Last week I was asked if I would look at a 1999 Vogue that had a permanent SRS light and Airbag Failure showing on the dash. The owner had recently swapped the driver's seat and assumed he had done something wrong but his Nanocom wouldn't connect to the SRS system so he was unable to identify the problem. As the MoT is due shortly and it wouldn't pass with an SRS light on, if it can't be fixed, a local breaker had offered him £500 for the car to break for spares.

I went to have a look at it and first thing was to try my Nano on it and found exactly the same as he had, a Nanocom would connect to every system except the SRS. As that uses a dedicated wire from the ECU to the OBD port, that was the first thing to check. Sockets on the port were spotless. So the next thing to check was continuity between the OBD port and the ECU. As that lives under the rear of the centre console, easier said than done but we got the centre console out and got to the ECU. Identified the wire and found continuity to the OBD port, so not that then. As diagnostics wouldn't connect to the ECU and the SRS light was on permanently, decided it could be a dead ECU. Took it out and took the cover off. Looked perfect inside with no signs of water ingress (difficult considering where it lives) or burning and the ball bearing in a housing that detects impact could be heard rattling around. A quick call to the local breaker (about 12 miles away) who told me there was a car in his yard, a 2000 so it would be the same, with the interior out so he was happy for me to go and help myself to it. Got there to find that although the seats were out, the centre console wasn't so set about removing another one. Got the ECU, went back to the car, fitted it and no different. Nanocom wouldn't connect and SRS light on all the time. Checked the plug for the ECU and didn't find power on any pin, only a 3.8V signal on one wire which I assumed was a data line. As I didn't have my laptop with RAVE with me and the owner of the car had printed workshop and overhaul manuals, but not the ETM, gave up on it.

Went back today with the SRS diagrams printed out as well as the laptop in case I needed to look at any other parts of the ETM. Identified the pin that should have an ignition switched supply to the ECU only to find it was the one with 3.8V on it. It goes via the RH footwell connector so that was the next place to look. Contacts all clean and not corroded and 3.8V on both sides of it on the wire to the SRS ECU. The feed comes directly from fuse 23 in the fusebox, checked that and found 12V on both sides. Lifted the fusebox to check what was coming out of it and while there may be 12V at the fuse, there was only 3.8V on the connection on the bottom. There's actually two separate wires coming out of the fusebox from fuse 23, one to the SRS ECU and one to the SRS circuitry in the instrument cluster. There was continuity between both and both showed 3.8V. Connected a piece of wire to one of them, turned the ignition on and connected that wire to the battery. SRS light went out immediately, Nanocom would connect and all it showed was a historic fault for the drivers seat from when it had been swapped. That said the problem was definitely inside the fusebox. Another call to the local breaker, another 24 mile round trip and came back with two fuseboxes, both from 2000 models and both had been working fine, I was assured.....

Off with the original fusebox, on with the better looking of the replacements, fired it up and everything worked. SRS light went out, no Airbag Fault on the dash and went through all the electrics to confirm we hadn't fixed one problem and put another one on in its place. So fusebox failure isn't always obvious, it doesn't always cause a burning smell but can cause all sorts of odd problems. A new one on me but somewhere to check in case of an SRS warning coupled with no communication with diagnostics.

Thanks for the thorough explanation Richard.

Tnx, good to know

That is an interesting story indeed ... now imagine how to explain this to a "normal" customer in our "modern", click-away tik-toked world.
A car to be thrown away for the want of a fuse connection ... a normal service/workshop would have gone crazy on this, because you need to be persistent and knowledgeable on what you are dealing with ... having the 'illuminated' mind to go pursuing alternatives - and have other replacement parts ready at hand ... truly a rare feat nowadays!

An interesting story & slightly worrying as a THOR fuse box is NLA as a new part although those for the diesel & GEMS are still available (the latter 3x the price of the former).

I must admit, on the way home I thought I should have asked the owner if I could take the fusebox so I could pull it apart and repair it, or at least investigate where the fault had occurred. I've done one before and got a GEMS one in the garage waiting for the day when I've nothing better to do. It does seem to be an odd failure though as it isn't as if that circuit is going to be pulling a huge amount of current to burn something out.

That would have been a good idea, it seem fuseboxes as they become more expensive or unavailable they will become more scarce in the breaker market, and from what I hear of people who have repaired them, it is worth the try if not completely burnt.

And to be fair to the “normal workshop”, troubleshooting can eat up a lot of man hours. Most customers don’t want to pay hundreds of dollars/pounds to figure out their issue.

Agreed but this was probably the simplest bit of troubleshooting possible. Low voltage at the ECU, low voltage at the only connection between the ECU and the fusebox, low voltage at the output of the fusebox but full voltage at the fuse. Had I had the ETM with me on the first visit, it would have saved the trip to collect the, not required, replacement ECU. It must have taken far longer getting access to the ECU that to actually identify the fault. Its a case of following things through in a logical order, something many seem to have forgotten how to do (if they ever knew in the first place) these days.