Since I bought my 2014 a couple of years ago it has been amazingly solid. I have put about 13,000 miles on it since then with no problems. So boring compared to my 2005 Maserati, which was a bit of a lemon if I'm being honest about it.

Recently, though, there have been several instances of "gremlins" in the electronics. In one case, the dashboard went black. It came back to life after a few seconds. In other instances, I got serious red alarms which went away so quickly I couldn't even read what they were for. An ABS system failure, which went away immediately. These happened at full speed on the highway with cruise control engaged, and they caused the cruise control to be canceled, so it can be dangerous in traffic.

Could this be a battery ground connection? I had some issues with that in the Maserati which "bricked" one of the control units.

 

The dashboard blacking out has happened to me once and it was very confusing. You may try checking the large battery cable connections underneath the car. If you are missing any of the under-trays, these can be exposed to water and corrode.

 

My first stop with an issue like that is to check the battery. If it's more than a few years old, it's likely due for replacement. Modern cars are heavily dependent on a quality electrical connection so a weak battery can manifest all kinds of gremlins.

 

+1 on starting with a check of the available voltage.

With the ignition in the OFF position for at least 30 minutes, use a voltmeter to check the available voltage across the battery terminals. There should be a minimum of 12.6 volts indicated on the voltmeter, if not, charge the battery for three to five hours at 2 to 5 amps and retest. If the voltmeter still shows less than 12.6 volts, the battery is suspect and should be replaced.

Many electrical 'gremlins', random DTCs and instrument cluster warnings are the result of a failing or weak battery.

 

By my experience the instrument dash blackout is a sign of low battery.

 

I agree with above member comments. Anomalies like you're experiencing are likely the result of a weak battery low voltage condition. We assume you didn't change your driving habits such that the battery doesn't get a full charge,

 

OK this is interesting, I suspected it was possibly a battery connection but I also thought this battery is less than 2 years old. In any case, a further thing that has begun to occur is that the ECO stop-start function is failing. Sometimes it turns itself off and reports a fault, other times it doesn't show anything but it just won't shut the engine off. I don't know if all these behaviors are connected to the same problem.

 

Safulop, here are all the various system relationships that inhibit the stop start from functioning. Any one of these can be not within limits or beyond thresh hold and cause the eco stop/start to not function. A low battery voltage at any of these sensors can take it out of acceptable range and inhibit the stop/start. Did you check battery voltage as recommended?

Auto Stop/Start System Inhibits



•  
  • Auto stop/start off switch has been operated by driver
  • Transmission not in neutral
  • Hood open detected
  • Driver door detected open
  • Driver seatbelt not fastened
  • All terrain progress control special programme selected
  • Climate control demand exceeds calibrated threshold
  • Heated windshield operating
  • Trailer connected
  • Brake booster vacuum below threshold
  • Engine coolant temperature below threshold
  • Engine oil temperature below threshold
  • Battery state of charge low
  • Battery cold cranking capability below threshold
  • Ambient temperature above 40°C
  • Ambient temperature below 0 degree C

 

There is a common misconception that the small Aux battery is used for the Stop/Start system, it is not. The engine is always started from the main battery. The Aux battery is there to maintain electrical integrity when the engine is off during a Stop/Start cycle. If the main battery, or the Aux battery, are not performing to spec, the Stop/Start will not work.

wombat

 

Yes this points to a battery problem for sure. That is a good thing!
As NBCat posted above can you post what the resting voltage is after the car has sat all night?
.
.
.

 

If you leave an ELM327 Bluetooth scan tool dongle in the OBDII port, and don't remove it when you turn off the engine, when you start it the next time your dashboard might light up like a Christmas tree. Mine did, and the cure was simple. See posts 6 through 10 here.

 

The auxiliary battery was 8 volts so, not good. This is the likely cause of the issues, it is getting replaced now. The main battery tests good.

 

safulop, If you disconnect the ground lead of the small battery that bolts to the chassis, you will disable the eco stop/start and the small battery won't matter anymore. Of course, if you want to keep the stop/start feature that battery must function at the right voltage. I don't like the stop/start feature and feel it is an unsafe nuisance. I have the same 3 liter 2014 model year and have had my stop/start disabled for years with no consequence to any other function,.

 

The aux battery is replaced now, the part cost $478. I'm not interested in any life hacks or cheapskate fixes and workarounds I only want the car to be original spec. With that battery on low voltage, there were a ton of low voltage related faults in various modules.

 

Be aware that the small battery has nothing to do with low voltage errors? It's only function is too keep the car alive with the engine off.
The car uses and runs off the main battery. Including ALL starting functions. That little battery could not start an engine no matter what!

Think about for a minute? If what you say is true how can we remove the small AUX battery completely without any problems? Because the car totally does not need it to operate.
.
.
.

 

As the Zen Master said, "we'll see."

 

Since the auxiliary battery was replaced, the car is much improved in many ways. The Eco stop-start feature works very well now and is easier to manage and predict as a result. All the other gremlins and alarms have stopped happening. Furthermore, the car performs much better as well. Previously, in D mode it was very sluggish, refusing to downshift unless the gas pedal went all the way down, and even then only after a huge lag time. I was always driving in 'S' mode to get better response. Now the performance of the transmission shifting in D is much more as would be expected.

 

Here is what an AI internet search says about it:

Symptoms of auxiliary battery issues in the Jaguar XJ X351, such as morning low voltage warnings, delayed system initialization, and stability control or battery management warnings, are commonly linked to a degraded auxiliary battery, especially after periods of inactivity or cold starts.A reading of 10 volts on a newly replaced auxiliary battery is not normal and indicates a faulty battery, potentially with a dead cell, rather than a charging issue.The auxiliary battery is crucial for the vehicle's start-stop system and other electrical functions, and its failure can lead to the "Auto stop start not available" message and other system warnings.Replacing the auxiliary battery, which is typically an Exide EK151 or a similar specification, is often the most effective solution to restore normal system function and eliminate these warnings.

 

Dump AI it's garbage!
The AUX battery has nothing to do with any of that? The purpose of the AUX battery is to keep the car alive with the engine shut off at stop lights. That's it. You can easily prove this to yourself because you can completely remove that small AUX battery from your car with zero problems. Many of us have done this and drove our cars for years just fine.

I would focus on the main battery as that's where all the problems come from.
.
.
.

 

The AI cites all its sources, mostly posts on Carly to furnish that information. I use AI to assist in my work, it's quite capable of assembling and summarizing under the right circumstances. It didn't invent these ideas, it's merely repeating what has been found online.

Quite often "experts" have diagnosed my car online, or rejected a diagnosis, because of their own biases and turned out to be wrong. When I had suspension noise in my Maserati, my shop found the front shocks had broken down and replaced them. A technician on a forum insisted that "those shocks will never break down at 60,000 miles, they are wrong with their diagnosis".

Why would I believe someone telling me to remove a functional part of my car permanently for no reason based on a theoretical understanding of the system? I believe what I've seen actually happening with this car.