Hi all. 05 XJR here. I've had the car for four months now. When I first purchased it I would get this message and occasionally the DSC message upon startup. Especially if I turned the ignition on and then let the car sit a few seconds before starting. IE, bad battery, makes sense. So I put a new battery in it two months ago and all was well. No more issues at all until today. Today the cruise apparently stopped working mid commute to work. I had used it in the first half of my commute, had to deactivate it, then at some point of the way along the second half, I went to set it again and got the "unavailable" message. This is the first time I've ever seen it happen mid drive. I pulled off at a gas station a couple miles before arriving to work and checked the front. No obstructions that I could see at all. I also cycled the ignition 4-5 times and it was working perfectly again every time. Write it off as a fluke or something more to look into?

 

 

What is the voltage across the battery terminals with the ignition in the OFF position when measured with a voltmeter?

 

I’m starting to think something fishy is going on with the battery or alternator as well. I used a foxwell scanner that the previous owner had and it came back with all sorts of faults. ECM faults, parking brake faults, ground faults, and a few low voltage faults.

With car off, voltage is 12.59-12.6. But that was after just getting home from my commute as well.

The parking brake message has never been present and only presented itself after trying to use the scanner.

 

Since charging is being discussed, the charging maintains ~14.5v from startup for exactly 5mins then drops back. (and comes on again as necessary who-knows-when)
I used to think it dropped back to "13.5v", but have since seen below 13v in the "idle period", so it might even stop for a while.
Anyone with a Classic Car (bit of a plug here!), should have at least a cig lighter voltmeter, which costs nothing on eBay.

The 5mins charging from startup is precise (unless you live in a climate where I wouldn't)
(That's with an already full battery. I wouldn't know what happens with a car left outside to it's own devices and wolves and scoundrels, as mine is on a permanent charger)

 

Running voltage with lights on, radio on, immediately after startup was reading 13.5

 

Update: just went back out and checked the car after being parked for a couple hours. Voltage with car off was 12.38. This time, voltage running with all accessories on was 14.3. So seems like the alternator is working? But is that resting voltage low?

 

Dunno about the current drag you have on startup, voltage should be 14.5v, but 13.5v for your problem is "above good" especially has you've already put in a new battery.

I don't think it sounds like battery. There are a bunch of flakey earth points to the ali body, although I'd expect them to throw other random errors too.

I'm guessing on your problem, except that I do monitor battery/charging.

(I would not expect a problem with 12.38. Or even down to 12.2 or less. That's just a normal battery without charging)

 

Idk. I’m at a loss. Maybe I jumped to conclusions on the battery. Just seemed like the most likely culprit. I just can’t understand what could be making it shutoff mid drive on both trips but then immediately be working again with a restart.

Not sure if it’s relevant but the car did just come back from my Jag Indy for a timing cover gasket, spark plug replacement, and water pump. But none of that should have affected electrics or the cruise control module right?

 

Another update. Whatever is happening is related to forward alert and/or cruise control. I set a timer when I set off on my work commute today. 52 minutes into the commute forward alert turned off on its own and the cruise would no longer work. Pulled over, restarted the car, and all was working again. I get no lights or warnings on the dash, except a message that says forward alert off, and then neither will work again until the car is restarted. What the **** is going on? I would rather complete a major engine repair than go down rabbit holes for things like this.

 

For all systems to function correctly, the available voltage at rest with the ignition OFF should be a minimum of 12.6 volts. If you have a reading of 12.3, charge the battery for five hours at 2 to 5 amps and retest the battery. If it's less than 12.6 volts, the battery is suspect.

Further diagnosis cannot be successfully carried out without minimum voltage being available.

 

I can understand that, but I've confirmed voltage is 14+ with the engine running. The problem is occurring with the engine running so how would the resting voltage be presenting itself as the issue here?

 

This sounds like iffy earth's to me. Check the ones behind the headlights, one of mine failed and the dash lit up like a Christmas 🎄.That was a brown trouser moment!

 

Perform a load test on the battery.

+1 on inspecting the earthing points (grounds) across the vehicle.

 

I’m not familiar with where they all are but I can do some research. What exactly am I looking for on them?

 

Are you charging your phone or other device in the car when the cruise acts up? The cruise in my 07 XJR stops working and says not available when I charge my phone in the car. When I unplug the phone charger the cruise starts working again.

+1 on checking the grounds for obvious corrosion. Be careful not to snap them off if you take a wrench to them.

 

Hi All. I had a similar problem a few years back on a 2004 model with Adaptive Cruise Control (ACC). When the ACC stops working or is intermittent the normal Cruise Control (CC) will not work and you get the 'Cruise Not Available' warning on dash. The early cars (2004 & possibly 2005) had a one-piece ACC system fitted behind the front bumper, whereas later cars had a split system. Early units are extremely rare and problematic. In the end my only solution was to fit a main ECU from a similar car (without ACC) and program that unit to the car (there is no way to delete the ACC from the system using SDD). I appreciate that voltage is important on these cars but, from the problem description, I think your ACC (if fitted) is the most likely culprit. Good luck either way.

 

Not charging my phone or anything of that sort. However, weirdly, for the last two days its been working fine. Looking for an opportunity to check the grounds and also load test the battery. Will report back.

Thanks for all the suggestions thus far.

 

Hey JTA,
You're running down the problem in a perfectly logical fashion; compliments.
Just a thought: if you do have adaptive cruise, try thoroughly cleaning face of the radar unit (behind front lower grille; LHS)
My car evidences same symptoms as yours when the radar face gets dirty (eg, long trips with lots of bugs, etc). It will work for a while after restarting, but then flag error message again, etc.