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.