E38 728i Airbag Issue

[mbwoy84]

I’m having a bit of a nightmare with the airbag system on a 1999 E38 728i I’ve got.

It uses the old 20-Pin under bonnet OBD port (it does have a newer-style OBDII 16-Pin socket inside the car, but only works with the engine management system). I read the codes some time ago (can’t find the list I made, but there were a couple of codes) and the first logical step following research was to change the NSF seatbelt pretensioner as everything pointed to that side of the car. I swapped that out for a supposedly good, used replacement. The light stayed on and other cars became a priority, so it got left for a few months.

I really want to get the car done though, so plugged it in again today and it has a list of codes, longer than before I’m sure, which won’t clear. These codes are as follows:

11 - Power Supply
16 - Sensor for side airbag, left
0B - Battery Positive Terminal Isolation 1
F0 - Internal Control Unit Fault
1A - Seat Occupancy Recognition (SBE1)
05 - Side Airbag, Front Left
03 - Belt Tensioner, Front Passenger

What does that point to? One of the codes suggests an ECU Fault. I can get that out and set away for testing/repair, but everything else points to something specifically in the NSF area. Pointing towards the seat and side airbag set-up in particular.

As said, the NSF pretensioner has been changed, but it was a used item.

The car was jump started today and hadn’t been started for probably 2 months, so guess anything power-related may or may not be connected to that.

I’m hoping that combination of fault codes strikes a chord with someone before I painstakingly have to go through every mm of the electrical system connected to this.

TIA!

[330ti]

Have you cleared all the codes ? Do that and see which ones come back

[mbwoy84]

330ti said:

Have you cleared all the codes ? Do that and see which ones come back

They won’t clear. Maybe that hints at a Fault with the ECU also perhaps?

[d_a_n1979]

What are the battery voltages (at idle and with engine off)?

Have you checked to see if the under-seat connections are connected properly and corrosion free?

Has the replaced seat belt tensioner been fitted right, free of corrosion and was it coded in to the relevant module?

Have you tied to disconnect the battery; let it sit overnight completely powered down and then reconnect the battery and see if the codes are there and can be cleared?

[RUSSELLM]

“Power supply” would have me checking if the module has a fuse.

[OldGermanHeaps]

ignition switch on these can cause spurious airbag faults. you can also check all power feeds and grounds at the airbag ecu pins
you can also check each circuits resistances at the ecu to check for wiring faults.

[LunarOne]

I don't know if it's the same on the E38, but my E46 driver's door and curtain airbags deployed after hitting a huge alloy-wheel-destroying pothole at speed. Apart from the two wheels, the car was undamaged so I replaced the airbags myself using parts from ebay. I discovered that replacing the airbags and turning the key is not enough to reset the system and neither is disconnecting the battery and waiting. The SRS system will only re-initialise with a special airbag code reader and reset tool, which I bought for about £10 again on ebay.

I needed that tool again when I removed the driver's seat to replace my final stage hedgehog resistor, which believe it or not is even more of a pig to do than the curtain airbag. Anyway I don't know if there's a similar tool for cars that don't have an OBDII port, but my point is that a normal code scanner isn't able to re-initialise the SRS system.

[mbwoy84]

I’m not using a code scanner. I’m using several thousands of pounds worth of diagnostic equipment, which is more than capable of reading and clearing fault codes and coding components on any ECU system.

I have one of the old 20 pin airbag scanners I bought when I had an E39 many years ago.

As for all the other suggestions here and on the 2 other forums I’ve posted on, I’ll be investigating further on Monday. Firstly, by removing the battery and fully charging it, re-running various tests, then remove the seat, door card and main airbag ECU and checking, cleaning and testing along the way, checking relevant fuses etc.

I’ve never heard of coding in a pretensioner before. Certainly not on a car of this age. Coding in a new ECU/module, yes, but not a part like that. Yes, all connections were checked, cleaned etc upon fitting. The part number was an exact match. Can it 100% be confirmed to be a working item? No.

[d_a_n1979]

mbwoy84 said:

I’m not using a code scanner. I’m using several thousands of pounds worth of diagnostic equipment, which is more than capable of reading and clearing fault codes and coding components on any ECU system.

I have one of the old 20 pin airbag scanners I bought when I had an E39 many years ago.

As for all the other suggestions here and on the 2 other forums I’ve posted on, I’ll be investigating further on Monday. Firstly, by removing the battery and fully charging it, re-running various tests, then remove the seat, door card and main airbag ECU and checking, cleaning and testing along the way, checking relevant fuses etc.

I’ve never heard of coding in a pretensioner before. Certainly not on a car of this age. Coding in a new ECU/module, yes, but not a part like that. Yes, all connections were checked, cleaned etc upon fitting. The part number was an exact match. Can it 100% be confirmed to be a working item? No.

Sometimes parts re-fitted may need to be coded to that necessary module or ECU; so it was just to cover all bases

Having owned a 728i myself; I know they can be awkward with codes at times and I know you've said you're using several thousands worth of a diagnostics machine, but it's known that they're not always right and the likes of INPA will get a much truer read out of codes and issues with these motors

But I'm not an INPA expert; so you'd need to find someone local ideally that has access to INPA and a laptop and knows how to use it etc

[Touring442]

Very often the airbag ECU is scrap once the bags have been fired. On any E38 it's worth removing the drivers seat, lifting the carpets and inspecting the fusible links under there. It's common to find a sea of red rust that causes a lot of trouble.