Airbag light

    Hi, I really hope someone can help me with this.


    On my 850cia the airbag light is on and the horn is not working. Untill now I've suspected the slip ring to be faulty. Today I took the steering wheel apart to check the slip ring before buying a new one, but to my suprise it was fine. Checked it on all connections with a voltmeter.


    When I assembled everything back together I started noticing that one of the relays in the engine room is making a click sound when I press the horn, which I guess is proof that the slip ring is fine, and it leads me to think I got two separate problems here.


    Now the horn is probably a minor problem. The alarm works fine, but I think I got two horns installed and maybe one of them is faulty. I'll check into that the next time I'm under the car.


    The airbag problem confuses me though. I've run diagnostics with Carsoft and tried to delete the error several times. But the same error comes straight back.
    Error 2 - Ignition Circuit - Drivers side - Defective


    Can anyone point me in any direction on as where to look and what to check next? Are there any relays for the airbag I should look at? :hmmmm:
    All the fuses are of course fine


    Any help is appreciated here!



    Edit: also if anyone can point me to where the main SRS controller/unit is located I could start checking connections from there to the steering wheel

    The alarm system has a seperate horn, so you're right that these issues are unrelated. The horns are located underneath the car just behind the front bumper. It's worth checking them, as their location means that they are pretty exposed to the elements and failure isn't uncommon.


    Assuming your car is LHD, the control module for the SRS (airbag) is installed in the right hand side of the dash behind the glovebox. It's pretty high up, conveniently difficult to access as always!

    Thanks for your reply, been a while since I had time to work on this.


    I fixed the horn, and as you said it had just taken a few bumps to the ground. Just needed replacement.


    For the airbag I've now tried swapping the airbag with the same result, so I started checking the wires. I just started wondering about one thing. The airbag wire going through the slip ring has two connectors on the front side going into the airbag and a larger connector on the back side with four holes/pins.
    I noticed that I got a cross connection on both pin 1 and 2 on the connector. So both pin 1 and pin 2 gave a connection to both the pins on the connector going into the airbag. Is there supposed to be a paralell/cross connection here?

    Pins 1 and 2 go to the airbag, if these are measured with a meter the reading should be 47 Ohms (this is from memory so I could be an order of magnitude out). If the airbag is disconnected, measuring the pins that go to the SRS module should show a high resistance. If this shows a low resistance then there is some problem with the SRS module or the wiring that goes to it. The airbag connects to pins 10 and 11 on the SRS module.

    If the wiring to the airbag is either open circuit or short circuit the airbag light will be illuminated......

    Yeah, it seems to be a short circuit in the slip ring.


    I just removed the steering wheel from the car and with the airbag removed I meassured the wires going through the slip ring again. I'm no electrical expert, but it seems to be giving the same amount of resistance from either pin 1(10) or 2(11) to both the pins on the airbag connector which I guess suggests a full short circuit..?

    To be sure, it will be necessary to get to the SRS module which (according to the WDS) is under the ashtray. If, when the plug is removed from the SRS module, the resistance increases to infinite then the SRS module is probably faulty. If the resistance remains low then the fault will be with the loom or the slipring assembly. The sliprings are usually found on Ebay as separate items, at worst you may need to buy the steering-wheel as well. The same slipring is used on the E36 wheel and there are loads of those, usually going for £50 at most........

    Alright, thanks for your advices :)


    I think I know someone who have an extra four spoke wheel laying around, so I guess the easiest thing to try first of all it to borrow that one. If that doesn't help I'll try to locate the SRS module.

    Hello Trono,
    look closely at the plugs!


    In order to prevent anything serious from happening during removal of the airbag plug, both pins are shorted mechanically as soon as the plug is pulled. This is why you read the same measuring from pin to pin.
    In order to assess the slip rings´ resistance you need to push a tiny piece of paper in the plug to remove the bridge between the pins.


    Yes, I was confused a while ago as well :D
    If necessary take a magnifying glass.


    The airbag resistance ought to be below 4 Ohms , not 47 as pointed out with a caveat by Timm.


    The message received from the ECU "normally" means a bad connector but since you replaced a lot of things already you certainly made sure the connection is well if there was any "corrosion". I´d definitely again focus on the slip ring first.


    Your control unit´s location varies with the build date. In case you have the orange impact sensors in the engine compartment then there is no ECU under the ashtray. Changes took place for MY 1994.


    Cheers
    Reinhard


    Addendum: below is the excerpt of the wiring diagram (clickable):


    Aha, that's nice to know! I was confused there myself of why I got those readings. Going to check this as soon as I get home from work today.


    My car is built Nov 1993, don't know what color the sensors are. Any ideas where the module might be located on this?


    Thanks :)

    Nov. 1993 as a build date or 1st registration?
    MY1994 starts around Sep 1993. Just have a look under your hood to see whether those two orange boxes are present:



    If they´re not there then the airbag ECU is underneath the middle console like so:



    I have not yet heard of someone whose airbag ECU was at fault.


    Cheers
    Reinhard

    Build date is nov '93. First time registered is '95 sometime..


    Anyways, the pictures are really helpful, I'll check when I get home :)



    Edit: and agreed, that the ECU itself is broken sounds little likely, but who knows. The only thing that have had me wondered for a bit is that carsoft returns error 2 "ignition circuit driver side defective", but I also see that error 2 could be a defective controller unit in some pdf's. But I'm guessing/hoping that this is for the older SRS systems. As far as I know I should have the SRS3 system in mine with the dual airbag setup. I guess maybe Carsoft detects this itself and gives the correct error message too?

    Carsoft is not exactly the most reliable tool for those old cars.


    Once you have found out whether the slip ring is the culprit or not also check the connection on the passenger side airbag (if installed). The dashboard lid can be pried open by hand and the connector IIRC is underneath the bomb on the left hand side. Just unplug and plug in again a couple of times - preferably with batteries disconnected ;) This in fact is a necessary and prescribed precaution for all work concerning in-car explosives!
    The airbag itself (if uninstalled) should always be stowed away face up - preferably in the boot where the explosion will stay contained :lol:


    Don´t risk a thing - if for any reason an airbag is ignited you only have about 10 milliseconds to get out of the way... bullet-time ;)


    Cheers
    Reinhard

    Hehe, I've taken all precautions and let the car stay disconnected for atleast an hour before even starting to work on the airbags. Nowadays she's parked for the winter anyway, so no need to have the batteries connected.


    And yeah, hehe, tell me about Carsoft. It doesn't even recognise any modules except the airbag and zke modules. The rest "doesn't respond" ..


    Anyways, I guess I have a few things to check out now. Hopefully I'll be able to fix this soon, been working on it for a while, but it's first now that the car is parked I've had time to go beyond the basics.

    Apologies for not noticing the links on the circuit diagram...obvious now. I guess that they are there to stop the possibility of static firing the airbag in your face whilst fiddling with the wiring!

    Well, it seems it wasn't the slip ring either .. :hmmmm:


    As the connection is shorted when you disconnect the cables I realised I could just hook everything back up except the airbag and measure from the airbag connector. As long as everything was connected like that there was infinite resistance, so if I'm not completely mistaken I guess that proves that the wiring is in order. As soon as I disconnected any part I got an open circuit


    Going to check the passenger airbag asap and if not that helps I'm going to dig up the control unit.



    By the way, I've head someone mentioning on another forum that you could try connecting a small light bulb, like the interior light bulbs in the car, instead of the airbag. That should give enough resistance that the control module won't report an error if everything else is in order. Anyone heard about this?

    Infinite resistance does not really prove anything other than that the two wires don´t touch :D It does not say whether the wiring in the slip ring is still intact. Both could be broken and you´d have infinite resistance ;)


    You need to measure both ends of the slip ring and both wires separately.


    The slip ring´s contents are best imagined as a 6m long curled thin plastic strip with embedded wire traces.
    That snake looks like so unwrapped:



    and if broken like that:



    Both pictures taken from here



    Cheers
    Reinhard

    Zitat von Trono;70720

    By the way, I've head someone mentioning on another forum that you could try connecting a small light bulb, like the interior light bulbs in the car, instead of the airbag. That should give enough resistance that the control module won't report an error if everything else is in order. Anyone heard about this?


    I have not heard about it, but I can see why it would work. I'm not sure how sensitive the airbag system is and what margins there are on the resistance, but I know a 3 ohm resistance works in fooling the system. Reinhard points out the resistance should be below 4 ohm, so the tolerances may be quite tight - I don't know I've never tested it.


    An incandescent light bulb is very much like a standard resistor at low currents. This is called the "cold resistance". It's not until the current increases and the filament heats up, its electrical characteristics start to deviate from what you'd expect from a resistor (actually a resistor would do the same thing, but unlike the lamp it will burn and fail after a short period). The airbag system - apart from the actual airbag ignition perhaps - is obviously a low current system.


    Now you can't simply take whatever bulb you have around. A 12 V 1.2 W instrument cluster bulb for example will have a cold resistance of around 12 ohm. The higher the power rating (at the same 12 V), the lower the resistance. I have not tested this, but I estimate a resistance of 3 ohm to be somewhere between 12 V 5 W and 12 V 10 W bulbs.


    For testing purposes you could try a lamp (verify its resistance with a multimeter), but if you wish to remove the airbag permanently (some people want to intall an airbag-less wheel), get yourself a 3 ohm resistor from an electronic components store. It's precise, reliable and very cheap. A standard low power 1/4 or 1/8 W resistor suffices. Note that I do not encourage to remove the airbag - after all it's a safety device that may save your life, but it's questionable a 15 years old airbag will still deploy reliably (airbags have an expiry date). There are several reports of cars of that age getting involved in a collision where the airbag did not deploy...

    Zitat von reinhard;70723

    Infinite resistance does not really prove anything other than that the two wires don´t touch :D


    Haha, yeah. Sorry, really wasn't thinking clearly there. Too much work and too little sleep I guess :)


    I'll do a proper reading later on.



    Revtor: Thanks for you input. Good to have a bit more technical information on the subject than the other thread I read. I guess it can't hurt the system to try atleast, even though I've tried two different airbags already.

    Still no go.. :help:


    I found the Airbag control module under the middle console and I did a resistance check on the wires, but they are fine.


    So, to sum up.


    - I've tried two different airbags. I know atleast one of them was functional in the old car.
    - I've tried to use a 12v 10w bulb instead of the airbag which gave about 5 ohm resistance
    - Checked, double and tripped checked the slip ring
    - Checked the whole wire harness from the steering wheel to the airbag module.


    Also made sure that Carsoft is working properly and I see that erasing the airbag memory clears the SRS lamp for about 5 seconds before the error is back. (The airbag also reports as OK if I do another diagnose before the light come back on again.


    So I guess that leaves me with the module itself beeing faulty..?
    And a new control module will have to be coded, right?


    The only thing I've noticed is that according to the vin number my car is produced 9/93 (not 11/93 as I first thought), but carsoft reports 12/93 as the production date for the SRS unit.. Don't know if this is normal, it means the unit has already been relpaced once or if it's just carsoft though..

    Could this be one of Carsoft's failings? It uses a simple set of error-code translations (the English description of a numerical error number). However, dependant on which airbag system is fitted, the error number has different descriptions as listed here:

    AIRBAG1
    $F0
    0 Unidentified error

    $H1
    1 Ignition Capacitor Defect
    Before starting to work on the airbag-system please
    disconnect the battery. Please replace the ignition capacitor.
    The airbag system will otherwise not function.

    $F2
    2 Diagnostic-Unit-Fault-Function
    ---------------------------------------------------------------
    AIRBAG2
    $F0
    0 - Unidentified error

    $H1
    1 - Front Sensor locked once.

    $F2
    2 - Front Sensor locked several times.
    -----------------------------------------------------------------
    AIRBAG3
    $F0
    0 Unidentified error.

    $H1
    1 Internal fault in Control Unit.

    $H2
    2 Ignition circuit drivers airbag is defective.
    --------------------------------------------------------------------
    AIRBAG4
    $F0
    0 Unidentified error.

    $F1
    1 Internal control unit fault


    $H2
    2 Failure warning lamp
    -------------------------------------------------------------------------
    AIRBAG5
    $F1
    01 Driver Airbag / E36 : Airbag Control Unit

    $H2
    02 Driver belt tensioner
    Airbag Control Light (Before mod. 2000 and All E36)
    --------------------------------------------------------------------------


    I've been caught out by this a couple of times, once looking for an ASC actuator where the car definitely didn't have one (fly-by-wire).....

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