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      03-08-2011, 08:17 PM   #1
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Possibly Solved! Airbag Light w/ Aftermarket Seats

So we all know the situation, right? You install some really awesome new bucket seats knowing fully that the pesky airbag light will be staring at you when you finally take your seat.

Because you cannot buy the sensor mat from a Z4 -- there is simply no part number for it -- a while back I bought a sensor mat from an M3 hoping it'd connect. No such luck as the connector on the mat was roundish and didn't line up with the squarish harness in the Z4.

Fast forward to a few moments ago after removing the sensor mat from a Z4 donor seat. The plug coming from the Z4 sensor mat is the same as the one found on the M3! But that sensor mat first connects into a small black box mounted to the underside of the seat. That small black box then outputs to the squarish harness found on the Z4.

So, buy the M3 occupancy mat, buy p/n 65778367242 (the small black box), connect it all up and you should be golden! (After a code/reset, of course.)

I'm going to undertake this project this weekend so stay tuned!
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      03-08-2011, 09:00 PM   #2
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      03-08-2011, 09:10 PM   #3
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      03-08-2011, 10:33 PM   #4
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      03-09-2011, 10:38 PM   #5
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The occupancy sensor is available for Z4's, it is part number 52107244135 for Z4 sport seat. The OC3 comes with the replacement foam pad.

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      03-10-2011, 02:22 AM   #6
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Quote:
Originally Posted by dyerkes View Post
The occupancy sensor is available for Z4's, it is part number 52107244135 for Z4 sport seat. The OC3 comes with the replacement foam pad.

Regards,
David
Right. The only problem is that it costs around $500!

Instead, you can get the E46 M3's sensor mat (and I'm sure others work, too, but I've only put my hands on the one of the E46 M3) for just over $100 and the control box unit for around $50. So $150 vs $500... clearly, the majority of that $500 is for the foam we don't need.

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      03-10-2011, 10:24 AM   #7
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How many wires is it ? I haven't looked at mine yet, however with most other cars I've had great luck just jumping the harness with the appropriate resistor after measuring the voltage change, not possible on the M ? (haven't installed aftermarket seats yet but will tackle it when I do soon).
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      03-10-2011, 10:53 AM   #8
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Quote:
Originally Posted by onelove View Post
How many wires is it ? I haven't looked at mine yet, however with most other cars I've had great luck just jumping the harness with the appropriate resistor after measuring the voltage change, not possible on the M ? (haven't installed aftermarket seats yet but will tackle it when I do soon).
I see three pins and I'm sure you could rig up an in-line resistor to get the desired effect but I prefer to stay away from DIYing safety-related items, airbags especially.
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      03-10-2011, 12:47 PM   #9
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      03-10-2011, 01:23 PM   #10
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Very interested. How does the weight sensor generally work? Does the resistance change when weight is added in certain areas? I guess I'm just hesitant to say that it will work exactly the same way as the Z4-specific occupancy sensor.

Let's say, for example, the M3 seat was wired in reverse of the Z4. From what I understand the airbags determine if they should deploy depending on the occupant's weight, and how that weight is positioned (correct me if I'm wrong). If the M3 was wired the reverse of the Z4, the computer that determines if the airbags should deploy will think the occupant is on the wrong side of the seat. The car won't throw an airbag light, but the system still isn't working properly.

If the M3's sensor works, it will be excellent for the Z4 community. I just think that in an area as critical as airbag deployment, you need to make sure everything works as it should.
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      03-10-2011, 06:21 PM   #11
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Quote:
Originally Posted by pokeybritches View Post
Very interested. How does the weight sensor generally work? Does the resistance change when weight is added in certain areas? I guess I'm just hesitant to say that it will work exactly the same way as the Z4-specific occupancy sensor.

Let's say, for example, the M3 seat was wired in reverse of the Z4. From what I understand the airbags determine if they should deploy depending on the occupant's weight, and how that weight is positioned (correct me if I'm wrong). If the M3 was wired the reverse of the Z4, the computer that determines if the airbags should deploy will think the occupant is on the wrong side of the seat. The car won't throw an airbag light, but the system still isn't working properly.

If the M3's sensor works, it will be excellent for the Z4 community. I just think that in an area as critical as airbag deployment, you need to make sure everything works as it should.
/start disclaimer

I'm not an expert nor are my comments in this thread anything more than an amateur's attempt to offer observations and opinions. Sorry, I'm sure you know this, but being in the legal field and this also being a discussion about a very integral safety system, I had to say it.

/end disclaimer

Very valid points.

So if by "reverse" you are referring to polarity, I suspect all of BMW's plugs are "keyed". BMW probably doesn't make female and male connector pairs that connect up fully that reverse things. Notice how every electrical plug in our car only fits one way and often locks? If by "reversed" you mean the physical route the sensor mat takes, you're right, it is possible but we can probably look at pictures of both and assume that if the route the flat wiring takes across the thin mat is generally the same then we're OK.

I'll snap a picture of the mat from the Z4 and will look for a picture of the M3 part.

Now, I do not *think* that the airbag system determines which airbags fire based on your location in the seat. I think it fires airbags based on presence which is a function of determining if the approximate weight of the passenger exceeds a certain amount. Perhaps by a change in the current? If you're over the weight, the yellow Passenger Airbag Off light is off. Now, the rate at which the airbags deploy is adaptive which I assume is, at least partly, based on the weight of the passenger. Not location of said weight in the seat because, though I'm no electrical engineer by any measure, I can't think of a way that you could gain that data from this mat.

I think we can also be comforted a bit by the fact that many occupancy sensor mats are used across multiple model lines. This tells me that the range of function is wide enough to work with different seat/cushion configurations which may impact the detection of weight detection and distribution.
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      03-10-2011, 07:00 PM   #12
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BMW is likely supplied the connectors by an outside source such as Tyco or one of the other electronics manufactures who assembled them, it may be possible that polarities are reversed from car to car depending on how the diagram, plug location, etc is designed for that specific vehicle.

Most manufactures in the past (up until a few years ago) simply use a voltage based sensor that tells a separate airbag computer "look there is someone sitting in the seat", which assuming the Z4's is similar will do the same. If it were reversed you'd likely know right away by a seat belt warning light when no one was sitting in the seat, well before the air bags ever got deployed.
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      03-11-2011, 02:04 AM   #13
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By "reversed" I meant the location of the sensors and not necessarily the polarity. I'm not a EE either, nor do I claim to be an expert. However, I do want to voice my concerns so some of the more experienced forum members can chime in and correct or agree with me.

I think a lot can be learned by the type of harness connected to the seat. If two or three pins are present, it's more likely that the seat senses an overall weight of the passenger and the computer DOESN'T determine the position of the occupant (analog system wired in series, resistance changes with weight, and no microcontroller sending digital signals via a single wire unless one is visible in the diagram). If many pins are present, the sensors are more than likely wired in parallel... which would require further investigation.

Thanks again for taking the time to do this. If it works out you will have saved me several hundred dollars. An SA told me the computer determines the occupant's seating position based on weight distribution, so take that for what it's worth. But, as ignorant as some SA's are with our cars, it's still a concern that should be looked into. I'd say press on without looking back if a potential safety issue wasn't involved.
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      03-11-2011, 10:09 AM   #14
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Quote:
Originally Posted by pokeybritches View Post
By "reversed" I meant the location of the sensors and not necessarily the polarity. I'm not a EE either, nor do I claim to be an expert. However, I do want to voice my concerns so some of the more experienced forum members can chime in and correct or agree with me.

I think a lot can be learned by the type of harness connected to the seat. If two or three pins are present, it's more likely that the seat senses an overall weight of the passenger and the computer DOESN'T determine the position of the occupant (analog system wired in series, resistance changes with weight, and no microcontroller sending digital signals via a single wire unless one is visible in the diagram). If many pins are present, the sensors are more than likely wired in parallel... which would require further investigation.

Thanks again for taking the time to do this. If it works out you will have saved me several hundred dollars. An SA told me the computer determines the occupant's seating position based on weight distribution, so take that for what it's worth. But, as ignorant as some SA's are with our cars, it's still a concern that should be looked into. I'd say press on without looking back if a potential safety issue wasn't involved.
Roger that and thanks for the information.

There are 3 wires leading from the mat to the control box and 3 wires leading from the control box to the under-the-seat harness. The control box, or whatever it is, is used in cars like the E34 5er and even the E81 8er which I doubt had airbags that adapted to a person's location in the seat. So by deductive logic, because the unit was used in cars dating back to the late 80s we can assume that the control box isn't involved in any sort of determination of where a passenger is located in the seat.
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      03-11-2011, 09:53 PM   #15
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Sounds like you're probably good to go then... especially if guys are rigging up a single inline resistor to simulate the occupancy sensor.
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      03-25-2011, 05:47 PM   #16
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So! I'm almost there.

It turns out that pre-2004 (I think) Z4's (of all types) used a sensor mat that is different from the one used in post-2004 Z4's. The pre-2004 mat uses an external control box of sorts with p/n 65778367242. The post-2004 Z4's mat has that control box integrated into the mat itself. However, the wire that ultimately connects to the yellow harness attached to the seatbelt buckle apparatus has the same pins in the same location with the same colored wires.

So, if you use a sensor mat from a pre-2004 Z4 you'll need to make sure that you also either have (or buy) a small wire with p/n 52108240500. The sticker on the wire has the number "8240500" on it. This is the wire that connects from the small control box to the wiring harness attached the seatbelt buckle apparatus. I'm going to be connecting everything up this weekend and will do a reset and, hopefully, that pesky red airbag light will be gone for good! Stay tuned for an update at the end of the weekend!
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Last edited by JCz04Bimmer; 03-26-2011 at 03:17 AM..
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      03-25-2011, 07:04 PM   #17
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      03-25-2011, 07:18 PM   #18
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Great! Looking forward to it!
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      03-25-2011, 08:08 PM   #19
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Great! Looking forward to it!
Same here!
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      04-06-2011, 08:10 AM   #20
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I just got an idea for another (much cheaper) solution to this. I'm going to work it up in the next month or so. I'll post the results if it works.
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      04-06-2011, 08:15 AM   #21
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By the way, I'm still very interested in your results! I'm not trying to thread crap
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      04-06-2011, 12:45 PM   #22
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Quote:
Originally Posted by pokeybritches View Post
I just got an idea for another (much cheaper) solution to this. I'm going to work it up in the next month or so. I'll post the results if it works.
awesome!
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