Another update, sort of definitive. And long, but hope some of it is of interest.
I decided to try as many of the relevant "Pinpoint tests" in the workshop manual as I could. That requires unplugging the various ECUs in the chain from the OBD socket to the Air Suspension Unit (which is the one I was concerned to contact). That, of course, is behind the rear seat. The manual says to remove the boot lining, the rear seat bench and then the seat back. Turns out that only applies if you want to replace the ASU; to unplug it you can ignore the boot.
Next in the chain is the J-gate module, so you need to remove the centre floor console. While that was out, I also removed the Instrument Cluster, as that's a vital gateway between the CAN bus and the SCP bus (which does stuff like seats, interior lights and doors). (I also removed the connection to the Yaw Rate sensor, which is by the J-gate, but that turns out to be on a private CAN bus directly to the ABS ECU, and therefore irrelevant.)
Next is the ABS/DSC module, which is really just the brake master system box in the engine bay, with a somewhat weird connector that latches and locks (probably to keep water and muck out really well). (From there it goes to the egine control module and the gearbox, but I wasn't interested there).
The pinpoint tests check for continuity of the bus wires, and insulation to earth, battery +ve and switched +ve. All of which seemed OK. So I plugged everything back, and tried my Foxwell scanner again. Same as before
but at least I'd cleaned the contacts with all this unplugging. What's worse, I now had DSC faults, and the information display scrolling through loads of messages and the instrument panel flashing like a demented Christmas tree.
At this point I decided to scrap the car. I used motorwise.com - who's web page said it was worth just over £1,000. Then when they phoned me and I said I hadn't started it recently because the battery was flat and my charger had died (water ingress) they said only £500 as a non-runner, but that was still well better than the actual scrappers would offer. I reluctantly accepted.
The following day, my new charger arrived, so I filled my boots, or rather the accumulator. And the engine now started and ran happily, and the warning lights and messages all vanished. Hmm... A document on the web caught my eye - the Jaguar dealer training "Multiplex Advanced Diagnostics" for 2002, and I later found "Advanced Electrical Systems and Diagnostics" from 2008 equivalent. The former had a rather simpler test of the CAN and SCP buses:
Disconnect the battery, unplug the ABS ECU, reconnect the battery, and switch on to position II. You should see a number of warnings, DSC and so forth, that show various modules no longer able to communicate. I did. Switch off, reconnect the ABS and battery, switch on, and the messages should go. A pass!
For the SCP, simply use the remote control to unlock the doors and open all the windows, and check the "door open" warning lights work. That covers rather a lot of the network. Another pass!
Next you can test the bus rather more simply using a multimeter. From the OBD connector the resistance between the two bus lines should be 60 Ohms. It was, so that's continuity to the ends of the bus where the terminating resistors sit. Then, with power on, the voltage from one of the bus wires to earth should be about 2.4 V, the other about 2.7 V ("about" because it's actually a mean of the %age of the time the wire is low and the % age high, so it can vary). (If it fails, you can repeat the tests from each ECU until you can narrow down the problem to one bad length of cable or ECU) Drat! 1.36 V, a fail.
Nothing on the Internet fitted this symptom. I perused the wiring diagrams once more. Ah, there are two earths in the UBD connector: Pin 4 is the chassis earth, but Pin 5 is the signal earth. I tried again, using Pin 5, and lo and behold, 2.37 V and 2.7 V, another Pass! (According to the wiring diagram, both earths go to the same tag behind the passenger airbag (where I don't date venture), but maybe that's just diagrammatic, or maybe the OBD socket is damaged. Whatever.) It appears my communication problem with the ASU is in the Foxwell scanner, not the CAN bus!
I phoned up to cancel the truck to take the car away. I'm going to fix the suspension (liberal application of dosh for two Bosch front struts from SNG Barrett), and get a replacement engine bay loom to hopefully fix the EGR problem - though I'll do the relevant Pinpoint tests first to be sure it's the loom and not the EGR valve.
