recently about this same issue The Real MSH!?...
iirc - member was experiencing dash alarm sound after start/cut out and non-restart, the immobiliser was ruled out cuz it prevents the vehicle from even turning the engine over, problem was diagnosed after two inspections by said member as being a brake fluid leak, which once fixed, shut the darn alarm up and the car started normally again without cutting out.
(nb. if your thread was a test, did I pass? )

[Hanuman]

First.

One of the V8s presents an occasional random oil warning; either the sound or the oil lamp symbol. Very occasionally, both. In every case, these are random and false as the engine is not running when it begins.

The V6 sounded an alarm (high pitched tone), and I stopped the engine. But there was healthy oil pressure and sufficient oil.

When I tried to start it again, that's when it wouldn't start.

On the matter of immobilisers. Most of these things are old, now. I'm no expert on the current state of the market or with what Audi installed back then, so much of my thinking is best guess based on aftermarket systems.
In most cases, if you wanted to disable a circuit, you would put a break in the circuit, which would need something like a key to make the circuit work. Most cars have a very basic immobiliser- the ignition switch. When you turn the key, you 'make' one or more circuits.
For instance, the starter motor solenoid.
Power to the ignition circuit, etc.

Now. Add in an actual immobiliser as a specific set of electronics.
You still need a key, which in this case is the little fob on your key ring, or a transponder in the key itself.
With the factory alarm, the immobilising function is incorporated in the alarm. So if the alarm is deactivated by opening the doors with the key, then the immobiliser will also be deactivated.

But there is a good chance that several circuits are being 'protected' by the immobiliser- fuel pump, starter solenoid, etc.
The immobiliser function will simply provide a break in the circuit for each of these. Quite likely, each circuit is broken using a relay. So when the relay closes to make a circuit, you need it to be a good contact, just like any other relay.
A bad contact in the relay will result in a bad circuit. Perhaps the internal relay in the immobiliser is controlling the main relay for the fuel pump, which now fails to switch properly.
If the immobiliser is protecting 2 or more circuits, the immobiliser itself may well be working as expected, but for whatever reason, one circuit is failing to 'make' properly, and it only requires one bad contact in the immobiliser to cause a 'no start' situation.

If you look inside many older alarms, you will find several relays, and these are generally not so rugged as the relays found in your normal fuse box/relay panel.

[Gib]

C4's all have stand alone immobilisers. The immobilisers are single signal direct to ECU with confirmation. Due to the time lag they are not required when starting a car but are required with in a few mili seconds to keep it running, hence the start then immediate stop you get when the immobiliser has not worked. Alarms are a completely different circuit and not linked.
Unless this is a non factory alarm / immobiliser?

[Hanuman]

So what exactly is the immobiliser doing in an Audi system?

[vorsprung durch technik]

Originally Posted by Gib

C4's all have stand alone immobilisers. The immobilisers are single signal direct to ECU with confirmation. Due to the time lag they are not required when starting a car but are required with in a few mili seconds to keep it running, hence the start then immediate stop you get when the immobiliser has not worked.

Thats what happened to mine Mark as i said earlier. If the car starts and then cuts out it's the imobiliser. You mention you cannot even start it. If it were your imobliiser which it seams it's not it,s a white box that sits behind the dash.The ignition switch also has coil reader wire that connects to the immobiliser that can sometimes go according to what Audi told me. I bought the said wire and never got any further with it. HTH somewhat.

[Hanuman]

It's not been a problem since.
It does seem a little counterintuitive for the alarm/immobiliser to allow the car to start, and then stop it. But then, I've never had a VAG factory system before, nor had to fault find one.*

The behaviour of my car is exactly as I'd expect a single channel of a system to fail, and is one of the main reasons why I tend to rip out any aftermarket alarm/immobiliser if it demonstrates even the slightest of problems.


*actually. the dead V8 which was scrapped had one, but it didn't survive long enough to be a problem.

[Gib]

The alarm does exactly what it says it alarms people to the fact the car has been opened incorrectly or some thing is in the car which should not be there (microwave button on the b pillar , multi flash of the hazard indicator when locking the car ).
The immobilizer, immobilises the car so it will cut off after a second or two. It should be immediate but practically adding this to what where ecus with no provision for an immobilizer meant an external signal to prove it should run. To make this safer these needs to talk to each other which means a slight delay. As mentioned these can be taken out very easily permanently or as an emergency need.

[vorsprung durch technik]

Originally Posted by Gib

As mentioned these can be taken out very easily permanently or as an emergency need.

Did i miss that part? How do you remove the immobiliser? Does it involve removing the white box i mentioned and thats it?

[Hanuman]

It's hard to tell from that description, but it seems to imply that the immobiliser doesn't work in the same sense as most of the immobilisers that I've dealt with. i.e. it actually allows the engine to be started, then decides to stop the engine after a short while, by signalling the ECU.

From past experience where a car will run for a short while, its because there is sufficient fuel pressure in the system to run for a few seconds, but this is exhausted quickly as the pump is not running, or not at full speed, due to a malfunction (such as a relay fault) There is a priming function when the key is first turned.

So, at the risk of repeating the question- what is the immobiliser actually doing? There is no hint of this behaviour in the handbook, or snippets of manuals that I have.

[Hanuman]

Originally Posted by vorsprung durch technik

Did i miss that part? How do you remove the immobiliser? Does it involve removing the white box i mentioned and thats it?

Well, that seriously negates the entire point of the immobiliser, surely?