I have a 2009 Jaguar XF with the 4.2. I kept getting a bunch of weird faults like gearbox and apply brake yellow fault on the information center. I heard it could have possibly been a low battery so I checked the battery, and tested good but it needed recharged. I then proceeded to check the car while running and it was only putting out 12.2 volts instead of 14.7 So I understood that the alternator had probably failed. So I replaced the alternator, car is doing the exact same thing. I read somewhere that it could be the battery monitor system sensor that's on the negative cable. So I bought one from a junkyard and replaced it, I was under the impression that the battery monitor system sensor would not cause the car to get so low that it would die? Any help on this would be beneficial.

 

Might want to plug in SDD or an Autel and see what codes come up. If you got a new battery but didn't reset the battery system then it doesn't know. Yes, it's supposed to do this by it's self but...Jaguar.
Also, if you didn't test the battery with a Medtronic or similar dealer level tester then it doesn't count.
Also, if there's a mismatch in the battery type the car expects to be charging, that can cause issues. ie My '09 isn't an AGM.

 

The only codes that come up is a P0A1A which is the generator control module. I'm not sure if they're talking about the alternator or the battery control module on the negative cable? I did just replace the alternator, I also just replaced the battery sensor on the negative cable. So far it's doing well, but I'm afraid to drive it too far from home. I tested the battery with a solar battery tester, it tested good but just needed to recharge but when the car was started it was only reading 12.2 volts so it wasn't getting a proper 14.7 volts. Which is why I immediately thought alternator. The battery was changed about 18 months ago, zero issues after changing. Didn't need to recalibrate anything. Used an AGM.

 

This explains things, take note of the fuselink as if this fails the bms can't monitor the voltage.....https://ibb.co/2sKSSHt

 

Although you have not given a full code, there is only one extended code for P0A1A87 so the possible cause is unequivocal. Better hope it's not ECM failure!




The only modules in the control system are the BCM and the ECM as the generator has an in-built regulator:





Your vehicle showed the classic symptoms of a partially discharged or failing battery but remember generators are matched to the battery which is why Jaguar recommend a wet battery should be replaced with a wet and not AGM. However, several members have gone AGM on other Jaguar models without reporting any subsequent problems so this cannot be absolutely critical.

Regardless of a battery being supplied as 'charged', it should still be put on charge overnight before fitting. If the BCM is not recalibrated, it will calibrate itself over time.

Graham

 

I came out today and the battery was 11.5, so it lost pretty much all of its charge overnight. So it makes me think the battery is not any good, ended up swapping it out. I'll continue to drive it locally to see if it is fixed or not. Hopefully it's not a ECU issue, but if it is, do you know are those plug and play? Or am I going to have to have Jaguar reprogram a new one?

 

Battery. These 4.2's seem very sensitive to anything being off in the electrical system. Whenever I get the gearbox fault message it's always a sign the battery is flat/dying. You'll also notice the instrument cluster flickering sightly and occasionally the infotainment system may randomly restart. It's most noticeable when it's cold when all the heating comes on, it seems to momentarily draw extra current from the battery whilst the generator is sorting itself out but the bad battery can't deliver so there's a slight deficiency and that can cause the electronics to do random things.

I had to replace mine last week due the same issues. It's fine now.

You are supposed to tell it there is a new battery, however allegedly it is supposed to work it out itself eventually, but it's less than ideal.