www.jec.org.uk

I'd noted that my 2008 XJ TDVi had been "levelling up" (to borrow a phrase) when stopped at traffic lights rather more frequently than usual. Then after a spirited drive on some of Oxfordshire's not-so-pristine B roads, the AIR SUSPENSION FAULT message appeared on the instrument panel. Switching off, the light would clear, though sometime reappear perhaps 20 minutes later on a long journey.

Driving round town with the windows down, I noticed on both sleeping policemen and nasty undulations of several inches that there was a whooshing sound from the front. Now the light comes on after a few minutes, and the car is resting at what I'd guess is its lowest suspension level, and doesn't rise.

My guess is a blown front suspension air spring. The car's only done about 125,000 miles, so it should be OK, but with the state of roads these days.... Probably the frequent levelling up was an incipient failure, and a large jolt was the straw that broke the camel's back.

As per another thread on the X350 Electrics forum, my ODB tools can't get any DTCs. I will try to fix that first, but can anyone confirm my diagnosis above? Also, any hints about replacing the air spring - looking at the workshop manual, the torque settings are pretty high, and I assume everything's corroded up and never lubricated before assembly.

Reading the manual, I also need the ODB gizmo to repressurise the air suspension after replacing the spring, so I guess fixing the ODB issue is essential.

I also have an air suspension compressor piston ring kit I bought some time ago on the assumption it would be needed soon, so now would appear to be a good time to install it!

My commiserations if it is a blown suspension bag.
I recall some threads discussing the air suspension and seem to remember that the suspension may need to be recalibrated. However I think that this can only be done with dealer level diagnostic equipment and not our had held OBD II readers.

However I would be very interested to hear that I am wrong and it can be done.

Good luck with it all and please keep us informed of both confirming the issue and it's rectification.

All the best
Tony

My CANBUS-SCP problems will be crucial, as I suspect even Jaguar's tools won't be able to calibrate the suspension if the bus is broken. If I can fix it, my Foxwell handheld definitely has JLT-specific plug-ins that are supposed to handle the suspension - indeed, it has bits that claim to cover all the ills of a modern Jaguar. For some time I thought it didn't actually work, but now I realise the problem is with the bus, not the handheld.

The good news is I've found a section in the workshop manual covering testing and diagnosing the bus, and it's not all oscilloscopes (though some of it is, it's possible an old logic probe I bought from Tandy (remember them?) might do the job). It's buried in the Electrical chapter, section 418.00 Module Communications Network. This could be fun, or frustrating. I anticipate a day or so cursing while I try to poke multimeter probes into connectors that can only be reached by a contortionist!

I'll try to take notes an if relevant photos for a write-up, if I don't throw the towel in first...

Paul,

Good luck with your investigations. I admire your efforts. The principle of the Canbus is great, but it's the one area that you don't want to have a fault!

Looking forward to your results!

Cheers

Paul

Stage 1 following the workshop manual was checking the relevant fuses, all of which are in the main passenger fuse box, the one at floor level under the steering column and in front of the pedals, so you can indeed only reach them lying on your side, and all were reluctant to come out, having never moved, and all were fine!

A friend pointed me at the "diagnosedan" youtube channel, with loads of relevant stuff including, particularly, diagnosing CANbus. Bad news is it's easier with an oscilloscope, but the video has many useful pointers. First of which is the ECUs that my Foxwell device could contact may have stored DTCs, but the auto-scan wouldn't find them because it stalls when it hits the first device that it can't contact. In particular, the Foxwell could see the Instrument Pack, which, as mentioned, forms a comms hub with the other buses, so it might have some relevant bus DTCs; I'll look at that today. Also, I need to make come back probes for my multimeter (see that youtube channel to understand).

An update; I wired up an ODB plug with test leads and measured various resistances and voltages across the various bus lines, as per the workshop manual's pinpoint tests, just starting in order from the first, as I don't have sufficient DTCs to make a sensible start. It's amazing the car runs at ll, and is a credit to the CANbus designers. The bus is apparently able to continue to operate in all sorts of degraded states, and mine is pretty obscene! The bus wires themselves are OK, but measuring the bus to battery +ve gives 10.5 V, whereas it should be floating, so there's a short somewhere, though not actually to +12V, rather to 1.5-2.5V (depending on my battery's charge).

Actually, I did get a whole raft of DTCs from the Front End Module (FEM) and Rear End Module (REM), but they look a bit like ones I got from flat-battery-itis, though there are messages about shorts and failed connection to the SCP bus. I need to clear them, run the car a bit, and rescan. Also try the Instrument Cluster (IC) to see if it's logged anything, though I'm not sure it will talk to me. If not, that's serious, as the IC is a gateway between the various buses, so it ought to be communicating properly.

That might just tell me where to look - I think on the previous X300 series the IC was a weak point; it certainly was on the XJS.

A progress report:
Things stalled while I tried to get my XJ6 Series III to run - the ball joint in the gear selector rusted up solid and snapped when I tried to put it in Part to start; sorting out a replacement took time.
As to the XJ TDVi, I was seriously considering giving up and going electric - still an option - but I realised I hadn't tried unplugging the engine bay harness, which I know is damaged from OBD errors and the RESTRICTED PERFORMANCE, It's a faff to get to - you have to remove the cabin air filter box, which means taking the back engine bay cover off, and that in turn means removing the wipers. Whatever, unplug it, and there's no longer a short in the CANbus, so that's one problem resolved. My Foxwell OBD reader still can't see the Air Suspension Unit, but it can talk to the Parking Brake Module, which is on the same bit of cabin wiring loom, so it looks like a fault in the wires near the back seat. More stripping to be done. However, it's less crucial than I thought - I misread the manual to think changing an air suspension strut requires recalibrating. That's only needed for a change of level sensors or the ECU.

I looked on the Jaguar Classic Parts web site for a new engine bay loom. Over £2,300, but thankfully No Longer Available. eBay has second-hand ones for less than £100. I'll try our local breakers, who are on the breakers' part location web thingy, as they'd give a guarantee.

Onwards and upwards.

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.

Well done Paul, great perseverance!

I guess that's always going to be a risk on electronic systems, that if you don't use the expensive designated equipment, you run the risk of variable connectivity and results. But at least, you've got a positive outcome, apart from the need to still resolve the engine bay / EGR issue.

Good luck with the last step in resolving the issue!

Cheers

Paul