Today oi mostly be playing with explosives.

[CJJ]

Well, I thought I had fixed my SRS, but over the last month or so it has been coming on more and more again, to the point it was almost on all the time.

I have suspected the drivers side pretensioner since it used to come on when cornering hard, or lately, just when I knocked the seatbelt clip.

Since I had 3 different warning lights on yesterday, I vowed to sort them. MIL eliminator sorted one (I just had to reset the MIL), one was caused by the remap on the rolling road (apparently it confuses the hell out of the ABS system when the back wheels are going round and the fronts aren't) which reset itself after a short drive. So that just left the SRS.

I bought a second hand pretensioner but it had the wrong plug on it. Mine had a yellow plug and the new one had a red plug. The original belt clip had always been twisted as well.

So I decided I wanted the new clip, with the old cable.

WARNING: ALL OF THE MANUALS AND A WARNING LABEL ON THE PRETENSIONER WARN YOU NOT TO CARRY OUT ANY WORK ON THEM. THEY CONTAIN EXPLOSIVES. If you do follow any of this it is at your own risk.



NEVER use a multimeter on the pretensioner or airbag system as they generate voltages and this could cause it to deploy.

I am stupid/brave/inquisitive (Please delete as appropriate).

For those of you that are interested, this is how the pretensioner works.



To remove the unit you must diconnect it electrically (The book says you should disconnect the battery and allow the system to discharge). Definitely DO NOT disconnect the unit when the ignition is switched on.

You have to remove the seat for best access (4 torx bolts. one at each corner). Disconnect the plug under the seat. You don't have to fully remove the seat, but I found it easier.

Remove the Torx bolt holding the pretensioner to the seat and it looks like this.



Notice the clip twisted through almost 90 degrees.

If you do work on the unit, never cover the end of the cartridge, never point it at any part of your body, never hold it by the cable etc. If it goes off it will go off like a shotgun cartidge and all moving parts will operate in milliseconds (Ouch).

I also wore safety glasses and ear defenders.

Slide back the rubber outer to reveal the plug.



The plug simply pulls out with a bit of force. This leaves the nasty bit so that you can put it out of the reach of children and small furry animals.



I would love to know exactly what is in there. I expect it is just an explosive cartridge and a detonator.

This leaves the cable and connectors to work with.



I found some evidence that the cable had been crushed somehow, which is never good.



Also, after taking the cover off the plug I found my least favourite connectors, the ones that you push the cable into and they cut into the wire creating a connection. These are renowned for working loose and causing bad connections.




So I removed the cable (Metal cable restraint clips in/out of the casing) and cut it back past the damage. The metal cable clamps are crimped onto the cable and have to be removed, widened and fitted back onto the cable where it can be crimped back on with a pair of pliers. The newly cut and stripped ends are now placed back in the plug and the cables pushed into the metal cutters to create a new contact.



I checked the connection out with a multimeter (the explosive bit isn't attached at this point). Put the cover back on and reassembled the pretensioner with the new clip. Fitted it back on the seat and fitted the seat.

I have been out for a drive and so far the SRS light hasn't come back on. Only time will tell.

I suspect the push in connections rather than the crush damage, but I fixed both so hopefully that will be that.

If anyone knows the purpose of the inductive coil, or how the system works as a whole, please let me know.

[NandoDave]

I am not an electrical person, but I would expect the coil is somehow there to prevent accidental deployment in some way - i.e. by only allowing a 'real' trigger signal from the SRS system through?

[CJJ]

It's in parallel with the contacts, so I was thinking it was to do with the monitoring system. If it was a capacitor it would shunt spikes away from the detonator preventing deployment.

It might be that the SRS sends some sort of waveform down the line and expects an out of phase return due to the inductor.

Sorry if that just read blah blah blah.

[HotTubRepairer]

Is it definately parallel??

[CJJ]

Originally Posted by HotTubRepairer

Is it definately parallel??

Ooh no. Good point. Will teach me to look properly at my own pictures. It is in series.

[HotTubRepairer]

Originally Posted by CJJ

Ooh no. Good point. Will teach me to look properly at my own pictures. It is in series.

That'll teach you to pay attention.

What was the rod made of that the coil was wrapped around? Is it a Ferrite rod by any chance?

[CJJ]

Didn't really pay that much attention at the time, was too busy checking connections. Just started wondering after when I was doing the photos.

[HotTubRepairer]

Originally Posted by CJJ

Didn't really pay that much attention at the time, was too busy checking connections. Just started wondering after when I was doing the photos.

I'm not much for electronics but if its wound around a ferrite rod it helps to reduce static and electrical interference, ultimately its to prevent any unwanted detonation of the pretensioner.
I could quite easily be wrong though but it seems logical.

[CJJ]

Yeah, as a choke, like Nando said originally.

[HotTubRepairer]

Originally Posted by CJJ

Yeah, as a choke, like Nando said originally.

Pretty much as Nando suggests.

I use amateur radio equipment and I use a larger version of the ferrite rod and wind the 13.8v + around the rod. This helps to reduce interference from charging systems/ignition and other electronic devices.

Well done for braving the pre-tensioner, you wouldn't catch me doing something as risky as that.

[CJJ]

I had my kevlar vest and WW2 steel helmet on as well

I was just sick of the light coming on. It was either it or me.

[HotTubRepairer]

Originally Posted by CJJ

I had my kevlar vest and WW2 steel helmet on as well

I was just sick of the light coming on. It was either it or me.

You were one desperate man then.
Hopefully it's going to be ok again now. Have you checked the other pre-tensioner for the trapped cable? I doubt that it was the cause because it didn't look bad enough but even so it was well spotted.

[CJJ]

I still think it is the splice type connectors at the end. They are the same quick and easy blue/red plastic crap that you get with aftermarket vehicle electrical devices. You clamp them on the cable and they cut into the cable. Cause no end of electrical failures in alarms etc when they work loose.

[HotTubRepairer]

Originally Posted by CJJ

I still think it is the splice type connectors at the end. They are the same quick and easy blue/red plastic crap that you get with aftermarket vehicle electrical devices. You clamp them on the cable and they cut into the cable. Cause no end of electrical failures in alarms etc when they work loose.

I know what you mean and you may be right. They're awful things, I got something similar with a towbar electrics kit. They were thrown in the bin and out came the trusty crimping tool.

Did the yellow connection onto the pre-tensioner seem ok? Its frustrating that one device can have so many connections and therefore potential breaks within such a short distance!

[CJJ]

The actual yellow plug that plugs into the pretensioner body (the one with the inductor in) is actually a very positive fit. It is difficult to test the actual connection due to the ohmeter make thing go bang scenario.

It is interesting to note that the leads are interchangeable on the new and old units.

[HotTubRepairer]

Originally Posted by CJJ

It is interesting to note that the leads are interchangeable on the new and old units.

I thought that too. Has the F/TF got the usual wiring harness connector under the seat, if so you'd expect the pre-tensioner to be hard wired instead?

[CJJ]

Yes, but mine are yellow plug/socket with slightly different pin configurations. I don't think you are supposed to know about the other plug as it is hidden under the protective sleeve.

[HotTubRepairer]

Originally Posted by CJJ

Yes, but mine are yellow plug/socket with slightly different pin configurations. I don't think you are supposed to know about the other plug as it is hidden under the protective sleeve.

There are a lot of things we're not supposed to know about, thankfully we are gaining more and more knowledge on these forums. I'm still gobsmacked that you took it apart. You must have good life insurance.

Well done CJJ. I'd better get to bed, the OH will start complaining otherwise.
Kind regards,
Ian.

[CJJ]

This is the plug off my TF



And this is the plug on the pretensioner that I bought.





I have noticed since that nearly every one that is for sale on ebay is the red plug. Managed to track down a breaker who had some with a yellow plug but they wanted £75 each.

[Tim.]

Great work CJJ