Clearing Airbag Codes has deployed drivers airbag.

M

Magicarcher

Active member
 Warwickshire
I have just inadvertantly deployed the drivers airbag on my beloved Z4 and I am in a state of shock. I had been having issues with error codes that wouldn't clear:
  • 0095AB SBSR Control Unit Fault (which I believe could be bing caused by the one below)
  • 0098D3 SBSR Short Circuit, firing circuit, front airbag, right (E85), or passenger airbag (E6x), stage 2, to ground
I decided to remove the passengers airbag module and take a look/clean the connectors using contact cleaner. I unbolted the drivers seat and moved it forward a little to give me more space to get to the module. I then removed the battery negative lead and unbolted the passengers airbag module, removed the connectors and set about inspecting the pins which were squeeky clean, I sprayed the connector itself with contact cleaner, left it to dry for a few hours. I then reconnected the passengers airbag module. I decided not to bolt it back in in case I couldn't clear the codes and needed to replace the module. I reconnected the battery plugged ISTA+ into the OBD2 port, turned on the ignition and I had an additional code: SBSR under voltage. Figuring the vehicle battery may be low I hooked up a charger and then proceeded to "delete" the fault codes usign ISTA +. As soon as I clicked the delete button, the drivers airgbag deployed. I have cleared codes many times on numerous vehicles with ISTA + without issue.

Does anyone know why clearing airbag error codes would cause the airbag to deploy?
 
I think part of the cause could be because the module was unbolted/free from the chassis anchor points
 
I would suspect clearing the fault codes has caused the safety system to reset and go through the self check cycle. Thats how it’s suddenly become energised and any short that was there has triggered the airbag.
 
I have cleared the fault codes a few times in the past (well tried to, they came back) without the airbag firing. The two things that were different this time was:
  • I had the 12V battery hooked up to a charger, so the voltage would likely have been around 13.8V and there may have been ripple on it.
  • The passengers module was not bolted to the car. It was floating on top of some rags and not in it's normal orientation
The only consilation is I have now removed the airbag so will be able to check the resistance to ground of the wiring independantly of any electrical attachments.
 
The module may have been grounded though the car, as it was isolated from the chassis the return current may have found different path.
 
Isn't that what Mat Armstrong did on one of his videos? He had the airbag control unit laying lose as well and that went off
 
I am coming to the same conclusion. If so it was my own fault, I was deliberately isolating the case of the unit becuase the fault code was suggesting a leak to ground on the SRS firing circuit. That said two things to me either:
  • There was a leak to ground through the wiring in the harness
  • There was a leak to ground internally in the SBSR
My though process was if I could electrically isolate the case of the SBSR from the car, then if the leak to ground was internal to the unit, the fault would dissapear.

However, I have now found the full wiring diagram https://bmwteka.com/wds/en/e85/8c981848 and the SBSR only has one ground which is a clean ground. The pyrotechnic igniters must use the SBSR case for ground as I would expect them to be on a dirty ground. As suggested by Street and Rockhopper the clear code procedure likely used the short to ground previously reported in code 0098D3 as a case ground and in doing so triggered the airbag.

Lesson to be learned: Don't try clearing airbag codes unless the satelite modules are in place and are cleanly grounded to the car. Probably best not even connecting the battery until they are in place to be honest.

On a positive note: If any of the airbags was to go off, the steering wheel is the easiest to replace; replacement used airbag on order; during my next round of sorting this problem I will make sure the SBSR is bolted in place before I connect the battery. When I eventually get this sorted I will post an update.

One thing worth mentioning. I had already opened up the SBSR to look for obvious signs of internal shorts, there was no sign of corrosion and the PCB had a conformal coating albeit only "laquer", but it wasn't practical to check for component shorts as most devices were surface mount.

Question for PVR if you could locate the video you mention in "Isn't that what Mat Armstrong did on one of his videos?" I would like to see it.
 
Magicarcher said:
I am coming to the same conclusion. If so it was my own fault, I was deliberately isolating the case of the unit becuase the fault code was suggesting a leak to ground on the SRS firing circuit. That said two things to me either:
  • There was a leak to ground through the wiring in the harness
  • There was a leak to ground internally in the SBSR
My though process was if I could electrically isolate the case of the SBSR from the car, then if the leak to ground was internal to the unit, the fault would dissapear.

However, I have now found the full wiring diagram https://bmwteka.com/wds/en/e85/8c981848 and the SBSR only has one ground which is a clean ground. The pyrotechnic igniters must use the SBSR case for ground as I would expect them to be on a dirty ground. As suggested by Street and Rockhopper the clear code procedure likely used the short to ground previously reported in code 0098D3 as a case ground and in doing so triggered the airbag.

Lesson to be learned: Don't try clearing airbag codes unless the satelite modules are in place and are cleanly grounded to the car. Probably best not even connecting the battery until they are in place to be honest.

On a positive note: If any of the airbags was to go off, the steering wheel is the easiest to replace; replacement used airbag on order; during my next round of sorting this problem I will make sure the SBSR is bolted in place before I connect the battery. When I eventually get this sorted I will post an update.

One thing worth mentioning. I had already opened up the SBSR to look for obvious signs of internal shorts, there was no sign of corrosion and the PCB had a conformal coating albeit only "laquer", but it wasn't practical to check for component shorts as most devices were surface mount.

Question for PVR if you could locate the video you mention in "Isn't that what Mat Armstrong did on one of his videos?" I would like to see it.
Click to expand...
That was a different issue. The airbag module in the Ranger Rover SVR also houses a yaw sensor. He didn’t secure it down and moved it. The movement triggered the curtain airbags thinking the car was rolling.

From about minute 40:
 
Bit of an update on this. I replaced the drivers airbag with a used part, bolted the B post SBSR unit back in place, checked continuity between the aluminium case and the battery negative which was 0 ohms. I then ran diagnostics and cleared the codes (keeping well clear of the steering wheel just in case). This time the airbag did not deploy. This seams to suggest the airbag deployed because the case of the SBSR module was not grounded. Unfortunately the two airbag codes 0095AB and 0098D3 persist.

Before reassembling, with the steering wheel airbag and ECU disconnected I checked the resistance to ground of the four wires between the ECU and airbag and they were all open circuit suggesting it isn't a wiring issue. I checked the resistance between the four pins on the ECU and the case to see if there was a hard short, two measured 2.6kohms and two 6.8kohms, so there is no hard short, but it is hard to know if these value are right as there could be all sorts of parallel circuits. It is pointing to the two codes being caused by a faulty SBSR ECU on drivers B post.
 
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