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... +++

Despite the mass of hot glue, I assure you that I am not the maker of this device. Does this mean the values are different on a BMW or are they selling this as a device to turn of the front airbag for child seats?

Ian, higher up the page you wrote "the correct values for the pressure mat are well documented". Doesn't this mean you can get those values without having to measure the resistance at the contacts?

I dont know... They are marketing it as a solution to help you fix the airbag light, there is no mention of safety or whether the airbags will still function.... :-(

Yup, and thats come back to haunt me - I know I've read the values somewhere because we used them to manually check a load of seats once that were already out of cars, but can I find that information now... :Confused:

I might have to downgrade my statement to 'documented', not 'well documented' +++

I discovered the cheapest solution to my airbag warning light problem was to buy a whole new interior and fit it. Problem solved. Also gone form alcantara to full leather for less than the price of getting the job done at Audi.

I wonder if you could use a variable resistor in place of a fixed resistor? That way you can use vagcom to test the dummy pad, turning up the resistance until it reads "occupied" - then measure the resistance across the resistor to give you your fixed resistor value or thereabouts?

Or is that more long winded than the aforementioned... :Confused:

:D

Not a bad idea at all that, using that I could find the upper and lower maximum values, so that we could pitch it right in the middle...

Otherwise, sods law would prevail and we'd 'test' and occupancy sensor for resistance that was on its way out, and end up on the fine line between working and failing..

hmm. +++

Would it be feasible to trace the wires to an accessible point in the car, then solder some insulated wires in serial andmake them long enough to do the measurements?
Ian, you could be on to a sleeker Audi variant on Ebay for the bargain price of 30 quid a pop!:rolleyes:

Sorry to be so dopey - if the airbag sensor light is on will all the bags deploy or just the drivers? I also care about my passenger. I can turn it off with the gadget that came with the car, but it always returns!

lads this has been done, I used a 1Kohm pot and connected to vagcom, I can measure the resistance that shows seat occupied , if anyone needs to know, let me know.
Regards Ali

Just send you a pm

here is a new recaro heated seat element with airbag sensor mat and foam padding assembly,it also contains the heating thermostat for all the 5 heating elements in the seat, normally £300 only £100. I know because my heated drivers seat went cold this winter. disassembly is not easy. from germany ebay
http://cgi.ebay.de/Audi-A8-4D-S8-Rec...item58885ec361

Thanks John,
I also saw that one on ebay. but its that the one with the broken sensor or is it this on http://cgi.ebay.de/ws/eBayISAPI.dll?...=STRK:MEWAX:IT ?

Ali measured the resistance for me and it shoud be 470 ohms with the seat occupied so now i can also put a resistor between it.

I've just found this thread having decided to do something about my airbag light ( been ignoring it for a while as the wife doesn't drive :rolleyes:).
Just to confirm this 470 ohm resistance will tell the car the seat is permanently occupied and hence mean the airbag will be fully functional ??
After my alarm horn pcb shenanigans this should be a walk in the park......
Also my passenger seat heating isn't working either so is that likely to be a similar problem of a blown component or dry joint etc I've had a vague wiggle of all the connectors with no joy so far.....:Confused:

I have replaced the 1Kohm pot that I had in my car for a 470ohm resistor,fitted it a couple of months ago, The airbag light has not returned.Make sure you connect it up to Vagcom and check it reads "Passenger seat occupied".This can be done without removing the seat from the car, if you adjust the seat up as high as it goes plenty of room to fit the resistor.Sorry cant help with the bum warmer.

Thanks to this post I have inserted the 470 ohm resistor and cleared the airbag light . :)

Once it cleared I had the bottle to tell the wife her airbag was now working lol

and ditto- thank to all of you- I am now a Maplins fan ( the 24p purchase sent their shares soaring....not!:ROFL:) and the 470 ohm resistor has done the business- so I am fully airbagged up.

Airbag light on again
 

I fitted my resistor over a year ago, its been great, but the airbag light has returned.After further investigation it appears if the passenger heated seat is on, on "6" after a long period the airbag light came on again, ( remember I fitted the resistor to my car its been through 2 MOT s ) if the seat heater is turned down to "5" the airbag light goes out again, and vagcom shows the seat as being occupied, so all is good, no need to connect to vagcom to reset the light. Just thought I would make you all aware of my experience.

When the sensor has failed is the default position for the ecu to assume occupancy and discharge the airbags if required? I thought it was but best to check eh? Wifey is only insured for 220k after all :). As it is, 470ohm resistor is being spliced in next week. Not sports seats but I assume that brown connector and blue white wires are still the right one? I have a 3, 7 and 10w 470ohm resistor. Any of them useable?

Bill

I used 1/3Watt 470ohm resistor, I dont think the wattage is important here.

Hello, I would like to know if a member of this forum had a picture of the resistance connected to the detection sensor ?

thank !

As I now have this problem on Sports seats I've started to have a look underneath. :(

If it is indeed the front (moveable) section that has its own small pad and that usually is the one that fails I thought I'd start there as it's more accessible.

Looking under the seat there is a thin cable sleeve connecting the front section to the rear, held on by a cable tie.
I can't think it's there for anything other than the detector; I'm sure there's no heater in that section.

As it's much more accessible than the main plug and wiring I'm going to start there, cutting the sleeve open first to see what's inside.

If there is a main and secondary pad I can only assume they're wired in series. Can anyone confirm that?
If that is the case it might even be possible to bypass the front pad with an intermediate resistance value so that the main pad operates the system normally.

Edit: Just opened the sleeve - one blue wire, one white wire.
I'll be cutting through that and measuring the resistance of the disconnected front pad shortly...... +++
Might it even be worth shorting them together, simplest solution of all? :rolleyes:

OK, this is the EVEN CHEAPER FIX, cost effectively nil if it is just the thigh pad sensor at fault! +++

Move the seat back and tilt the front up to gain access. Move the thigh support forwards as far as it will go.
Look underneath and you'll see a thin black cable sleeve that runs from the thigh support back under the seat, remove the cable tie to ease access.

Carefully cut open the sleeve open to confirm it contains the blue and white pair of wires for the occupancy sensor; trim back the sleeve to bare a good length of wire and cut through it nearer where it enters the pad so there's a decent length to work on emerging from under the rear of the seat.

Tape up or cable tie the spare wires going into the thigh pad, this part of the sensor is now inactive. Move the pad back now to make access easier.
Join the blue and white wires together at the end of the cable coming from under the seat.
I actually used a "choc-block" connector and small loop of wire, but that was for convenience of testing resistances. You could just strip off some insulation and crimp them together.
(I'll probably do that some time if the fix continues to work OK)
Tape or cable-tie out of the way and return seat to normal position.

Finally plug in your laptop running VCDS (or equivalent) and clear the codes from the Airbag Module.

All seems to be working normally so far...... :D

I did do some vague resistance testing, as follows:
The thigh pad was open circuit when unloaded, pressed as hard as I could manage (in some places) I could get it down to maybe 20K ohms.
The main seat pad was in the "several K ohms" range when unloaded and dropped to maybe 200 ohms (IIRC, not sure exactly what range my meter was on) with my big fat backside on it. :o

I'll report back if there are any untoward developments.....:o

And after several trips with and without front seat passengers large and small it still is. +++
No seatbelt or airbag warnings of any sort.

Before I go and remove my "choc-bloc" temporary lash-up and properly crimp the short-circuit into place has anyone with more knowledge any thoughts?

No comments on the airbag warning, but you have mentioned seatbelt warning a couple of times now... Never seen a seatlbelt fault, not even sure they were wired up on Preface ? On Facelift there are ignitors on the seatbelts to pre tension, but no sensors that I'm aware of.... Later Audis do, and by default do that annoying binging noise when you drive with no belt on...

Only in response to others comments, I don't think there are any either.

My Ford Focus of very similar vintage definitely has pretensioners though.

Hi, my airbag light has just come on. When I moved the passenger seat I think.

I looked under the passenger seat and there is a variable resistor fitted, presumably by the previous owner who experienced the same fault.

I need help! How can I get the light to go off? Do you need VAG COM to reset it.

I have the sport seats (D2 S8 PF).

See my thread elsewhere, I'll find it if I can.

If it's the front thigh pad (most likely) then you can literally just short it out as it's in series with the main one.
It will need resetting via VCDS though.

Here: http://forum.a8parts.co.uk/showthread.php?t=4873

And a few posts back here: http://forum.a8parts.co.uk/showthread.php?t=114&page=4

Once fixed you will need vag-com to clear the fault code before the light will go out.

Sometimes these things can be intermittent - mine was initially. I used VCDS (VAG-COM) to clear it and see if it came back and when/how.
After a few goes at this at closer intervals I gave up and fixed the wiring as noted above.

As yours has been "fixed" once already you'll need to look carefully to see what has been done and what might have gone wrong (broken joint maybe?).
As noted elsewhere if it's just the thigh pad sensor at fault you can just short that out.

I took the car for the MOT and the light was on, this was recorded as an advisory so the car passed. (I asked them to clear the fault but it came back on).

I can short the wiring in my own time now!

I believe that under the "new" MOT rules that were supposed to be in force earlier this year that would be a definite fail.
However, my mate who is a qualified MOT tester did mention to me just last week that implementation and enforcement is in a rather "unclear state" at present, to put it mildly. :tuttut: