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  1. 2001 525d E39 Airbag warning light stays on 
    #11
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    Quote Originally Posted by Smart Motor Works View Post
    Ok, from a technician point of view, if you are certain your diagnostic equipment is capable of the task and connected correctly with the ignition on, but still not giving any information, you need to check the basics. First of all power supply to the control unit. (Disconnect the control unit!) This can be supplied to more than one pin in the connector, and can be permenant, terminal 'R' and terminal '15' ('radio' and 'ignition on' positions of the ignition switch). You must also check these supplies relative to the earth terminal(s) in the same connector, preferably using an old style, non LED test lamp - i.e. with an incandescant bulb. This will show up any high resistance wiring connections that a multimeter or LED tester will not, due to the current draw. If these supplies and earths are satisfactory, you must now check the K bus communication to the unit. I believe this is a grey/white/yellow or similar. To check this, you need a multimeter. First of all, with the ignition on, and the meter set to DC volts, check the voltage level of the K bus relative to chassis ground, it should be around 12 volts. Next, check the AC voltage level on this wire, it should vary depending upon how much digital communication is taking place - i.e if you operate the passengers window from the drivers side, you should see a brief increase in the AC voltage level. If this is all satisfactory, and you still have no communication, then the unit is pooped.

    N.B. this all depends on your diagnostic equipment being up to scratch!

    N.B.2 NEVER REMOVE THE AIRBAG ECU WHILE IT IS PLUGGED IN, OR PLUG IT IN WHILE IT IS NOT BOLTED DOWN! Any false moves in this situation will deploy a selection of airbags, and you will be severely injured at best!

    Good luck, and I hope I was of some help.
    Hi m8, that could be a lot of help and thanks for taking the time to post it and you sound pretty clued up.

    Just a couple of questions..

    When you say 'disconnect' the CU, what do you mean please?

    You are talking about the 'airbag' CU and not another CU are you? Is not, where can I find that one? Secondly, if the Airbag CU was faulty or not working at all, wouldn't that be flagged up by the diagnostics tools? And in that case we are talking about the main CU? Any idea where the fault codes are stored? I presumed it wasn't in the Airbag CU but the main CU if there is two on the car or am I mistaken and there is only one that is under the centre console at the rear?

    I have got a multimeter but not the other which I can rig up.

    I would disconnect the battery before removing any equipment including the airbag CU but will need it left in position to check the voltages anyway.

    Is the K bus comm unit part of the Airbag CU or somewhere else? Sorry for me being dim.

    Kind Regards and thanks

    Mike
     
     

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  3.  
    #12
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    By 'disconnect the control unit', I mean the airbag control unit between the front seats. If the unit was dead, not correctly supplied with power, or not correctly connected to the K bus, it would not show any faults to your code reader, because it would have no means to do so.

    The faults are stored within the airbag control unit itself.

    The K bus is a single wire communication system, used for 'body' modules (i.e not drive or chassis), whereby several control units are connected together with the wire 'visiting' each unit at one pin of the connector plug. The control units all 'talk' to each other using this wire, and the instrument cluster (also connected to the K bus wire) can decode these messages, and if ness, create other messages and send them onto the D bus, which is a wire which goes to the diagnostic socket, and is where your code reader is told of faults on the car.

    If the airbag control unit has been replaced with a new one, it will need to be coded to the car for it to work.

    If it has been replaced with a secondhand one (even if the part number is the same), it may never work, because in some instances, the control units are coded to a chassis number. These units will need specialist treatment to recover them to new status, and I have no idea who to speak to in this case, as I come from a main dealer background.

    Good luck to you both!
     
     

  4. 2001 525d E39 Airbag warning light stays on 
    #13
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    Hi SMW,

    Thanks again for that information which is very useful. It really sounds as if my airbag CU is up the creek and wants changing and if as you say no fault codes can then be read, then taking it to either the Indie of the Dealer will yield the same result in that they will say no fault found as David as found out as well. I suppose their first response is for them to supply and fit the new ABCU and if that should then enable the diagnostic reader to be used to clear the fault light or identify any other faults. Not sure if that option was offered to David though?

    My other option is to buy a second hand one and fit it myself and ask them to recode it, but I guess that won't be cheap either. But they are on ebay at very cheap prices compared with brand new. I wonder if that is why David can't get his to work? But I will have a go at checking the ABCU first. Then if that's duff, the cheapest option seems for me to fit another ABCU and then get them to recode it at the same time as they did the diagnostic check. Otherwise its let them do the diagnostics. Which will involve another cost after the ABCU is fitted.

    Of course it is possible that my code readers weren't up to the job and they find it was something else? I did read somewhere that it wouldn't want recoding if the ABCU was taken off a similar car?

    Where is the airbag light fed from if the CU is dead?

    Thanks once again.

    Mike
     
     

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    As I said, a secondhand one might never work on your car, because the chassis number will have been burned into the electronics, so beware.

    I am unclear whether both of you are suffering the same fault. It sounds like neither of you are not getting any response from the airbag unit when interrogated by your and your dealers code readers, if this is the case, then you need to perform the basic checks I mentioned previously. If the units are responding, but are saying there are no faults, then the situation is a little different.

    Even if a secondhand unit works, it will be prudent to get it coded, as there are a myriad of detail changes from model to model, and year to year.

    If the control unit is dead, has no power, or has no means to communicate, the airbag light will probably come on, because it is internally fed inside the instrument cluster. The earth path is through the airbag control unit.
     
     

  6. 2001 525d E39 Airbag warning light stays on 
    #15
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    Quote Originally Posted by Smart Motor Works View Post
    As I said, a secondhand one might never work on your car, because the chassis number will have been burned into the electronics, so beware.

    I am unclear whether both of you are suffering the same fault. It sounds like neither of you are not getting any response from the airbag unit when interrogated by your and your dealers code readers, if this is the case, then you need to perform the basic checks I mentioned previously. If the units are responding, but are saying there are no faults, then the situation is a little different.

    Even if a secondhand unit works, it will be prudent to get it coded, as there are a myriad of detail changes from model to model, and year to year.

    If the control unit is dead, has no power, or has no means to communicate, the airbag light will probably come on, because it is internally fed inside the instrument cluster. The earth path is through the airbag control unit.
    Hi mate,

    What's the difference between the dealers coding a brand new CU and a second hand one? David has disconnected the earth wire from his CU to get his airbag to go out and so that ties in with your comments above.

    Cheers once again.

    Its good at least to see what the alternatives are even if it has to be rectified by the dealer/Indie. The problem though is if they can't fix it as appears to be the case for David's car, then what next? Scrapping the car as he suggests?

    This is what happens when they go from a simple seat belt restraint to one full of electronics monitoring. My local Dealer didn't even want to know about using a passenger seat bypass even though that was in the fail safe position. ie airbag to operate in any event whether passenger is in the seat or not.
     
     

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    #16
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    Some control units, once coded to a specific vehicle, cannot then be re used on a different vehicle. This is mainly a money making idea on behalf of the manufacturers (BMW probably leading the way mind you).
     
     

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    DAVID please note the reply from the VOSA re airbag lights..


    The full reply is shown below and most significant is the last paragraph..ie

    "In respect of the SRS (Supplementary Restraint System) warning lamp; a
    vehicle will fail if this lamp indicates an SRS system malfunction.
    However, there will be no MOT requirement for the lamp to actually work
    once these changes become 'live'. "
    -----------------------------------------------------------------------------------------------
    Presumably and hope you all concur in that as long as there is NO airbag light showing, the car will NOT fail the MOT test.

    ie no requirement for the light to come on and go out after a few seconds?

    ---------------------------------------------------------------------------------------------------------------------------

    Their reply..

    Thank you for your email enquiry dated 22nd August 2012, concerning an
    airbag.

    Until recently, the content of the MOT test was the domain of the
    Department for Transport and is enshrined in legislation. The test also
    complied with the European Framework Directive on 'Periodic Technical
    Inspection' as the MOT test is known in Europe.

    However, this Framework Directive was recast in 2009 (2009/40/EC) and
    amended in 2010 (2010/48/EU) to include numerous new mandatory test items.
    These changes are aimed at bringing the MOT test more up to date by
    including the inspection of electronic systems which are now a fundamental
    part of vehicle design. They are also a step towards harmonising the test
    standards across the EU.

    As an EU Member State, we are legally obliged to comply with this Directive
    and to introduce the new test items by 2012. The reason that these new
    items have since been 'deferred' is because there have been delays in
    amending our domestic legislation to accommodate the changes.

    Failure to comply with the Directive is not an option and would lead to
    infraction proceedings being taken against the government, resulting in
    fines, potentially running to millions of Euros and continuing until such
    time as the Directive is complied with.

    Whilst the Department for Transport would have commented on this Directive
    on behalf of the government at the proposal stage, and may well have
    opposed some aspects, we now have no option but to comply with the
    requirements of the Directive.

    In respect of the SRS (Supplementary Restraint System) warning lamp; a
    vehicle will fail if this lamp indicates an SRS system malfunction.
    However, there will be no MOT requirement for the lamp to actually work
    once these changes become 'live'.

    I hope this information has assisted you with your enquiry, but if you have
    any further questions please do not hesitate to contact us again.
     
     

  9.  
    #18
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    OK, just had the car checked at my local Indie on his diagnostics programme and he tells me he can't communicate with the OBD2 airbag system but can all the other systems on the car which points to the Airbag Control Unit being duff and needs replacement and recoding. The other alternative is to get the light out to pass the MOT later in the year.
    So Smart Motor Works seemed spot on with his theory it may well be that as well, so good man..

    I will probably do the voltage checks he has suggested though and see how that goes..He said he can recode one as and when.
     
     

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