SRS Light after seat swap; Seat Occupancy Sensor override

[shu328]

Hey folks,

Swapping seats from my old '91 into the '95 caused an SRS light which cannot be reset -- codes point to the "supplemental restraint system control module" // seat occupancy sensor.

This is obviously because the '95 SRS control module is looking for an input from the occupancy sensor in the seat (which didn't exist in the '91 seats).

I wouldn't be worried about the light, and initially I assumed both airbags would fire anyway as a safeguard, but after some research my understanding is now that the airbags will NOT fire at all if this light is on. Can anyone verify?


Assuming that is true...

In doing some research in the ETM for my '95, I pulled together this info and have narrowed it down to what wires are responsible for the occupancy sensor. However I'm not quite certain the best/easiest/safest way to override or jump it out...





It appears the seat occupancy detector is a 3-wire transistor type of switch.

Violet/White +12 INPUT from
White/Red OUTPUT to SRS module
Brown Ground





And as you can see, the SEAT connector has nothing where the VT/WT and WT/RD pins are on the car side.

I would think I simply need to install a jumper on the SEAT connector (currently empty) which connects VT/WT to WT/RD to send +DC to the SRS module constantly. However, is that what the SRS module needs? Is that the same as if the seat were occupied constantly?

Also, is it wise to install a fuse in that jumper just so I don't short/overload the SRS module?

Electrical experts, please chime in!

Best,
Matt

[0reo535i]

i have the same issue after swapping what i guess are later sport seats into my 91? The light flashes for a while then stays on. Ill be watching this thread for sure

[Grush]

I believe you`ll have to emulate the resistence that the weight sensor (assuming it is one in the E34, as nowadays) have when there is a load on it.
You can try to measure it by seating on your old seat with the multimeter connected to the pins and checking for the Ω value, and solder a equivalent resistor as a jumper and done. or you can try to find it with a device like a "dimmer", and lock it in place after you found the sweet spot.

[shu328]

I believe you`ll have to emulate the resistence that the weight sensor (assuming it is one in the E34, as nowadays) have when there is a load on it.
You can try to measure it by seating on your old seat with the multimeter connected to the pins and checking for the Ω value, and solder a equivalent resistor as a jumper and done. or you can try to find it with a device like a "dimmer", and lock it in place after you found the sweet spot.

I like the idea except I don't have the old seats ... I did the swap literally over a year ago.

The question really seems to be what signal is the seat occupancy sensor sending to the SRS module? Is it a variable (resistance/voltage/mA) signal or simply a discrete on/off?

If the occupancy sensor has a switch-point or threshold value (i.e. weight) it needs to see before giving the SRS module a "go" value, then it's likely just a simple 12V signal. But if the SRS module does some sort of math to determine (a) is the sensor functioning, and (b) is there a passenger, then I need to come up with a much more "fancy" solution using a resistor etc.

[Layne]

Any idea where the detector is located or what it looks like? I dismantled a E32 seat and may still have it laying around. I was thinking the buckle had some wires coming out of it...?

In lieu of examining an actual sensor, I would try passing the sensor wire to ground before applying voltage to it. The transistor could work either way, it's hard to guess which, but going to ground is unlikely to damage something if it needs a voltage, but the opposite is not true.

- - - Updated - - -

P.S. There must be more to it than just switching on and off. Otherwise your SRS module would just believe the seat is unoccupied rather than triggering a fault. There must be some way to confirm the function of the switch.

[Grush]

Actualy, rechecking the component in the electric diagram you provided, it seems to be a Control Electronics part, not a variable resistence sensor or likewise. So now I believe the component should send a signal to the SRS module. Could be digital, like 0 and 1 or on and off (where jumping it directly might solve), and in this case the calculation is made inside the component, or a specific voltage value (where jumping with the specific resistor might solve), in this case the calculation is made by the SRS module. lol, sorry, but I think your back to ground zero now.

[shu328]

P.S. There must be more to it than just switching on and off. Otherwise your SRS module would just believe the seat is unoccupied rather than triggering a fault. There must be some way to confirm the function of the switch.

I was thinking the same thing ... the SRS module must need something back (on the WT/RD wire) at all times. If not, you'll get the fault which will not reset.

My guess:
Signal below X = SRS Fault
Signal above Y = seat occupied

Does that make sense?

However, I'm not familiar with what type of variable signal it would most likely be sending. Is it V or mA?

Also not entirely sure where the occupancy sensor is located either. Any way you can trace the wires back from the connector??

- - - Updated - - -

Thinking about just buying one of these to see if I can figure out how to integrate it... Even though my seats don't actually have the sensor, perhaps it would still connect into the car side connector somehow?

http://airbaglightoff.eu/bmw-5-series-e34-passenger-occupancy-seat-sensor-emulator-bypass-unit-67-p.asp

http://www.ebay.com/itm/BMW-E31-E34-E36-E38-E39-E46-E53-325i-535i-740i-850i-X5-Z3-Seat-Occupancy-Sensor-/251465158656

[Layne]

Weird, it seems that part fits everything but an E32. I don't remember seeing one either. I thought all the dual airbag systems were the same.

[demetk]

The seat occupancy detector is on the passenger seat?

[shu328]

http://www.realoem.com/bmw/partxref.do?part=65778367242

I'm guessing this is what you just looked at, but it tells me several things:

1. The seat occupancy "sensor" is actually a 2-wire switch -- it's a mesh/mat which closes a set of contacts when it deflects enough (due to weight)

#17 in the diagram:
http://www.realoem.com/bmw/showparts...5&fg=65&hl=146

2. The "mat" connects to this "control electronics -- seat occupancy" module (#18) via the small round plug labeled 349, shown here:
http://www.ebay.com/itm/BMW-E31-E34-...-/251465158656

3. The other end of the control electronics module must connect to the 12-pin connector and give some sort of proportional or variable signal to the SRS module.



That being said, I wonder if I purchased one of these control electronics modules (new or used) and jumped the wires going to the "sensor mat", to read what kind of signal the SRS module receives on the WT/RD wire?

The only thing I don't have right now is a 12V power supply to test this out accurately...?


EDIT:

It appears these eBay sellers are simply doing what I just said ... jumping the two wires together which normally go to the "sensor mat" to simulate a passenger. The top connection shown in the picture is what you pull out, and insert the bottom "simulator". I'm going to buy just a used control electronics module and do some of my own hacking. If that doesn't work, I can always buy one of these "simulator plug" setups to get rid of the issue once and for all.

http://airbaglightoff.eu/bmw-5-serie...-unit-67-p.asp

[cddallara]

I got the sensor override thing off eBay and have yet to install it.

I just live with the srs light.

Actuslly, I don't, because I haven't driven the thing in months...

[shu328]

I just live with the srs light.

I wouldn't have a problem doing that... But does the SRS light mean your airbags are disabled? Even though I rarely drive the car also, I'm not entirely comfortable with that lol

[Binjammin]

Guys, this shouldnt be too tough to figure out. Find someone with a working seat and srs system and put a meter on the connection (with it plugged in, of course) and see what changes you get.