Airbag warning light - Passenger seat

[David's8]

Originally Posted by IT

From memory, there are two occupancy pads. One on the knee bolster, and one on the main seat squab.

Iirc they are wired in parallel, so the resistance of each will affect the net resistance measured by the airbag module.

It looks like even if you short out the smaller knee bolster, the resultant resistance across the main seat is sufficient to bring the overall resistance down within tolerance.

I guess its possible if you then have a passenger that covers more seat than normal you could push it outside tolerance again.

This is where the 'fix' of hardwiring a plausible resistance across the main seat connector, bypassing both sensors might prove more reliable overall.

I've never understood what the correct resistance would be though to show the seat as occupied, as permanently wiring it as unoccupied would result in no passenger airbag deployed in a crash, which is not a position I'd want to be in.

I guess there will be a measuring block for occupancy in the airbag module, it should just be a case of turning up the resistance until it shows occupied, or measuring the valid resistance off a perfectly working D2 and emulating.

So, no hippo sized passengers allowed ......SWMBO is ok then
I see what you are saying Ian and I think a bit of VCDS work and a trip to Maplins is coming on.

[HPsauce]

Originally Posted by IT

From memory, there are two occupancy pads. One on the knee bolster, and one on the main seat squab.

Iirc they are wired in parallel, so the resistance of each will affect the net resistance measured by the airbag module.

It looks like even if you short out the smaller knee bolster, the resultant resistance across the main seat is sufficient to bring the overall resistance down within tolerance.

Ian I just went back and reviewed what I posted here several years ago and what I remember (not a lot!).
Parallel is not logical (given how resistors in parallel work) and would not produce the results I measured and solution I used that was totally effective.
The main squab and thigh pad are almost certainly in series and resistance reduces when the seat is occupied.
My solution was to short the wires together for the thigh pad, reducing that measure to zero. The main pad, which seems more reliable, will (OK, should) still show the seat as occupied.

If the thigh pad was faulty, giving a permanently high resistance (as mine was) then just sitting on the main pad would show the seat as occupied.
That's not what was happening. And the VERY SIMPLE wiring suggests otherwise.
Measuring both separately would need a different electronics module for that seat. Maybe it exists, maybe not....

[David's8]

There are, from observation and as described discussed here earlier, two separate pads but the wiring schematic, (probably for simplicity?), just shows a single sensor with 1 resistor and 1 diode. There are unfortunately no resistance values shown.
Can I assume that by shorting out the thigh pad, the seat occupied signal will work? I will try and check this today using VCDS - i might even ask The Laird to do a stint as a crash test dummy.

[David's8]

So, having put VCDS on it, the measuring block show "Occupied" even when empty. (It showed sensor fault before). With The Laird gracing the seat it shows - as you might expect - that its occupied too. What does this show (if anything?) about the resistance?

[moltuae]

Originally Posted by David's8

I have looked at the circuit diagram and it shows (I cant copy the diagram as its a secured pdf)

I can probably de-secure that pdf for you if you'd like to send it to me

[David's8]

Cheers Mark, attached.

[moltuae]

Done

Wiring Diagram 2001-audi-s8-wd-unprotected.pdf

[HPsauce]

And this is the sum total of what it says (Diode parallel to resistor). Note blue and white cables as expected:

[oldnick]

Originally Posted by oldnick

anybody tried MJ's theory that the thigh pad is on a separate circuit to the main seat ?

for a while have had a very slow response to the light going out , but now is staying on , so as this is a fail in the MOT [ since 2013 ] have got to do something !

now andrew's idea is that the two pads are in series ; so I started up the engine,light stayed on , put all my weight on the thigh pad , light stayed on;
now transferred all my weight to the main squab , light went off ! weight off ......on again

so what does this mean? what actually IS the circuitry ? maybe MJ is correct , maybe there is a minimum resistance on each circuit [ hence the 470ohm referred to ? ] ; suffice it to say that I would welcome any thoughts before I start cutting wires ! or not !

http://forum.a8parts.co.uk/editpost....tpost&p=119236
just had a thought ; as everyone will have noticed it is possible to deactivate this airbag with a key ; so I wondered what happens if I do this ;
so I deactivate and carried out the same test

result ...exactly the same as before ...so what does this tell us ?

just to be clear ......unmodified wiring ..... airbag activated code 656
airbag deactivated code 656

and to be extra clear , ~I just want the airbag working and the light off ...if the airbag goes off with no passenger , hard luck ...the insurance company is going to write off the car come what may

[moltuae]

Originally Posted by HPsauce

And this is the sum total of what it says (Diode parallel to resistor). Note blue and white cables as expected:

That looks an EOL (End Of Line) termination, commonly used in things like fire alarms and intruder alarms.

From an electronics point of view, the purpose is usually to provide a very simple means of detecting different fault states. The resistor allows for differentiation between open/short circuit (if a simple switch were used, it would be impossible to tell the difference between a correctly functioning switch and broken or short-circuited wiring). The diode is probably serving as wiring polarity detection and further confirmation of a correctly functioning circuit.