Unusual fault on diagnostic - help!

The airbag light is on on my 525i

I had it in with Logic Automotive for ABS module repair and the following came up on the diagnostic (no code)


Restraints : Airbag
"battery positive terminal isolation 1: resistance too high'

Anyone any idea what the fix for this is? Peter at Logic had never seen this fault before

This relates to the BST, or Battery Safety Terminal. There is a pyro-technic charge built in to the positive cable that connects to the battery. In the event of an accident the airbag module sends a signal to the BST & the charge explodes cutting the power to the car (the cable breaks with the mini explosion).
"Resistance too high" probably means the little black cable on the BST is disconnected, you will see a black/yellow connector on this.

When you power on the car the airbag module "checks" all the satellite devices (e.g. Airbags, crash sensors, seat mat, seals belt tensioners & BST). It does this by measuring the resistance in ohms of each of the devices, if it comes back greater than 6ohms from any of these circuits then it will trigger a fault as being present & illuminate the airbag light in the instrument cluster.
If it is to low it will also trigger an airbag light, eg if someone used a plain piece of wire to bypass the airbag or sensor.
There also could be a problem with the software version on the repaired module. I hit this issue on a car a few years ago.

This fault is actually quite common, second only to a broken webbing in the passenger seat (occupancy sensor).

Here is a diagram of the BST:

http://i.ytimg.com/vi/c26Jq1O1m0M/hqdefault.jpg

The fault may be with the igniter capsule too.

http://www.bimmerfest.com/forums/att...3&d=1316667714

Here's the unit that might need replacing:

http://i568.photobucket.com/albums/s...0-%20BST/2.jpg

http://www.bimmerfest.com/forums/att...8&d=1326418113

Thanks for your input Rebel Ranter very informative. I guess I need a new BST then. Do you know if they need to be specific to the 525i E39 or will any e39 one do?

Hi, they are the same part number for most RHD e39 unless they have some really obscure options; 12 42 1 436 885. Part #1 below:

http://bmwfans.info/parts-catalog/E39-Sedan/Europe/525i-M54/R-A/nov2002/browse/engine_electrical_system/battery_cable/

http://i175.photobucket.com/albums/w...pspzj9gdrg.png

Price: € 194.60 EUR (on www.carparts-pros.com, add €17 for delivery, plus €5 for PayPal)

Thanks ....that's quite expensive . I'll try and pick up a second hand one

^^^ why this forum exists! Nice one Mark!

will be nice to change it :hehe!:

Quote:

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Originally Posted by Billy Greenbush

Thanks ....that's quite expensive . I'll try and pick up a second hand one

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Did be very tempted to try connect up a seat occupancy bypass sensor to the two little black wires. If it doesn't work then you would be buying a replacement unit anyway.
If it does work just bear in mind that when you crash the battery won't disconnect automatically.

It was suggested by BavarianCare on the other forum that I could disconnect and code this out ? Know where I can get this done in Dublin?

I called into to Courtney's and they sat they would need the car for the day to find out exactly what's going on with it. Probably needs that new part but they need to check it themselves

If it's possible to get it coded out I would....have spent a bleedin fortune on her recently so would like to avoid anything else if possible