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