Understanding SRS / Airbag specific parts

There is no seat belt fastened circuit. That is not fitted to UK cars. You need diag to check SRS and clear any faults.

N151SDW said:

Yes, I realise there are recurrent threads that go over the same ground here although I do have a particular spin I'd like to understand on my P38's current issue! It's a 1996 'N' (28 July 1996 reg so almost a 'P') 4.6 HSE auto.

After a baptism of fire of EKA / key lockout / battery drain / immobiliser issues after a recent purchase I now need to make sense of the SRS airbag fault the car is displaying to get it through a new MOT and into use.

I have taken the dash instrument cluster out and changed the warning bulbs as it definitely had one bulb gone which I read as being a trigger for the warning lamp as a failsafe. In doing so I also checked all the yellow airbag feed connections under the dash and subsequently along to the SRS DCU.

Whilst I had the dash apart I took the opportunity to swap out the hacked about rim steering wheel and also changed the rotary coupler and wheel airbag with apparently GWO one's off ebay - this may be the first banana skin I've decided to jump on as I now realise upon buying an official LR parts catalogue that there are at least x2 versions of these according to the part numbers! Would there be any difference in the functions that these control - I've got the wheel with all the ICE functions and the replacement rotary coupler was said to be off an identical age / spec car...

My main question is whether there are significant differences between these: the one I have now removed is AMR 5631 and I'm about to bite the bullet and dismantle the column again if only to read what the part number is for it's current replacement. Some sold on ebay don't even list the part number and clearly state by what appear to be LR breakers that they'll work on all models or is this just lazy sellers?

Just to be totally clear - if there is a break in the SRS circuit, will the warning light clear off the dash once it is rectified or does it need clearing with a diagnostic set up / laptop - I understand the fault code will remain but my task is getting the car passed through it's MOT. Does full connection of the DCU remove the warnings on the dash or is the BECM to wise for that?

To know whether the passenger airbag needs to deploy, does the system use the seatbelt unfastened circuit - surely it wouldn't know if a passenger is sitting without a fastened seatbelt so would need to deploy automatically? I'm just wondering if there are wires I've read about here that need checking on my car which won't use side airbags or seatbelt pretensioners.

To put all the above another way, what are the usual top fixes that could nail this stumbling block. After the new bulbs I hoped the 'new' rotary coupler was the fix as this has worked many times before for me with other marques of car.

And.

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Not sure the 96 has side airbags, there will be warning tags on the seat backs and a yellow cable under the seats if it does.
Side bags (if fitted) are triggered by side impact sensors, front bags by front inpact sensors, all included in the SRS ECU. The SRS ECU is different on early cars with the Impact sensors remotely located on the chassis rails at the front. Once an airbag has fired, the SRS ECU has to be replaced.
Low battery voltage can trigger a a fault and this can only be reset with diagnostics, You are chasing rainbows without diagnostics.

Datatek said:

Not sure the 96 has side airbags, there will be warning tags on the seat backs and a yellow cable under the seats if it does.
Side bags (if fitted) are triggered by side impact sensors, front bags by front inpact sensors, all included in the SRS ECU. The SRS ECU is different on early cars with the Impact sensors remotely located on the chassis rails at the front. Once an airbag has fired, the SRS ECU has to be replaced.
Low battery voltage can trigger a a fault and this can only be reset with diagnostics, You are chasing rainbows without diagnostics.

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Seat airbags only fitted MY1999 up.

Datatek said:

Not sure the 96 has side airbags, there will be warning tags on the seat backs and a yellow cable under the seats if it does.
Side bags (if fitted) are triggered by side impact sensors, front bags by front inpact sensors, all included in the SRS ECU. The SRS ECU is different on early cars with the Impact sensors remotely located on the chassis rails at the front. Once an airbag has fired, the SRS ECU has to be replaced.
Low battery voltage can trigger a a fault and this can only be reset with diagnostics, You are chasing rainbows without diagnostics.

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DCU is same on both systems with an internal crash sensor. On distributed systems two additional sensors are mounted on the chassis legs. DCU has to be changed after activation of internal crash sensor which should not be reset.

wammers said:

DCU is same on both systems with an internal crash sensor. On distributed systems two additional sensors are mounted on the chassis legs. DCU has to be changed after activation of internal crash sensor which should not be reset.

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SRS ECU is different on early cars without side airbags, it does not have the side impact sensors.
I already said that a fired airbag ECU has to be replaced.

N151SDW said:

Just to be clear - the system on my car hasn't been deployed but clearly believes there's a major fault from the second the ignition switches on as it brings up the 'air bag failure' message on the dot matrix along with the SRS warning lights.

I just wonder if an incorrect rotary connector could trip the warning? The system is the multi point front detecting crash sensor layout with the big large yellow DCU.

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Yes. Diagnostics needed to locate the problem.
So it's the old system. I have a spare ECU if you find you need one.

Datatek said:

SRS ECU is different on early cars without side airbags, it does not have the side impact sensors.
I already said that a fired airbag ECU has to be replaced.

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Yes i know that, but this is a 1996 vehicle so will not have the later unit. The early cars all had the same unit but some had additional sensors on the chassis legs, some later ones before MY1999 using the same DCU did not. Yes you did already say the DCU had to be replaced i just explained why.

wammers said:

Yes i know that, but this is a 1996 vehicle so will not have the later unit. The early cars all had the same unit but some had additional sensors on the chassis legs, some later ones before MY1999 using the same DCU did not. Yes you did already say the DCU had to be replaced i just explained why.

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As did I, the clue is in the statement "fired airbag" The actual sensors are latching devices, pretty sure they could be reset if I can get them apart.

Datatek said:

As did I, the clue is in the statement "fired airbag" The actual sensors are latching devices, pretty sure they could be reset if I can get them apart.

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Yes i am sure they could, just a simple momentary inertia switch were a roller is displace tripping a contact. Putting the roller back to rest would sort that but they are sealed units. Could be done, but beyond the average Joe.