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Being new to Jaguar ownership experience the wife and I are pleased to join and learn from those who have gone before us and as a modern-car novice and computer incompetent I hope you will excuse my errors and ignorance.
That said I have usually done my own car maintena2nce and repairs for 45 years and hope to continue doing so with our latest transport : a 2010XK, I will try my best with the technology.
We bought the car privately late last year, promptly SORNed it for the winter and carried out subframe corrosion proofing.
Taxed for the road from April we've had some good runs and also discovered some quirks.
Quirk 1 - battery rapidly discharges when the car is unlocked. Is there a way to get around this?
Quirk 2 - I connected a charger to maintain the battery charge whilst working on it whilst unlocked and the battery remained connected to the car. During this time I switched the ignition on to check lights.
Am I correct to think this upset the battery management system?
Because the next time the car was started the alternator stopped charging the battery after a couple of minutes engine running.
Stopping and restarting the same short period of charging happened, and the same again next day. (i have a voltmeter thats plugged into the cigerette lighter socket)
After disconnecting the -ve battery clamp for a few minutes the alternator seems to work again as normal.
I hope thats normal and anyone else experiencing apparent alternator failure after turning the ignition on (not starting the engine) could try disconnecting the battery to reset the Battery Management System.
Attached (I hope) pics: our XK on axle stands because theres an oil leak and the ebay cigarette lighter socket voltmeter
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The alternator is controlled by the battery providing positive current to the single lead wire from the ECU to the alternator
this wire activates the electormagnet inside alternator.
The ECU can turn the alternator on and off as needed based on engine load and charging requirements however
a bad alternator would give you an error on your screen "not charging"
I'd say your battery is probably needs replacing, or a full charge at best.
Unless you’ve recently replaced the battery with a new known good one, the odds are in favour of a failed battery. Quite common regardless of age. So check that first. Many threads here about spent batteries.
Appreciate the info.
It's a new battery and there's no message to say not charging.
I'm pretty sure I confused the battery management system and since "rebooting" it has been ok.
If someone else finds themselves with the same issue the following account might help.
Here's how things went -
The led volt meter in the cigarette lighter socket read approx 14 volts for a couple of minutes after engine start before dropping suddenly to 12.8 then reducing to 12.4. That told me there's no alternator output current so I stopped the engine and resorted to our ford Mondeo for the day 😕
I'm very glad of this £5 eBay led meter as it saved getting stranded a few miles down the road.
The next day when restarting (the battery is still good) there was the same initial couple of minutes charging from the alternator before it stopped providing current. No message on the dash again.
The same symptoms had occurred once before and been cured when i changed the old battery which had seemed rather weak.
I also recalled that prior to both these occasions I had been charging the battery whilst connected to the car and had, on the most recent occasion and possible the other, turned the ignition on to do something or other.
So, I figured, the "fault" wasn't the battery or wiring/connections but maybe the alternator electronics or the BMS.
I read up on the BMS and whilst not taking it all in realise that it's a really clever system that takes a voltage and ampere history into account in its control of the alternator output.
Doing the easiest thing first, I reproduced the previous changing of the battery by disconnecting the negative terminal for a couple of minutes.
Pleasingly the engine started up fine, the alternator output raised the voltage to the normal 14 to 15 volts and it stayed there. We've done a few journeys today.
Hopefully that's it sorted out by a simple "turn it off then on again" with the use of a 10mm spanner.
In the meantime I read that the BMS goes through a new battery learning process that can take many miles whilst it monitors voltage and current during a full battery charge from the alternator and controlled discharge. I really don't know how it uses this learnt knowledge but I do feel rather uneasy about interrupting it🤔
The lessons learnt:
1. Don't turn the ignition on whilst charging the battery. Maybe don't unlock - that's something to verify.
2. The Battery Management System is very sophisticated and I don't know how exactly it decides it's commands to the alternator. (A software thing)
3. The BMS program doesn't account for an interfering human with a battery charger.
As an electrical engineer this is all fairly interesting but really I like things simple and without the invisible software.
You're doing it the right way. Figure things out. Mechanically and electronically, this is a very straightforward car. Parts from JLR are stupidly expensive, but a good number of OEM replacements are available from Bilstein, Lemfoerder, Eurospare, Bosch and many others for half the price of the Jag parts.
Go to the stickys, get the maintenance manual and begin reading it. Also, get the 5.0 electrical guide in the stickys so you can trace relevant circuits. Finally, the JLR software diagnostic system is SDD. Very effective, but awkwardly written and clumsy to use. In the meantime, for $70 you can get the Autel AP200 with the JLR download. Does about 80% of the typical data gathering and a few of the service routines. Later on if you should ever want to reprogram or replace the various ECU's, SDD can give you the opportunity.
Thank you for this advice.
I found the Jaguar service manual which is really useful and includes good explanations of how things operate. The electrical guide too.
I've wondered if it's worth buying Haynes software manual; does it add or does it just repeat?
I have a THINKOBD 100 that deals with generic codes. I'm definitely interested in getting a more specific tool and will be researching in the future.
Been abroad for a few weeks so no XK driving for a while, and being of a cautious nature we took the t/rusty Mondeo to airport. Happily, upon returning home the XK started straight up😁, as it should, no trickle charging whilst away so probably would have been ok in airport parking.
To continue the BMS behaviour story: visiting rele's a hundred miles away yesterday and about half an hour in I noticed that volt meter is only showing 12.5v, it soon drops to 12.4 and I am worried and thinking where to make a safe stop if voltage continues to drop or engine dies.
However, we 'bravely' continue and the vehicle voltage stays at the 12.4 to 12.5 level for 15mins more and so I think, maybe this is the BMS doing its calibration thing - allowing the battery to discharge a bit. (Due to the meter inaccuracy, true vehicle voltage is probably closer to 12.3v.)
Feeling more confident now since the voltage had not decreased any further.
Then, an interesting thing: I had occasion to brake and saw the voltage rise, 13.5 to 14.5v. So, the BMS is telling the alternator to charge the battery when braking. When not braking the voltage reading returned to 12.5. Descending hills or deceleration without braking also resulted in raised voltage.
As the journey continued the 12.5v remained (apart from when decelerating) for about an hour when we arrived.
Restarting later everything is normal, voltage is initially reading 15v, drops to 14.5v after a while. Maybe as low as 13.8v on occasions but I was not checking very often.
So, there you are: BMS behaviour!
I don't know if BMS control is better than the old "dumb" alternator system. Is reliability and battery life better for it?
New member : alternator and BMS
