Airbag warning light - Passenger seat

[oldnick]

just run in to this problem ! warning light didn't go out [ had recently been sitting in front passenger seat testing battery voltage ] so went for a short drive and it continued to stay on ; shortly after restarted engine and it went out , albeit a little slowly

thanks to andrew, it looks like there is an easy fix

but what does the light being on actually say....if I understand correctly it is actuated by the pressure pad saying there is a passenger to protect in the seat so why is that an MOT fail ? or is the light saying something else as well ?

[David's8]

I seem to have been blessed with this fault I have a couple of questions with which I would appreciate some help.

My scan indicates:
00656 - Seat Occupied Sensor; passenger side (G128)
36-10 open circuit - intermittent

1. How do I find out whether its just the thigh pad sensor which has gone?
2. As a non electrician why does ITs solution of fitting a 470ohm resister work as well as HPs fix of shorting it out?
Any help gratefully received

[MikkiJayne]

On a more modern car the seat sensors weigh the occupants to decide how hard to fire the airbags. I very much doubt the D2 was that sophisticated though, in which case the sensors probably just have a resistance range to account for wear, of which 500Ω is the upper limit of 'occupant detected'.

Assuming the thigh pad sensor is on a separate set of wires to the main seat base sensor then you can just short them out with a paperclip at the brown plug.

[David's8]

I think I now understand I'll try shorting out the thigh pad sensor first as that seems fairly straightforward and then, if it doesnt work, i'll go for the resistor option which i now see is the cure if the main pad doesnt work.

[oldnick]

Originally Posted by MikkiJayne

On a more modern car the seat sensors weigh the occupants to decide how hard to fire the airbags. I very much doubt the D2 was that sophisticated though, in which case the sensors probably just have a resistance range to account for wear, of which 500Ω is the upper limit of 'occupant detected'.

Assuming the thigh pad sensor is on a separate set of wires to the main seat base sensor then you can just short them out with a paperclip at the brown plug.

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 !

[David's8]

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 !

I tried HP's solution which is to short out the thigh pad which is fed by a blue and a white wire from the area of the main seat pad. The airbag warning light has gone out and, since I cleared the DTC for the Airbags control module, has remained out. Result! So in my case it WAS the thigh pad tho' how it is connected i know not. I will look on the circuit diagram later tonight. Its now beer o'clock.

[oldnick]

when you say you shorted out the thigh pad david do you mean you cut the wires to it then sealed it off ...or just tucked it up ..then joined the ends of the feed cable together to close the circuit ?

[David's8]

Thats about it. Joined the feed blue and white wires and tucked the remaining wires to the pad out the way.

[David's8]

I have looked at the circuit diagram and it shows (I cant copy the diagram as its a secured pdf) and it shows the blue /white pair connected to the "Passenger's Side Seat Occupied Sensor" (note singular sensor). The sensor is shown as a box with a diode in parallel with a resistor.

[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.