Airbag controller fault
 

While working through outstanding issues with my "new" A8 I have arrived at the airbag system.....

The warning light does not come on at any point (poss. bulb out). Presumably to bypass the MoT.

The fault code list is fairly extensive:
1025 (diagnostic light malfunction)
00588 (Drivers airbag ignitor resistance too high - intermittent)
00532 (supply voltage too low)
00654 (Drivers seatbelt pre-tensioner ignitor resistance too high - intermittent)
0065535 (ECU knackered)

Codes cannot be cleared.

So probably it is either that the multiplug is half unlatched and corroded (like most on this car it seems) or the ECU really is knackered.......

I gather that the ECU lives under the climate control unit and radio (which is a bugger to remove) so it seems that some dashboard disembowelling is in my future...... (deep joy) and most probably the need to find another ECU. I note that the ECU used in A4s seems to have the same part no - is this really true?! At least I can change the climate control back light bulbs while I'm in there!

Car is March '96 registered 4.2 Quattro Sport btw.

Cheers

Nick

Changed airbag ECU. Only one fault now:

00654 (Drivers seatbelt pre-tensioner ignitor resistance too high - intermittent)

Nick

The ignitor is part of the seatbelt reel. It's more than likely a poor connection at the 2 pin plug where it attaches to the reel, but requires suitable precautions to discharge any electric charge before removing :)

Thanks, yes that was going to be my next check. Battery will certainly be disconnected well before any fiddling - I'm fairly paranoid about airbags/tensioners. I'm working on the basis that I shouldn't test the tensioner with a multimeter! What would be useful to know is what the impedance of it should be so I can connect a resistor to the wiring and see if that clears the fault. Certainly there have been plenty of issues caused by grubby/oxidised connectors on this car so far.

Is there any definitive visual way of checking if the tensioner has been fired? The only reference I can find says that it will not reel the belt back in if it has been fired. Mine does, but without much enthusiasm.

Nick

If the tensioner has fired the belt is taut big time. You can get 2nd hand ones of the sponsors for a few quid - i also have some for around a 15 delivered from a FL D2 A8.

This being an early 96 car it is supposed to have the pre-tensioner integral to the reel (changed July 96 acc to Elsawin). Certainly it doesn't seem to have any other attachments. Can't see any wires on it either, but the trim cover is being troublesome as I'm trying to avoid removing the seat........

Nick

Ah, now I've worked out how to get the B pillar cover off I can see that my last statement was complete tosh - it does have the separate tensioner cylinder mounted low down along the sill. Carpet in the rear footwell is sopping wet (must have squeezed a couple of litres out of the backing foam) which certainly won't be helping matters.

Nick

Got the igniter freed from the sill to access the plug. Bit crusty but not that bad.

Cleaning and dosing with contact cleaner made no difference. Bending the prongs a little to increase the contact tension made no difference. Shorting the plug changed the error to "igniter resistance too low".

Bit of online research suggests that typical igniter resistance is 2 -3 ohms so I found a 2.2 ohm resistor and plugged that in. Error cleared and stayed clear. However, the airbag warning light stays on...... even though no codes at all are thrown with the resistor in place......

Seems that the igniter really is knackered but I'd like to see the light go out before buying another seatbelt assembly.

Nick

The seat belt assemblies are peanuts 2nd hand. If it doesn't fix the problem you can always return it for a refund and will be easiest way to solve. Just plug it in unmounted (on the B pillar) and see if the code is still present.

Yes, no problem doing that. Just concerned that it won't actually put the light out as when I fit the 2.2 ohm resistor I no longer get any fault codes but the light remains on, even when code is cleared, ignition cycled, battery disconnected etc.

The original ECU p/no. ends 655K. The "new" one ends 655C and was a spare I had for my C4 A6, but as I found it also cross-referenced against early A8s I thought it worth a go. It seems to be healthier than the original and even had the same soft code in it - just this outstanding warning light even when all codes have been cleared and stay clear through power cycles. Maybe it doesn't speak to the cluster in the same way.....

Probably need to get the seatbelt assembly and 655K ECU. Getting a bit of a shopping lost building up!

Nick

Replacement seat belt assembly arrived today. Great service from Prestige.

Fitted this evening without any difficulty. Outstanding fault code cleared no problem and stays cleared.......BUT airbag light remains on. Scanned again, still no codes, light still on. I was concerned it would do this following the above tests, so wasn't very surprised. Foxed now!

Ideas welcome!

Nick

Scanned again with full fat version of VCDS. No codes, light remains on.

I've ordered another K suffix airbag sensor in case that is the issue. The only other possibility is that the light I think is the airbag light (because it says "airbag" on it strangely enough) is not in fact the correct one and there is another somewhere else which also has the bulb removed - although I think that would show a code.... Irritating!

Nick

Correct K suffix Sensor/ECU obtained from Prestige and now fitted. However, I'm now officially disappointed as the saga continues to test my patience. :-(

Airbag light stays on
Fault code 1025 warning lamp circuit faulty 31-00 short or open circuit (this in spite of the fact the lamp is glowing brightly!)

Previous C suffix sensor showed no fault codes though the light remained on.

Questions/possibilities

1. Am I really looking at the right light? It's in the main group of warning lights in the lower centre of the cluster and says "airbag" on it - which seems like a strong clue, but maybe this cluster has come from another car. Certainly I doubt the mileage it shows relates to this car. Could it be that the ECU is looking for another light either in the cluster or elsewhere which isn't currently connected? There was no bulb in the space when I got the car - I assumed to confuse the MoT man - but it wasn't taped over as some of the other blanks were and looked like a bulb had been fitted before.

2. Is the ECU fussy about the wattage of bulb fitted? I just a picked a random one from my collection.

3. Does this indicate a faulty cluster? It doesn't show any fault codes (FWIW)?

Any clues appreciated - it's beginning to get wearing

Thanks

Nick

Am I fishing waters where only nutters go.......? :-(

Nick

I would suspect it may be sensitive to wattage. I'd probably try borrowing a different bulb from the cluster somewhere into the airbag slot and see what happens.

The prefacelift clusters, in terms of the airbag light should be re really very interchangeable, and the facelift cluster just wouldn't fit, so I doubt any sort of cluster swap is as fault.

The error message does seem really very specific, and in my experience I would definitely be concentrating hard in that area alone.

The only thing you don't mention, but I assume you did, is to check the soft coding on the replacement module to ensure its coded the same way as the original one you took out ?

Its seems an incorrect code is unlikely as you'd get errors about airbags being present not expected or vice versa potentially, but worth mentioning.

Thanks for the response.

Soft coding is the same (01022) as the original, sick, sensor and the new one. Even the C suffix A6 one I tried was the same.

I agree on concentrating on the specific fault area relating to the code. I have now tried 3 different bulb wattages (0.9, 1.1 and 1.5 IIRC), which makes no difference. I don't know what is meant to be there as it was missing when I go the car.

There are some test procedures mentioned in Elsawin which I will do more thoroughly when I've figured out how to get test probes on the bulb holder with the cluster plugged in (wires too short!).

Wiring diagram doesn't really give clues as to how the circuit works as it is unclear what goes on in the cluster between the pins that supposedly relate to the airbag light and the bulb contacts. One of them isn't a direct connection (or is broken).

It could be that the cluster feeds power to the sensor via the bulb and the sensor switches the earth.

It could be that power feeds in both directions and the light goes out when it has 12v on both sides (like old fashioned ignition light)

It could be that the sensor switches power to the bulb and the cluster provides a simple earth (least likely IMO)

I have checked that the earth switch in the sensor multiplug is working.

Tempted to break into the light wire near the sensor plug and try wiring a bulb directly to see what happens.......

Nick