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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 10-09-2008, 08:00 PM   #1
Vandad
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Thumbs Up How to diagnose the problem with your Airbag light

Hi, I have been suffering since 2006 when I bought my E46 318iSE as the airbag light was always ON and I could not work out how to fix this issue.
In 2007 I was told that the problem is with my seat occupancy sensor (using scanner tool) but I was also told that the problem could be the sensor itself or the pad in the passenger seat.
In the UK, BMW dealers charge 40pounds for the sensor and 75 pounds for the pad itself+ about 3 hours labour at 150 pounds per hour so I had to pay around 600 pounds ($1200) to fix this problem.

Now I want to show you how to find out the problem with the pad or sensor. you usually get only one error for the sensor and do not know whether the problem is with the pad or with the sensor.

Do this:
After scanning the ECU and getting passenger occupancy sensor error:
make a circuit like this, you will need:
2 resistors 100 ohm each (or 200 ohm) not less than 100 ohm.
1 diode (rectifier) 1N4001 or 1N4002 or 1N4003 or 1N4004 or 1N4005 or 1N4006 or 1N4007

connect this circuit to your sensor (disconnect the seat pad's cable and connect it to the sensor [the little black box]) and CLEAR the error with your scanner tool. if the airbag light comes back then the problem is with your sensor NOT the pad. If airbag light does not appear then you have to change the pad in the seat (what I had to do....) and it is a great deal of pain but it worths to see the car is fixed! I found this circuit from a website and as I am doing electrical engineering I could easily work out what it does!

WARNING: This should never make you tempted to connect this circuit to your sensor just to have your airbag light off. In this case your airbags MAY or MAY NOT operate, I am not sure to be honest, but I am pretty sure that the sensor is actually disabled as it can not measure the variable resistance. Just simply order a new pad and on a weekend remove the seat and fix it (it took about 2 hours for me). It feels very good when something is DONE and DUSTED!


Another way of checking the pad is to measure its resistance (you need to have an ohm-meter) :
A faulty pad has higher resistance (mine had 13Mega Ohm) but usually the pad's resistance should be between 9- 10M Ohm with no occupant and when someone seats on it, it should drop to under 100 Ohm. That is the way the sensor can sense whether someone is actually sitting or not.
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Last edited by Vandad; 10-09-2008 at 08:14 PM.
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Old 11-25-2008, 08:40 PM   #2
chrisca70
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Excellent informartion!!!

My mechanic just read the code for me today (for free!) and he said it was the occupancy sensor. I will follow your instructions to narrow down the problem.
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Old 11-26-2008, 01:09 PM   #3
ers51
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Quote:
Originally Posted by Vandad View Post
Hi, I have been suffering since 2006 when I bought my E46 318iSE as the airbag light was always ON and I could not work out how to fix this issue.
In 2007 I was told that the problem is with my seat occupancy sensor (using scanner tool) but I was also told that the problem could be the sensor itself or the pad in the passenger seat.
In the UK, BMW dealers charge 40pounds for the sensor and 75 pounds for the pad itself+ about 3 hours labour at 150 pounds per hour so I had to pay around 600 pounds ($1200) to fix this problem.

Now I want to show you how to find out the problem with the pad or sensor. you usually get only one error for the sensor and do not know whether the problem is with the pad or with the sensor.

Do this:
After scanning the ECU and getting passenger occupancy sensor error:
make a circuit like this, you will need:
2 resistors 100 ohm each (or 200 ohm) not less than 100 ohm.
1 diode (rectifier) 1N4001 or 1N4002 or 1N4003 or 1N4004 or 1N4005 or 1N4006 or 1N4007

connect this circuit to your sensor (disconnect the seat pad's cable and connect it to the sensor [the little black box]) and CLEAR the error with your scanner tool. if the airbag light comes back then the problem is with your sensor NOT the pad. If airbag light does not appear then you have to change the pad in the seat (what I had to do....) and it is a great deal of pain but it worths to see the car is fixed! I found this circuit from a website and as I am doing electrical engineering I could easily work out what it does!

WARNING: This should never make you tempted to connect this circuit to your sensor just to have your airbag light off. In this case your airbags MAY or MAY NOT operate, I am not sure to be honest, but I am pretty sure that the sensor is actually disabled as it can not measure the variable resistance. Just simply order a new pad and on a weekend remove the seat and fix it (it took about 2 hours for me). It feels very good when something is DONE and DUSTED!


Another way of checking the pad is to measure its resistance (you need to have an ohm-meter) :
A faulty pad has higher resistance (mine had 13Mega Ohm) but usually the pad's resistance should be between 9- 10M Ohm with no occupant and when someone seats on it, it should drop to under 100 Ohm. That is the way the sensor can sense whether someone is actually sitting or not.
can u come up to Glasgow and sort mine out. lol.. [ 4months now]

ers51
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Old 11-26-2008, 01:33 PM   #4
C-J
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I used indy, £30 to scan the fault and an extra £20 to cancel out the sensor. He said if he just clears the fault it may keep coming back, so instead he cancelled out occupancy sensor.....so assume airbag will go off if there is weight or no weight on the seat?!

Sold the said car now, left receipt in pack which states the above.
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Old 11-26-2008, 01:38 PM   #5
Ervin87
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I suspect this would work with other airbag sensors located in the car as well? Same method?
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Old 11-26-2008, 02:15 PM   #6
chrisca70
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After my mechanic cleared the code the light appeared again when I put a chick in the passengers seat. So the problem is still there. I will measure the resistance of the pad to narrow it down.
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Old 11-27-2008, 01:14 PM   #7
ers51
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Hi Vandad ,know very little about electrics but am keen to learn.

Questions..

Is the pic. on the left an actual make up of the right.

What is the black cable [from the car? ]

Do I have to get a scanner tool and if yes what and where

Where would i get resistors and diode


cheers ers51

is there no pm's on this forum?
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Old 12-31-2008, 08:00 PM   #8
Vandad
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HI, Yes, the picture is exactly what your circuit!

If you look behind your passenger seat, you will see a cable (two wire; one blue and one white) connects to the sensor. You must exactly connect the blue wire to the point shown in the picture, same for the white wire. As the diode basically stops the flow of current only in ONE direction.

Let me know how you get on.

Last edited by Vandad; 12-31-2008 at 08:05 PM.
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Old 01-10-2009, 04:56 PM   #9
chrisca70
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Hi there guys:

With the help of a mirror I managed to find the sensor just underneath the seat. It was attached to a plastic white holder/clip. I found the white and the blue cables, and I have my voltmeter ready. So here I am...Where exactly do I have to measure the resistance? Do I have to peel the white and blue cable to expose the wire and measure the resistance with everything connected? I attached some pics.

-Christian
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Old 01-10-2009, 05:17 PM   #10
mkodama
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I've had this info on my computer for a while. That circuit was originally intended to turn off the passenger side airbag in an E36 race car that had two racing seats and harnesses if I remember correctly.

Quote:
Originally Posted by Ervin87 View Post
I suspect this would work with other airbag sensors located in the car as well? Same method?
This doesn't work for airbags, only the occupancy sensor in the front passenger seat. The driver seat does not have one since there is always going to be a driver in the car so it is assumed, and no sensors for the rear seats.
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Old 01-11-2009, 11:44 AM   #11
Vandad
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Quote:
Originally Posted by chrisca70 View Post
Hi there guys:

With the help of a mirror I managed to find the sensor just underneath the seat. It was attached to a plastic white holder/clip. I found the white and the blue cables, and I have my voltmeter ready. So here I am...Where exactly do I have to measure the resistance? Do I have to peel the white and blue cable to expose the wire and measure the resistance with everything connected? I attached some pics.

-Christian
Hi Christian,
As you see that socket is connected to the sensor (black box). You need to measure the resistance of the wire connects to the sensor. It is a bit hard to put your ohm meter's probes inside that socket but that is exactly what you need to measure. I removed my seat and I could access that easily, but again if you go underneath the seat you can measure your resistance.

Remember what you need to measure is the pad's resistance, NOT the sensor's resistance. Pad is connected to the sensor via a cable (Black and White) and you need to measure the resistance of this.

FYI, the sensor connects to another connector through another cable which if I remember it is covered in a black plastic and has 4 wire.
Good luck!
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Old 01-11-2009, 11:46 AM   #12
S54DR
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it could be anything from a bad module or bad wiring..or it could have had the seat taken out without dis-connecting the battery that will throw a light
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Old 01-11-2009, 11:49 AM   #13
Vandad
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Quote:
Originally Posted by chrisca70 View Post
Hi there guys:

With the help of a mirror I managed to find the sensor just underneath the seat. It was attached to a plastic white holder/clip. I found the white and the blue cables, and I have my voltmeter ready. So here I am...Where exactly do I have to measure the resistance? Do I have to peel the white and blue cable to expose the wire and measure the resistance with everything connected? I attached some pics.

-Christian
This is the picture of the cable you need to disconnect first and measure its resistance.
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Old 01-27-2009, 09:14 AM   #14
chrisca70
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Update:

Peake code: Table 2 code 18 : "Seat occupancy detector, passenger"

Well I measured the resistance of the sensor mat and it did not give me high resistance values.
Empty seat: 75 Ohm
Occupied seat: 2.6 M Ohm.
I figured the change in resistance was good enough to conclude the sensor mat was fine so I ordered the sensor control unit (shown in the pictures above PN: 65776940191)

After replacing the control module I cleared the code and then I put someone on the seat with the car on and the airbag light came on again, DAMN!!!!!

My guess is that the faulty part is definitively the sensor mat, which is over $100, so I'll re-read the code today to confirm is still the same thing and order the sensor mat within a month or so. I think the part I just replaced is good, so I will sell here the control module that is brand new.
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Old 01-28-2009, 10:29 PM   #15
Jaxbimmer
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ANy progress

Did you replace the mat under the seat? Did it fix the problem? I live just north of you in Jacksonville (I used to live in G-ville which is where I bought my Beamer) and am having intermittent issue with my airbag light. I think it is the pad, but want to know more info. Does anyone have a DIY for this?

Thanks. -Tim
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Old 01-29-2009, 12:41 AM   #16
chrisca70
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Tim:

No, have not had time not even to order the mat. I have the peak airbag tool so if you need I can read the codes for you (for a six pack of course).

ttyl
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Old 02-14-2009, 07:50 PM   #17
chrisca70
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AIRBAG LIGHT GONE!!!!!

Finally ordered the occupancy sensor mat and decided to replace it today. Removing the seat was easy, and once the seat was removed I removed the gray covers that go on the sides (I have a sport seat which has all the adjustments electric, so it's a bit more tricky). Then all you have to do is unfasten the leather which is hooked to the seat metal frame by a continuous plastic part. Then it was the tricky part, opening a bunch of hog rings that attach the leather seams to the foam metal structure. Once you do this you will see the sensor, and just replace it with the new part. After this you can reuse the hog rings, I used a regular pair of pliers to place them back and close them, you have to be patient here. Then reinstall the plastic parts and reinstall the seat, plug everything back and clear the airbag codes. I turned on the car have a friend use the passenger seat and no airbag light!! voilŠ, re-read the airbag code and nada, so it seems that I didn't mess with anything else and the problem is solved!!

Will list the peak airbag tool on eBay tomorrow.
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Old 04-06-2009, 12:18 PM   #18
George300
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Hi chrisca70... I have the same (seat sensor) problem...do I need to remove the seat from the car to get this done?
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Old 12-20-2010, 08:22 AM   #19
floorjacker
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Quote:
Originally Posted by Vandad View Post
Hi, I have been suffering since 2006 when I bought my E46 318iSE as the airbag light was always ON and I could not work out how to fix this issue.
In 2007 I was told that the problem is with my seat occupancy sensor (using scanner tool) but I was also told that the problem could be the sensor itself or the pad in the passenger seat.
In the UK, BMW dealers charge 40pounds for the sensor and 75 pounds for the pad itself+ about 3 hours labour at 150 pounds per hour so I had to pay around 600 pounds ($1200) to fix this problem.

Now I want to show you how to find out the problem with the pad or sensor. you usually get only one error for the sensor and do not know whether the problem is with the pad or with the sensor.

Do this:
After scanning the ECU and getting passenger occupancy sensor error:
make a circuit like this, you will need:
2 resistors 100 ohm each (or 200 ohm) not less than 100 ohm.
1 diode (rectifier) 1N4001 or 1N4002 or 1N4003 or 1N4004 or 1N4005 or 1N4006 or 1N4007

connect this circuit to your sensor (disconnect the seat pad's cable and connect it to the sensor [the little black box]) and CLEAR the error with your scanner tool. if the airbag light comes back then the problem is with your sensor NOT the pad. If airbag light does not appear then you have to change the pad in the seat (what I had to do....) and it is a great deal of pain but it worths to see the car is fixed! I found this circuit from a website and as I am doing electrical engineering I could easily work out what it does!

WARNING: This should never make you tempted to connect this circuit to your sensor just to have your airbag light off. In this case your airbags MAY or MAY NOT operate, I am not sure to be honest, but I am pretty sure that the sensor is actually disabled as it can not measure the variable resistance. Just simply order a new pad and on a weekend remove the seat and fix it (it took about 2 hours for me). It feels very good when something is DONE and DUSTED!


Another way of checking the pad is to measure its resistance (you need to have an ohm-meter) :
A faulty pad has higher resistance (mine had 13Mega Ohm) but usually the pad's resistance should be between 9- 10M Ohm with no occupant and when someone seats on it, it should drop to under 100 Ohm. That is the way the sensor can sense whether someone is actually sitting or not.
Hi I see you have the diode in series with the resistors. is this the way it should be done? i have done it the way "profor" has done it in the previous post and it did not work at all. can you check this way to see were he went wrong? I think i paralelled a doide and a 3k3 resistor. anyway it does not work , can you help me out?.

Last edited by floorjacker; 12-20-2010 at 08:25 AM. Reason: word error
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Old 10-03-2011, 04:24 PM   #20
ste69
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sounds like something ive got now...
i get 02 with a fault code 18 seat occupancy sensor so looking around i think it maybe the mat but am i better off doing this test to check an is it really as easy as you all say...
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