Hi all
This may be my first post, I was sure I had joined in 2017 when I got my previous X350 XJR, but maybe not!

Anyhow I have for the past 2 years had an X350 4.2 NA as a replacement, which has been pretty reliable (certainly more so than the R). However I am now "up queer street" as is the British saying, with a totally dead car. The only code I ever managed to extract was "P1797 CAN TCM/ECM Circuit Malfunction", but now even the OBD port wont talk to me! I currently have the car booked for a very expensive trailer ride down to one of the few people who are willing to look at Jags of this age, but wondered if anyone had any wisdom in the meantime? I have a brief timeline of the breakdown which may assist in suggesting something that would enable me to at least drive it to the garage rather than trailering (which is going to be difficult as the air suspension has dropped leaving it very very low).

1. Halfway through a 2 hour drive back home the dash displayed the "suspension fault" message. I made it home but the suspension was very low.
2. I checked the ride height sensors to make sure they were attached securely. The air pump seemed to come on fairly often so I ordered a rebuild kit. The car was already booked into a local garage in the next couple of days for a front right suspension bush to be attended to so thought they might spot something.
3. In the meantime over the next couple of days the car started cutting out at low rpm, the dash showing the "gearbox fault" message. If I stopped the car for a short time, or did a hard reset at the battery, the car would start and drive, the suspension rise, but would after a few minutes/seconds stall. Again I thought if I could get the car to the local garage they might spot something. My hobby code reader couyld only get the error code P1797 from the vehicle.
4. At the garage they replaced the front right suspension arm, but the broke down on their ramp and would not start. They plugged in their code reader but just got a slew of error codes, not just the P1797 I got. They admitted they were out of their depth, so I managed to get the car home over several short trips, each time doing a hard reset at the battery until the next stall. At this point the dash was displaying every error message it had (engine, gearbox, DSC, parking brake etc etc).
5. At home I replaced the battery with a new one, as the original preceded my ownership (2 years) and was of unknown age. This did not help and the stalling/error messages continued, and the P1797 was all I could read on the OBD port.
6. I then rebuilt the air pump with a new piston seal and dessicant; there did not appear to be a noticeable leak when parked, but when moving the pump seemed to come on more often than I felt was usual, or was slightly noisier than usual, one or the other.
7. After rebuilding the air pump I could no longer get any readings at all from the OBD port (just read "linking error") and the car would no longer even crank, though all other electrics were functional.
8. I freed off the electric parking brake so the car is pushable, however the suspension is currently very flat. That is where I am at now.

I looked on a few old threads here and the respondents answers were varied and complex, and some didnt bother to respond at all after receiving help (I promise not to be one of those douchebags!) Some found wiring breaks, some found gearbox modules to be failed. Others mentioned that the gearbox wiring plug was contaminated with oil. There does not seem to be a single common thread on P1797.

• I have checked and cleaned the gearbox wiring plug with contact cleaner with no success.
• Some people blamed water leaking from the pollen filter area down to the ECU begind the glove box, but I always clean leaves etc out of this area weekly as my neighbour has a lot of foliage, and the drainage tested with a hose appears to be fine.
• We have had an extended period of rain these past few months, and the car is parked outside, but to the best of my knowledge has not been driven through any deep or fast water.
• I have also checked and cleaned all the earth points, which were fine, and this changed nothing.

At the very least does anyone have any advice on jumping the suspension pump before it gets trailered away? The previous garage just dragged it off their ramp while low, damaging some of the undershields in the process

 

I don't have any good advice on the P1797, as you mentioned there can be a wide range of issues causing that. The code itself is for a CAN bus failure, which if that happened would certainly explain no crank/no start/no run as the ECM, TCM, and J-gate all need to communicate with each other for the car to function.

Your new battery would be my first place to start as well. Any subsequent steps will require an OE capable scan tool, you need to determine which modules are online on the CAN network and which aren't. The CAN bus includes the parking brake and air suspension modules, so some of your faults may be tied to a common failure. Once you figure out what is talking, then you can start trying to isolate where the failure in the CAN bus is. A scope would be useful here to see what's happening on the CAN bus. However can also try some simple resistance tests to see if that shows any dead shorts or opens in the bus wiring at the physical layer. That won't help if you have a failed module that is bringing down the network, though.

What year is your car? The network topologies did change a bit between 04-05 and 06-09.

Unfortunately you won't be able to "jump" the air suspension pump to raise the car (you can easily make the pump run, but the solenoid block in the trunk has to be activated to raise the 4 corners). What you might try (this is a long shot) is jacking up the car at each corner with the battery disconnected. Let it stay raised for 5-10 minutes at each corner. I find that if you jack up the car and the ASM is off, the system will naturally pull air into each air bag, and when you let it down it will be above ride height. This will not really work if any of the airbags are leaky though.

 

It sounds like a frustrating situation with the X350, especially with the air suspension making transport tricky. The P1797 code often points to communication issues between the TCM and ECM, so checking wiring connections and fuses could be a starting point. Even temporarily restoring power to these circuits might let you move the car safely to the garage.

 

Hi Trophy,

Welcome back to the Jaguar Forums! It's great to have you with us.

My first thought as I was reading your account was that your ECM may be suffering from water ingress, so when I got to the part of your account where you suspected this too, I expected to see that you had removed the cabin air microfilter box, disconnected the ECM electrical connector and inspected the connector pins for corrosion. Personally, I would recommend this as your next step. Just because water will flow out of the ECM compartment drain doesn't mean that water has not pooled up around the ECM electrical connector and contaminated it.

To detach the electrical connector from the ECM, you will need a special 5-lobe security Torx bit (the type with the hole in the center of the lobes). You can buy an inexpensive set of these bits on Amazon or ebay. You need 5 lobes, not the common 6 lobes. Here are a couple of options I found with a quick search of Amazon:




The air suspension diagnostic trouble codes (DTCs) are Chassis (C-prefix) and Network (U-prefix) codes that cannot be read by generic OBDII scan tools that can only read Powertrain (P-prefix) codes. But it is highly unlikely that the air suspension issues have anything to do with the no-crank no-start condition.

The full list of DTCs that were scanned by the shop would be very helpful, if the shop can provide them to you and you can post them here, along with the model year of your car as mhamilton requested. To read all the codes in your car, P-, C-, U- and B- (Body), you don't have to have a Jaguar-specific diagnostic system. Today's high-end professional scan tools such as those made by Autel and Launch can read all the codes.

Cheers,

Don

 

Hi thanks for the reply. The car is a 2007 X356 and for the last 2 years has been very reliable. The previous owners were careful with it too.

I have got the wheels in the air at the moment hoping the bags may draw air in slightly, but I don’t have high hopes! It’s a bit of a design flaw really that there is not a method of gaining ride height for emergencies or breakdowns like this. The car is really vulnerable, on top of which I hate cars being pushed around especially with soft ally panels.

 

Wiring was never my strong point sadly, and with the advent of CAN I pretty much gave up. I wanted a simple lights on buzzer on my DB7 Volante recently and just couldn’t make it work! I felt like such a numpty.

 

Thanks for that. I didn’t even know a 5 point Torx existed! No I didn’t remove the cowl and pollen filter housing completely as the drainage seemed perfect, but I will give that an inspection as soon as I can.

I have some less than great photos of the garage scan tool readouts taken in the heat of the moment, there were pages of them. I’ll see if I can edit them to make them more readable.

 

Hi there, Just found the pictures of the scan codes. They actually make a bit more sense now people have explained some. The only ?legacy? fault I had was a non functioning rear parking sensor, which went off last year during a heavy rain spell.

Does this all mean anything to anybody?

 

One quick check you can make on the CAN bus is for proper terminating resistance and shorts/opens.

You'll need a digital multimeter, put it on the ohms scale. Disconnect the vehicle battery so your readings are not affected by operating voltages. Check between pins 6 and 14, the result should be 60 ohms +/- 2 ohms. If you are measuring 120 ohms, open, or zero, there's an issue.

Also check from each:

• Pin 6 to ground
• Pin 14 to ground
• Pin 6 to 16
• Pin 14 to 16

Let us know the results. Maybe there's an open you can track down with basic tools.

 

This can't make sense to us unless you post all of the codes and definitions. Otherwise we're all trying to diagnose in the dark.

 

I uploaded the pictures last night but they seem to have disappeared 🤷‍♂️ will try again.

 

 

OK reposted above, no12. Let?s hope they stay stuck!

 

Whenever I see that many seemingly unrelated codes, I suspect low battery voltage to the computer modules while cranking the starter. Since you now have a no-crank condition, have you checked all the fuses, including the megafuses in the boot? Is it possible something was shorted either while the shop was working on it or while you were rebuilding the air compressor?

I would try clearing all the codes and then see which codes recur when you attempt a re-start.

Regarding the grounds, you really can't tell anything by visually inspecting them. The corrosion that forms on the aluminum studs, eyelet terminals and nuts is microscopically thin, but it is aluminum oxide, which is an insulator (it does not conduct electricity). It appears as a very fine, white powdery coating.

Have you been able to check the ECM for water ingress?

I don't see that you have told us the model year of your car, which can make a difference in diagnostics.

Cheers,

Don

 

Most of these codes are saying the modules on the CAN bus can't talk to one another.

I would suggest checking the can bus terminating resistance as I posted in #9 above, then report back. If you find an open circuit or missing terminating resistance, then we have a path forward. Otherwise we're all just guessing. It is possible a module can lose a power or ground and cause similar symptoms, but this test will confirm whether it's a broken CAN wire or something else entirely.

 

Hi there

I was wondering if I shorted something with the compressor. I looked at the fuses in the footwell and bonnet, but forgot the boot, will check.I did see on another video of a Land Rover with problems that a visibly intact fuse failed under continuity with a multimeter. I'll have to dig mine out, continuity and earth is the only thing I understand how to do with it!

I cant clear any codes at the moment with my little hobby reader as the OBD wont communicate sadly.

I havent checked the ECM as the 5 pointed torx are still in the post!

I did clean the earth points and eyelets with emery paper even though they were already shiny.

Sorry forgot the model year, its a 2007 re-imported from Japan.

Cheers
Mark

 

Thanks for the advice and diagram. My,ultimeter is lost in the garage since moving house, I'll have to go digging in there today!

 

As Don mentioned every code you have most likely is related to a voltage problem. Have you checked the +ve connector under the car. It can get corroded. At this point I am hoping the garage has checked your battery and alternator output.

 

Hi there, I checked the earths and connections under the car, starter etc, all were tight and clean. As you will know corrosion here in Britain has always been a problem, which is why I bought this car as a Japanese import, it is very clean by comparison to a car that has spent its whole life here in our climate. I wasn’t expecting problems like this sadly.

Im afraid I reached the limits of my abilities and the car has been trailered away to an electrical specialist. They should be looking at it today so hopefully a diagnosis and solution is not far away.

 

I'm with Don B on this I'd start with a fresh battery, if that doesnt fix anything, then check as many earths as you can get to, especially the ones under the headlights.
far too many codes far too suddenly, unless it is a major module failure, then your in deep, So I'd start with a battery issue first and foremost