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I hooked up the scanner with the car at idle and this is the voltage test, no ac or lights or screen on. Does this suggest alternator is on the way out?
The centre screen started continually rebooting again and they wanted to try a hard reset. That seemed to do the job and I asked them to check the battery status which is when they came up with the alternator starting to be on its way out. Think I will hold off on that until I get some other things done first!
The centre screen started continually rebooting again and they wanted to try a hard reset. That seemed to do the job and I asked them to check the battery status which is when they came up with the alternator starting to be on its way out. Think I will hold off on that until I get some other things done first!
I done the alternator and all the belts and pullies whilst at it and it's a days work in a garage give or take, the supercharger is more difficult than the NA
The centre screen started continually rebooting again and they wanted to try a hard reset. That seemed to do the job and I asked them to check the battery status which is when they came up with the alternator starting to be on its way out. Think I will hold off on that until I get some other things done first!
Ah yes I recall you posted on this before and this was my advice. Still holds and you can do this.
It's a classic symptom of a failing module in the MOST bus fiber optic ring. The flashing symptom is the faulty module attempting to reboot. The fiber optic ring is being interrupted, There are two stacks within the ring. Trunk stack and center stack. The center stack is the stack of modules under your touch screen. They include the touchscreen, the ICM which is a gateway between canbus and fiberoptics (so they can speak to each other). That's the little black box on top of the CD player. Then there is the CD/radio Player. If you have premium audio, there is also the amplifier under the carpet in the passenger footwell.
In the trunk there are a stack of modules. Bluetooth module for you phone, The Navigation DVD module (maps) if you have satellite, that module, your iPod module. BT, satellite and Navigation are the usual suspects. You need a fiber optic bypass loop. Simply disconnect the fiber optic line from the module, pick one at a time, and connect the bypass loop to the fiber optic line. Turn on your car, if the problem stops, you found your offending module. If not, reconnect it and move on to the next one.
I done the alternator and all the belts and pullies whilst at it and it's a days work in a garage give or take, the supercharger is more difficult than the NA
Just a minute... Is there a noticeable squealing noise or a noticeable "rumbling" noise from the generator?
Mine was still kicking out the juice even when the bearings were completely shot. At first, all of us thought it was the supercharger snout coupling...
Just a minute... Is there a noticeable squealing noise or a noticeable "rumbling" noise from the generator?
Mine was still kicking out the juice even when the bearings were completely shot. At first, all of us thought it was the supercharger snout coupling...
Mine sounded like a turbine engine, worse when cold.....yep an extra supercharger
@alanroberts50 Your 2013 XKR came from the factory with a Battery Monitoring System module attached to the negative (black) terminal of your battery. If a battery charger or battery maintainer was ever directly attached to that negative battery terminal, your BMS Module could be damaged. Also, damage to that BMS Module may have occured if you ever hooked up jumper cables to that negative battery terminal.
A bad BMS Module could be the cause of your problems. See this thread on how to reset it. BMS reset
5.0L owners should always use a chassis ground. Never attach anything to the negative battery terminal. Doing so could brick your BMS module.
Stuart
P.S. I forgot to mention that most auto technicians are clueless about the BMS Module, unless they're Jaguar -trained. That module causes the alternator to behave kinda funky, which can lead a technician to believe that the alternator is defective when it isn't.
Last edited by Stuart S; Jan 10, 2026 at 10:26 AM.
Reason: Added P.S.
Guys, you're jumping on this alternator like it's something to check first. His output looks fine. He's not complaining of noise. He has a touchscreen that occasionally goes black. He should be testing the battery - must have done that because shop said he needs an alternator. Scan shows alternator output in his post. Makes little sense to chase the hardest and least likely cause first. He can simply use a bypass and check each module. Sure he could remove the alt belt and check the bearing but go for the easy stuff first. Let's try to save him some time and money.
The easiest way to see whether BMS + battery + alternator are working properly is to plug an eBay cigarette lighter socket voltmeter in and keep an eye on it as you drive.
The BMS behaviour means you will see "charging" voltage varies from 12.4 to 14.8. I've known it to remain at 12.4 for a couple of hours whilst BMS does a battery discharge test.
Like a smart battery charger voltage can read from 13.5 to 14.8 during "bulk charging", remain at 14.8 for desulfurication, but also runs an occasional battery capacity check by allowing a controlled discharge down to about 12.2.
Before knowing this, when I first read the charging voltage on a fully charged battery I was left thinking there's an alternator fault, as it read just 12.4 with the engine running.
The only way to properly appreciate how it works is to cast the occasional eye on the meter during a journey. For instance, BMS can increase the charging rate when you decelerate or brake and you will see the voltage increase.
I hooked up the scanner with the car at idle and this is the voltage test, no ac or lights or screen on. Does this suggest alternator is on the way out?
There is every likelihood that your alternator is working just fine and in unison with the BMS🙂
The easiest way to see whether BMS + battery + alternator are working properly is to plug an eBay cigarette lighter socket voltmeter in and keep an eye on it as you drive.
The BMS behaviour means you will see "charging" voltage varies from 12.4 to 14.8. I've known it to remain at 12.4 for a couple of hours whilst BMS does a battery discharge test.
Like a smart battery charger voltage can read from 13.5 to 14.8 during "bulk charging", remain at 14.8 for desulfurication, but also runs an occasional battery capacity check by allowing a controlled discharge down to about 12.2.
Before knowing this, when I first read the charging voltage on a fully charged battery I was left thinking there's an alternator fault, as it read just 12.4 with the engine running.
The only way to properly appreciate how it works is to cast the occasional eye on the meter during a journey. For instance, BMS can increase the charging rate when you decelerate or brake and you will see the voltage increase.
Shop says new alternator?
