C8 Airbag issue

[MixerFistit]

I have no comms with anything Airbag related.
No dialogue for airbag
Comms fault under Memory unit
BSI cannot see Airbag ECU

THE RAMBLE:
I'm working through some electrical issues on a C8 I picked up recently.
One of the major issues is a persistent Airbag fault.

Instrument cluster shows Airbag and the amber child seat symbol (presumably to say passenger side deactivated)
I also get "Airbag Fault" and "Passenger Airbag Deactivated" displayed as a message.
There's no signs of corrosion on the central Airbag module and I've had the passenger seat out to visually check the connectors which look fine- not testing with multimeter yet until I've found out the correct procedures. - I read there's a splice near the sill but I haven't gone undoing the tape yet due to the faults listed below making me think the fault is elsewhere.
Aside from the central module, I've also found a green connector on the back of the seat and there is a Timec module under a shield near the B pillar which is presumably a side impact sensor.

Now the big concern is that Lexia cannot communicate with Airbag ECU.
I get a big fat NO under dialogue and - - - under fault. So it seems the module itself is either dead or there's some other break in the comms.

The BSI reads a fault of Communication on the DATA wire of the Airbag ECU
(as well as DATAB of sunroof Ecu - probably irrelevant but I'll mention incase it's a sign of a common issue elsewhere)

There's also a fault under Memory Unit which Im guessing is the 'blackbox' part of the Airbag system. That will initially say Permanent Fault = Communications on Data wire.
But then Lexia auto refreshes and reads Intermittent Fault = No comms between storage unit ECU and BSI.
It'll then keep switching back to other message every 30s or so.
I've never seen Lexia do that with faults before.

So, do all these faults to something beyond a dodgy connector? Is there a common location that would show up all these issues? I'm wondering if this is beyond the usual suspects around the passenger seat

VF7**************[VIN obfuscated, can be read by forum staff]

[GiveMeABreak]

The lateral sensors are fitted inside the bottom left and right side pillar sills and are responsible for detecting lateral acceleration for determining side impact severity.

Has the vehicle been in a previous impact at all? If so, and the Airbags were triggered, the Airbag ECU will record the impact and lock itself. This often requires a new Airbag ECU for obvious reasons. If this is the case it may be why you are not getting comms with the unit.

[MixerFistit]

I don't know the history but I'd assume not given the owner.
I've got a repair history stretching 3 years and no clues to anything like that.

There were 3 glass cubes under the seat I'd assumed were from a break in but I also found an old £1 coin which would go back further than 3 years so I know the seat hasn't been taken out. Sliding doors work fine, none of the doors look any different or especially new, in fact the previous owner had it off his dad who for some reason put chrome on all the car's lines (internal and external). His son took most of the chrome off when he took charge of it but can see the sticky backing is still in parts on all doors so fairly certain no unexpected change. The dad also spray painted the wing mirrors a pale blue and theres flecks of spray paint on the door. No Airbag stuff picked up on MOT and looks like that centre were more than happy to pick up any issues as they were certainly very happy to charge the guy for repair work too..

Do you know where the fuse is for. The Airbag ECU? Everything I've searched points to F3 under the steering wheel but that is empty (and blank by the looks). This is a 2005 model so doesn't seem to be near a face-lift overlap but is a 6 seat exclusive so I don't know if things were done differently. Everything online has the fuse box as 2002-2008 then 2009-2014

Edit
Also, when an Ecu locks itself, would that result in no comms at all? I'd assume there'd still be dialogue rather than a data wire fault? Although, I know enough by now that I shouldnt assume anything with PSA.

Second edit,
OK, I'd heard it referred to as a Pull an but it seems only the 807 is called that (not sure what the c8 6 seat) version is. So thinking they followed Peugeot's electrics for this I can see on the 807 pullman, F3 is blank and the rest of the box seems to match so F16 seems to fit the 5A fuse of the Airbag (which of course is perfectly fine)

So if the Airbag ecu does need replacing, is it just the large module between the seats?
Is it plug and play?
Is the memory unit part of this module or does that need looking at too and where is it if elsewhere?

Sorry for all the questions, just trying to cover the bases

[wheeler]

Edit
Also, when an Ecu locks itself, would that result in no comms at all? I'd assume there'd still be dialogue rather than a data wire fault? Although, I know enough by now that I shouldnt assume anything with PSA.

Yes you would normally still get comms & the ECU would have fault codes relating to deployed igniters.

As for the splices you have read about, this was a verry common issue on the C8 however it relates to faults with the curtain & lateral airbags circuit. It doesent normally cause lack of comms.

As for the ECU if you were to get a brand new one then it needs basic setting up with Lexia however a used one of a car with the same level of airbags should be plug & play. If yours is an earlier VAN/CAN model you need to get one off another VAN/CAN model.

[MixerFistit]

Thanks for the input,
Is there any common things to check for no comms?
It's a big dashboard to start chasing wires without clues! Or is the module itself the most common culprit?

Just to add, I've just thought, my currently off road Dispatch has an Ecu in the floor, I wonder if connecting that could at least rule out a wiring issue. Trouble is, I don't know if it's CAN, VAN or CAN/VAN. It's an 06 face lift old shape. Only has a drivers airbag and a very simple (or at least tolerant) sensor network as the majority of sensors unplugged don't seem to make it complain.

[GiveMeABreak]

I wouldn't suggest mixing Airbag ECUs at all and no, the one from your dispatch is entirely different and not advisable to try...

[wheeler]

No, your 2006 Dispatch isnt even multiplexed so its neither VAN nor CAN.

If i remember right if its VAN/CAN your remote key is the oval shape, when it changed to Full CAN the remote is rectangular.

[MixerFistit]

That's correct, 4 positions on the barrel, cruise control and radio stalks. I've had a later model c8. The lock/unlock and side doors were in a + shaped arrangement unlike this one.

Thanks for the heads up on the Dispatch, I was going to wait for someone's say so first as even if the connector fit I wouldn't have put it past them to swap the pins and I somehow deploy all of my airbags on first crank

So, as nobody is jumping up with the magic: "it's 90% this wire in this location" I'm set to load the parts cannon and try grab an airbag ecu from a reputable source. Worst case it can be returned for a restocking fee. This 2.2 HDI has really grown on me in the last 36 hours it makes me want to put the effort in as on the surface it looks like there's only the rust on one sill and the Airbag to deal with for an MOT in a couple of months. Although, whoever put the thermostat under the egr pipe wants taking out the back and given a good hiding!

[wheeler]

First thing is to get the wiring diagram & check if the ECU has all its power supplies & earths. If these are all present then a new ECU is worth a punt.

[GiveMeABreak]

I'll get a diagram up shortly for you.

Most hardware (not the wiring) codes are here:

viewtopic.php?t=80489

This is the main Airbag and Pre-Tensioner ECU


These are the other connections:

[MixerFistit]

Thanks for that, certainly worth a look before getting an ecu. Are there any not so obvious DO NOTs when testing SRS?

As an aside, I have observed lower voltages than expected from the alternator but it's apparently within tolerance.
I get 13.9V from alternator casing to its post + that the strap attaches to. 13.8V across battery so it doesn't look like I'm losing anything of significance in the wiring, and it seems to be anything above 13.5 is within tolerance but I was just surprised as every (correctly functioning) Citroën I've ever owned has had a voltage of 14.4V so I always get a bit wary when I see less. (I've tried putting main beam and fogs etc to increase load but 14.0 is highest I've seen, also changed battery from the one in the Dispatch which happily turns over the upgraded starter in the van. The C8 on the other hand takes a few more cranks than I'd like and they seem slower (a bit like a week before my stock dispatch starter died actually) . Possibly a bad starter or connection - did the 2.2 have the rubbish starters like the 2.0?. I'm not too concerned at the moment, and I'm hijacking my own thread with other problems, but as most of my issues are electrics based (aren't most people's!?), I thought it worth a mention.

[GiveMeABreak]

CAUTION: All pyrotechnic components must always be disconnected before measuring instruments are used Ohmmeter or other measuring instrument to check the electrical components and wires.

CAUTION: Any of the pyrotechnic components may be triggered if a powered measuring instrument is used for fault finding purposes on this system.
An ohmmeter can be used for measuring resistances when the checking tool of kit (-).4181-T is connected to replace a pyrotechnic component.

Deactivation / Reactivation Central Airbag / pretensioner System

Disconnect the battery negative cable (1).
Protect the cable and the negative terminal in order to prevent any contact.

CAUTION: Wait for at least 5 minutes before performing any operations (discharging of the airbag ECU power reserve).
CAUTION: The ECU must not be removed with the connector still connected under any circumstances.

Disconnect the airbag ECU connector.

Activation

ESSENTIAL: The area of the airbags and pyrotechnic seat belts should be kept clear, both of objects and of people.

Ignition off: Connect the airbag ECU connector.
Connect the battery negative cable.
On the driver’s side, door open, ignition on, keeping the steering wheel module activation area clear.
Check the system is operating correctly by checking the airbag fault LEDs.

N.B.: After disconnecting the battery, some electronic systems (injection, safety auto-reverse system ... ) need an initialisation procedure ; Perform the appropriate initialisation procedures.

[MixerFistit]

I'm revisiting this as have had too many other things to sort out pre-MOT.

I realised I forgot to ask: where is the BSI actually located so I can begin testing/checking the wires and connectors? I assumed it was in the engine bay but that's of course the Injection ECU

[GiveMeABreak]

Be very careful with this - the BSI is the main vehicle computer that controls everything else and is mega expensive. On the C8, it is located on the front right hand side as shown by (1):

There is a storage compartment there that should have a cover with a screw that needs to be removed to access the fuses - the BSI will have several rows of fuses in several areas.

[MixerFistit]

Thank you that's a big help, I've had the battery off for a while so just going to look for anything obvious corrosion that could explain the no comms.

Also thanks for the paint code in the parts thread!