Airbag warning light - Passenger seat
 

Here is another very common fault, which seems to hit many D2's with the sports recaros after around 10 to 12 years of life, although it will certainly be based on the number of time the passenger seat is used.

How to diagnose:

The fault is diagnosed using vagcom on the Airbag controller, and expect to see a fault regarding the 'seat occupancy sensor' for the front passenger seat. It will likely be 'intermittent'

You can reset the fault, but it *will* reoccur, it might be a week, it might be a month, but it will.

What is the sensor for ?

The occupancy sensor is used to determine whether anyone is sat in a particular seat. The theory is, that if a seat is unoccupied, then in a crash it can deploy less airbags and reduce the repair cost. There is always a sensor on the passenger seat, and apart from the very early D2's, and the very last ones, there are two sensors for the rear seats too. The drivers seat has no sensor as its fair to assume in a crash that seat was occupied +++

Where is it?

The occupancy sensor is actually nothing more than a thin, flimsy pressure pad that is bonded on the seat foam underneath the seat covering. It is not a mecanical switch or push to make type contact, instead it is a pressure pad which gives a variable resistance depending on the load on the seat.

Does it affect non sports seats?

No, it doesnt seem to. The sports seats are unique in that the presssure pad is in two parts - It has one part on the main seat base, and another much smaller one on the pullout knee squab. It is this latter one that always seems to fail.

Can I just bypass it ?

Yes, See below

How can I fix it Properly?

The pressure pad is bonded onto the heated foam of the sports seat, and thus the whole item needs replacing. The part is approximately £200 from Audi. Expect to pay 2 / 3 hours labour to have it fitted.

Depending on how long you intend to keep the car, it may be cheaper and more cost effective to consider just buying a 2nd hand passenger seat without the fault. Check with us to see what stock we have.

Surely someone can up with a bypass schematic that can be built and plug in under the seat. I see folk have done this for BMWs. I imagine that when the seat is being sat upon it creates a steady resistance or a resistance above a certain point that can be permanently replicated.

Can you advise which plug it us under the seat?

Interestingly enough.. Mine used to do this all the time '98 with Sports seats.

Had the ECUs disconnected when trying to get to the source of a leak, and also tightened the connections on the airbag ECU behind the centre console and it hasn't came back again.

Maybe just got lucky, but might be worth looking at the ECU connections to make sure they haven't worked loose. Are there extra wires to the sensor in the sports seats?

My 97 A8 has this snag. I've thought of taking the bulb out, but that's not really my style!

I'll have a shot at the connectors, Psyche, but surely all that's needed is to convince the car that the passenger seat is permanantly occupied?

Surely this could be done by shorting a couple of wires with a jumper?

Failing that, how to reset the system?

Ta

You can reset the airbag light using VAG COM, have done this. But it just comes back on :(

Its not a simple make or break connection, its a resistance value that determines whethe its occupied or not....

I do agree that it should be possible generate a fake resistance equal to the airbag sensor mat -

There is a unit available for BMW's that does just that, and I bought one last week so when I get chance to test it, i'll let you know.

The correct values for the pressure mat are well documented, so if someone is really handy with their electronics and can knock up the right resistance we may be on a winner... I'm ok with electronics, but draw the line at trying to make any sort of circuit that looks professional +++

Ian, I'm the same, but find that if I immerse enought of it in hot glue and some kind of impenetrable case then no one will ever see my terrible craftsmanship.

I reckon we must be able to crack this. I've noticed the BMW one too selling on eBay for £35 or something. Someone's doing well out of that--the components must cost next to nothing.

So, I got the 'BMW' airbag pressure mat bypass unit through the post last week and thought I'd investigate a little....

Here it is, not particularly pretty but functional. Comes with a 2 pole electrical conector at one end for connecting to your car harness...

http://forum.a8parts.co.uk/images/uploads/abbp1.jpg


Oops, now I've invalidated my warranty.... :D


http://forum.a8parts.co.uk/images/uploads/abbp2.jpg


Might as well have a peek inside the magic box of tricks now....

http://forum.a8parts.co.uk/images/uploads/abbp3.jpg

Oh, so its just like a huge blob of glue..... :Confused:

Before we risk breaking it, check what the actual resisitance is:

http://forum.a8parts.co.uk/images/uploads/abbp4.jpg


...and now, de-glue it to see what we have....


http://forum.a8parts.co.uk/images/uploads/abbp5.jpg

Er, 2 resisitors, one diode and a very minimilistic bit of board.... !!

But, would it work on an A8 ???

This is the view looking from the passenger footwell at the underneath of the seat. We're interested in pins 2 and 3 of the big brown connector, which are the blue and white wires. These are the occupancy sensor connection that goes straight back to the airbag controller.....

http://forum.a8parts.co.uk/images/uploads/abbp6.jpg


Its far easier to work on if you remove all the connectors, then the loom will pull out to the front of the seat to work with. Here, I have located and snipped the blue and white wire, and removed the insulation to make the high quality connection with the electric block connector... :rolleyes:

http://forum.a8parts.co.uk/images/uploads/abbp7.jpg

And voila, box connected. The colour of wires on the bypass box (aka 59p's worth of parts from Maplins) bare no resemblence to anything, its just a trial and error to which way round works...

http://forum.a8parts.co.uk/images/uploads/abbp8.jpg

So, does it fool the airbag controller?

Yup, it sure does.... Once you've reset the warning with Vagcom or similar, its happy that the occupancy sensor mat is working with range.

However, if you read the measuring blocks in Vagcom, it clearly thinks that the passenger seat is unoccupied, which has the knock on implication that in a collision, the passenger airbags wouldn't deploy.

I wouldnt be fitting this particular box as a 'solution' to the problem as I generally value the lives of folks I call passengers, but as soon as I get chance to measure the correct resistance across an 'occupied' sensor mat then all we have to do is create that resistance instead, and probably make a far neater job, and make a far better connection to the car +++

P.S. If anyone has absolutely no mates at all, and wants a slightly out-of-warranty bypass box cheap PM me :ROFL:

Just get little dave to sit in it and measure the resistance?

Its a matter of being able to access the wires - You cant really get to the contacts on the seat without removing the seat, which wasnt possible today - Everything I did today was with seat in situ... +++