A few weeks ago the airbag light in my trusty 306 decided to stay on after after ignition. Oh no, MOT test due soon!
As of ~2012 the airbag system became a testable item, the internet has plenty of tales of botchers tricking the airbag ECU with resistors over the pre-tensioners or removing the bulb from the dash. This kind of botch job saves you a few quid, but endangers the life of anyone driving your car – and I bet they don’t own up when selling. DVLA likely got wind of this scheme and a couple of years later introduced the clarification that SRS indicators had to be lit when the key is at ignition and go out when the engine is running. This too lead to new fiddling, wire the bulb to the alternator lamp, or connect up a simple timer circuit (chip or capacitor). Again a botch job that’ll see your eyeballs in the foot-well.
Quick web search reveals a common problem with pugs in the mid/late 90’s, pre-tensioner harness (cable running from under the seat to the air bag ECU) connection directly under seat would become faulty, possibly from objects moving under the seat or arses flexing the connectors (the seats are soft and socket/plug assembly is attached to the springs of the seat. The official fix is to cut the connectors and solder the wires. This should be done as the first remedy and is said to sort 80% of cases. Of course I was in the special 20%. Not content with a bodge job and pressed for time, I performed the airbag ECU change along with squib ring, having tried and tested the seat fix.
Notes on jobs done
Job: Pre-tensioner harness wiring connection fix
Start by parking the car so you can open the driver door fully. Central lock the car, disconnect the battery. Now go away for at least 10mins, make some tea, prep your tools – whatever, just don’t fiddle with an armed airbag system – this includes the seats, pre-tensioners are basically fireworks attached to the side of the seat, they suck the seat belt socket down which tightens the belt holding you in place and also engages the locking mechanism. Additionally the airbag ECU will know the pre-tensioner has deployed and may decide to blow the bag too! The seats must be removed to get access, simple task with a torx driver, four torx screws hold the two rails of each seat to the floor of the car, start up front where the position is really awkward, then the back which is pretty easy. Disconnect the pre-tensioner cable and harness so the seat can be removed (or just snip the cable near the socket/plug to leave the most cable to reconnect. Solder wires (same colours together) when ready to refit.
Job: Rotary Coupling / Clock Spring / Squib Ring replace
Same deal with battery, ensure it’s been disconnected at least 10min. Remove the steering column shroud, for me three screws underside of shroud. From behind the wheel remove the (torx) two screws holding the airbag into the wheel, carefully pull the bag away from the wheel, a cable connector sits just behind the airbag, remove this pulling gently on the plug body. With the wheels straight ahead, put the wheel in lock and mark the relative position of wheel and dashboard to assist refitting. Using a long-armed wrench (the wheel removing one) loosen the nut but leave it attached (to avoid hitting yourself in the face with the wheel) Tap the centre boss of the steering wheel with a hammer and wiggle the wheel from sides (wherever it’s strongest) in short alternating push pull motions until the wheel can be lifted free of the steering shaft. The rotary coupling sits inside a plastic frame that holds the switching levers, first remove the two torx screws holding the light stalk body to the shaft, slide out by an inch or so to clear the auto-cancelling for the direction indicators. The squib ring has two connectors, the one going to the airbag, the other attached to the shaft is terminated with a socket. So in addition to unplugging, the slightly fiddly task of removing the socket is achieved by sliding a very slim object (blade/sheet metal, etc.) between the socket and the plastic mounting body from underneath, this disengages a plastic latch allowing the socket to be slid down and off. Replacement of new unit is the reverse process, ensure the wheel nut is tightened to the torq specified for your car. Haynes manual is pretty useless in this area, showing images of a hot of the production line phase 1 (mines the later phase 1) a confusing difference.
Job: Airbag ECU replace
Disconnect battery and wait as previously mentioned. This job looks impressive, but is really pretty easy. The airbag ECU is located at the base of the centre console, for me, just underneath the immobiliser. To get to this requires removal of the centre console, see Haynes manual. The airbag ECU is bolted to the floor and easily removed by lifting the black latch on the plug body then removing the orange plug. It’s good practice to reconnect the three other plugs in the area having sprayed with WD40. The brown socket in the picture where the grey bit is the plug, and two orange sockets sitting on spurs just below the photo hidden by a bit of carpet).
Friendly garage will charge £50 to try the pre-tensioner soldering fix, same again to replace the rotary connector. Though not quoted, wouldn’t be surprised to hear another £50 for replacing the ECU. So in excess of £150 saved by DIY. Parts which I’d have to locate anyway, or pay the premium, came to about £30.