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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 06-14-2010, 02:17 PM   #1
Scott87
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Exclamation Airbag Occupancy Sensor Bypass Unit DIY (pics)

Well, every time I had a passenger in my car I got the really annoying airbag light on my dash. This is due to the sensors in the passenger seat failing or having failed. The seat has a mat inside it that can detect the presence of a person. Now this has broken the ECU detects that and puts the airbag light on. The bypass units can be bought on ebay very cheaply, mine was £5 delivered. They are basically just a small resistor to imitate the resistance created when a passenger sits in the seat. They are used to control the deployment of the passenger airbags. A crash with no passenger means no passenger airbags deployed saving on repair costs. Now this unit is fitted the airbags will always deploy as it always thinks there is a passenger (better than the other way round). The only downside is now you can't put a baby seat up front due to the airbag deployment. This, however, wasn't an issue for me and is way better than the £480 BMW wanted to fix it.

Any way, on with the DIY!

You will need:

Airbag occupancy sensor bypass unit
Maybe some tape/zip ties
Snips

Okay first up you need to roll the seat forward as far as possible then tilt it forward as much as you can. You don't need to go mad and push it forward there should be enough room if you lay down on the back seats. In this picture my fingers are on the connector unit that you will be working on:


If you look under the seat its right on the back side closest to you, ignore all the cables in the middle underneath. This is the unit you need to find:


At the top of the image above are a blue and white wire. These wires come out of the box (left side as you look at the image) and in to a connector block. The right side of the connector block needs to be unclipped and pull as much slack as you can out. This is the connector hanging down here, excuse the really bad photo:


Here is the connector cut off, leave yourself a couple of inch of wire on it:


Here is the bypass unit I will be connecting:


Mine came with some crimping blocks that needed to simply have each wire inserted and crimped shut. You may need to strip and twist them together then insulate with tape. Either way is fine and should look like this:


Now simply plug the connector back in to the unit and zip tie or tape the wires out of the way. Enjoy no more airbag lights!!

Last edited by Scott87; 06-14-2010 at 02:19 PM. Reason: Image Link
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Old 06-14-2010, 03:50 PM   #2
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Great DIY. I did this a couple of weeks ago and it works like a charm and its super easy.
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Old 06-15-2010, 12:28 PM   #3
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if somebody is little bit more skilled, he can do it himself and not buying things from eBay.

I did some research by measuring the ocupancy sensor which was okay, and figured out which components You need to bypass the sensor if it is broken.

So You need soldering iron, not shaking hands and one 3k3 to 30k resistor and diode 1N4007.

schematic diagram is in the picture



CAUTION:

this is modded bypass modul that was previously posted on some forums, but that "OLD" one caused the airbag light off but the seat was in control unit of airbags NOT OCCUPIED. therefere I did this mod which will make the seat "OCCUPIED" even if nobody is there!!!!!

Last edited by profor; 06-15-2010 at 12:36 PM.
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Old 06-15-2010, 12:35 PM   #4
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I could have done with knowing the resistors rating before I bought it off ebay... oh well lol. I measured it myself out of curiosity when i got it.
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Old 01-14-2012, 11:16 AM   #5
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Reverse Occupancy sensor

Quote:
Originally Posted by profor View Post
if somebody is little bit more skilled, he can do it himself and not buying things from eBay.

I did some research by measuring the ocupancy sensor which was okay, and figured out which components You need to bypass the sensor if it is broken.

So You need soldering iron, not shaking hands and one 3k3 to 30k resistor and diode 1N4007.

schematic diagram is in the picture



CAUTION:

this is modded bypass modul that was previously posted on some forums, but that "OLD" one caused the airbag light off but the seat was in control unit of airbags NOT OCCUPIED. therefere I did this mod which will make the seat "OCCUPIED" even if nobody is there!!!!!

Hey buddy,

I want to turn the airbags off as i want to put my child in the front (he is a nightmare to keep entertained) Im thinking, if we can trick the occupancy sensor to say the is always a passenger, we can also trick it to say there isnt one.

Does anybody know the resistance of the sensor if no one sits in the seat?

Then i make a resistor up for that and off we drive.

Kind regards

Matt
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Old 01-28-2012, 12:33 AM   #6
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the pad's resistance should be between 9- 10M Ohm with no occupant and when someone sits on it, it should drop to under 100 Ohm. That is the way the sensor can sense whether someone is actually sitting or not.

make the same thing to read between 9-10mega ohms
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Old 06-15-2010, 04:04 PM   #7
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Todays mod - used 4k7 resistor





Last edited by profor; 06-15-2010 at 04:13 PM.
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Old 06-15-2010, 05:23 PM   #8
Scott87
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Cool. So you took the unit out and mounted it internally. Saves having an extra wire kicking about under there. Looks tidy.
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Old 12-08-2010, 04:50 PM   #9
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Quote:
Originally Posted by profor View Post
Todays mod - used 4k7 resistor




Hi profor i tried this just now with a "n14002" diode and a 3k3 resistor and the light is still on? can you make any suggestions what i have done wrong?
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Old 12-08-2010, 05:55 PM   #10
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You still have to clear the code. Otherwise the light stays on.
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Old 01-28-2012, 03:43 AM   #11
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I need to check mine out tomarow as simply re-setting my system didn't work. I though it was a simple pressure switch... Not so simple.

Last edited by RKiepper; 01-28-2012 at 03:47 AM.
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Old 01-28-2012, 05:54 PM   #12
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what is the tolerance of the 4.7 k resistor hes using....it looks like its brown at 1% 5th color? i dont think it matters though....im guessing this to be a standard ¼ watt resistor

the other set up uses 2 100ohm resistors (un occupied passenger) bag wont go off

this one uses 3300 3k3 to 4700 4k7 ohms....and hes using a 47000ohm resistor which is 47k (bag will go off whether there is passenger or not) why the dramatic jump in resistance from 3k3 to 47k i dont know

for those of you that dont know the k in the middle represents a decimal place. they use it for schematics to not make a mistake as decimal symbols are small.

the stripes represent values (ex...{yellow is 4) {purple is 7} and orange represents three zeros behind it meaning 47000)and can be checked by using a resistor color coder easily found online the first 3 stripes are the most important. the 4th one represents a + or - or tolerance and the 5th is just a quality standard and is sometimes not there

i just got a message from andrews2nd3....

"Hey man - I actually used a 1/2 watt, 5% tolerance resistor - radio shack part # 271-1130" read his post below....with the resistor part # and the diode you have all the exact info you need to do this job. You will have to use an srs code reader to turn it off. peake research sells them. but the better diagnostic equipments does alsom...like autoenginuity

Last edited by glhx; 02-02-2012 at 04:48 AM.
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Old 01-31-2012, 02:23 PM   #13
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The method proposed by profor will put the seat into an ‘occupied’ status, so the airbag will go off in the event of a crash. The method with the 2 100ohm resistors puts it into an unoccupied status.

Yesterday I had my car at the indy for an unrelated issue and I asked him to check on the computer whether or not the seat was coming up as occupied. I had the 2 100 ohm resistor bypass in place and the seat came up as ‘unoccupied’. I changed it last night to the profor's version (although I used a 47k resistor and a 1n4001 diode), and it was showing as ‘occupied’.

So, if you want your airbags to go off, you should use the 47k resistor and diode combo that profor explained.

Disclaimer: I’m not liable for anything you do or any damages you incur on yourself, your car, or other people….
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Old 03-20-2012, 01:36 AM   #14
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Quote:
Originally Posted by profor View Post
Todays mod - used 4k7 resistor




Thanks profor! To confirm, the above works perfectly using 1N4007 + 4k7 resistor as described.

And I recommend doing it this way as it's very easy to do, much cleaner and you definitely know the airbags will go off. Although being an engineer helps my motivation to mess around with electrics and why I love these cars.

I've added a few more pics if you guys want to see how I pulled it off, and opened it up. You can't really break anything (famous last words), but really, it does come off quite easily like most things BMW


When you've unhooked the cylindrical connector which is connected to the seat pad (top right of pic), you can twist the enclosure and it will pop off from under the seat. Then you can unhook the white connector. I used a small flat head to help nudge the locking off. Once this is off, you can use the same small flathead and just slot it in the end where the arrows pointing and draw it around and the enclosure will pop open and you can slide it out.



I cut the wires off, then just de-soldered the header (where the connector inserts into the board) off. Then just follow what profor has done - solder in the diode and resistor (check polarity of the diode is correct). Only thing I added was just a small precaution and that was to insulate this with a bit of tape or heat-shrink before closing back up.

And now I don't have any more issues with intermittent airbag lights when people sit in the passenger seat, nor worried it won't go off should it have too.
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Old 09-14-2012, 10:45 AM   #15
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Originally Posted by profor View Post
Todays mod - used 4k7 resistor




i did this mod and cost me about $3.
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Old 06-16-2010, 06:41 AM   #16
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Thank You,

yes is tiny, original blue/white cable is wound up inside, so it looks like nothing happened to it, also the sensor is connected.

Not occupied seat

Not okay - happens if the sensor is broken

Occupied - this state will have CU after installing the diode and resistor as i posted here

Last edited by profor; 06-16-2010 at 06:46 AM.
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Old 12-08-2010, 05:54 PM   #17
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Thank You,

yes is tiny, original blue/white cable is wound up inside, so it looks like nothing happened to it, also the sensor is connected.

Not occupied seat

Not okay - happens if the sensor is broken

Occupied - this state will have CU after installing the diode and resistor as i posted here
ehmm what is that program? how does it work?
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Old 07-10-2010, 09:49 PM   #18
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profor, you do some kick ass work man, are you a electrician ?
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Old 07-10-2010, 10:40 PM   #19
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subscribe. i will do this next week.
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Old 07-11-2010, 04:56 AM   #20
profor
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Yes, i am but not in car service, i am just interested in such stuff and i am not afraid to do these things on cars

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profor, you do some kick ass work man, are you a electrician ?
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