Battery going flat while driving

[davejag]

When starting my 97000 mile 2005 XJR today I noticed a slight whining noise which changed pitch as I pulled away. After around 10 miles the battery warning light came on and then the cruise control self cancelled and was unavailable. At this point point I decided to head for home. Just as well as by the time I got back to my driveway under limp home mode, all hell had broken loose. The message centre was warning of almost every electrical system failing (DSC; ABS; Power steering; Climate; Cruise; Air Suspension; Indicators etc. etc.) and the engine was running quite roughly. Even the indicators and speedo had stopped working. From reading the forum and general knowledge about the car, I am assuming it is the serpentine belt slipping, either from wear and tear or perhaps a coolant leak. Anyway, the car is now connected to the CTEK charger and I will be on the phone to the independent on Monday morning. Any thoughts on my diagnosis or other ideas in the meantime would be gratefully appreciated.

[andypryce]

Definitely points to battery not charging, possible alternator failure or as you say belt failure

[ptjs1]

Agree with Andy, sounds like belt or alternator.

Before putting the charger on, did you check the voltage at the battery?

Once the battery is charged, try the following:

- check the voltage at the battery terminals
- now check voltage at the bulkhead terminals and compare
- assuming figures are similar, start car and measure voltage at idle at battery terminals, should be close to 14v
- rev to 1500rpm and check again, should be 14v+
- check again at bulkhead terminals

If less than 14v, yiu could try and put a bit of belt dressing on alternator belt and see if that improves figures. If so, might point to belt rather than alternator.

Good luck

Paul

[PaulGover]

A whining noise suggests to me a seized bearing somewhere in the drive to the alternator, such as the jockey pulley.

[BrianG]

It may not be a mechanical fault in the alternator but an electrical one. I had a similar sounding problem years ago with a Rover 3.5 V8 where the battery light came on about 1 mile from home. When stopped at traffic lights I could hear a loud whining noise, which I ignored until I got home. Even with the ignition off and engine stopped the battery lamp was still glowing dimly; the drive belt was too hot to touch. As I disconnected the main battery cable at the alternator output terminal this produced a very big spark, so lots of current going in there from the battery. Once disconnected from the battery the alternator pulley was free to rotate.

The cause was a failed regulator board inside the alternator, and the fault allowed the battery power to effectively act as an electric brake on the alternator shaft. If you have this type of failure mode in your alternator then leaving the battery connected will not help and will simply prevent it from being recharged in situ. As the alternator connector is not readily accessible on the X350 you would be able to check for a similar fault by disconnecting at the battery itself - normally with everything off there will be very little current flowing so not much of a spark.

[davejag]

Thanks all for your helpful replies.

Well it turned out to be the alternator and a new one is being fitted this week.

Regards,
Dave

[andypryce]

Good, easy fix though probably expensive..