bought my fl2 3 weeks ago, 58 plate 38k FULL HISTORY,
since buying the car the battery light comes on and stays on!
under warranty its had new alternator, also new wiper motor as the wipers stoppped working after 3 days,
battery changed, i've owned the car 3 weeks but only had it in my drive 3 days
any ideas what it could be ?
ps earth straps ok too.
getting fed up now.

The first thing to check would be the fuse for the charging circuit "have a look in the hand book on fuses". Usualy the batery light comes on when the car is not charging the battery. This can happen when the alternator packs up or there is a loose connection on one of the cables. Check that the battery terminals are tight and that the earth straps are tight "neutral".
Another test you can do if you have a volt meter is to "with engine turned off"turn the headlights on, you will need the igniton on for this, for about three minutes then check the voltage at the terminals. You should get about 12.5 to 12.9 volts or there about.
Now start the engine and get some one to raise the engine revs to about 1500 rpm with the excelerator. The voltage now should rise to about 13.8 or there abouts, if this higher voltaged is recorded then the alternator is fine and doing its job and the fault lies else where and further investigation is needed.
If however when the engine rpm is increased and theres no rise in voltage your charging system is not working and then you have to investigate in that section.

It could happen that the car's communication wire with the alternator to rub on the metal parts of the engine and interrupt. Check it's integrity.
It's a blue wire usually. Here's a picture from our russian friends.

Click image to enlarge

Bet thats the answer, if the wires disconnected theres nothing to excite alternator.

cheers guys, will pass the messages on to garage,

May I suggest looking at basics.
A light bulb lights because there is a Potential Difference across it. In laymans terms you have 12 volt on one side and the other is earthed so the bulb is illuminated.
Now back to the car; with the engine not running, the 'ignition' on (odd term for a diesel), there is '12' volt at the bulb at the dashboard and the other side of the bulb is connected by a wire to the alternator where it finds an earth though the windings so the bulb illuminates. When the engine runs, the alternator produces the charge voltage, (probably about 13.8 ish volts), and this powers what was the 'bulb earth wire'. With the bulb now seeing 13.8 volts on each side of itself, there is no potential difference so the bulb will not illuminate.

Now practicalities.
1/. If the bulb does not go out, disconnect the single thin wire on the alternator top. now start the car; if the bulb is still glowing then the wire has rubbed somewhere and found an alternative earth so its not the alternator.
2/. If the bulb does not illuminate at all then there is an alternator fault to investigate.

The third option is that if you start the car and the bulb goes out initially and then starts to glow dimmly then the alternator voltage control internals have failed. When this happens the voltage produced exceeds the battery voltage somewhat so you now get say 16 volts from the alternator, 13.8 volts from the battery and hence a potential difference across the bulb filament so it will glow. Replace the alternator! 2010GS Auto ex Celeb, Santorini Black, 2011 facelift , spoiler, spats, mudflaps, LED footwell lamps, Witter detach,
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thanks for the advice, got the car back from the garage last night,
looks like it was a bad earth to one of the wires,
fingers crossed

@fredastaire

That was true for older type of charging systems and/or alternators. The generator/alternator on FL2 is completely digitally controlled.
That means that inside the alternator there is a small "computer" that drives the excitation windings based on the commands that are received from the car. So through that blue wire is exchanged data with the car in a completely digital way. Then, the car, based on various conditions, lights (or not) the battery light in the instrument cluster.

I was having the battery not charging light on 30 seconds after start up and staying on , I followed alex-pescaru post, after taking diesel filter off you can see the blue wire from wireing harness to the alternator and sure enough were it runs under the power steering pipe it had been chaffing and broken the copper wire,exactly has shown in the picture.[/b]

Confirmation of the same problem and cause. The Battery / Alternator warning light had been staying on.

This started 4 years ago, at about 110,000 miles. Battery was all good, so didn't really do anything about it. I saw this thread a year or so ago and reached down to blue sensor wire it was all intact.
Whilst I was doing some work recently I managed to get access to the wire and discovered that the wire plastic coating had worn and the copper wire had completely corroded. It then snapped. I cut out the section of corroded wire and remade it and out went the warning light.

Also worth noting that the heated seats won't work if the Battery/Alternator warning light is lit, or more accurately if there is no communications to the alternator.

J.

Great source of information, and indeed my wire had broken at 120,000 miles just as it exits the plastic covering of the loom.

Great post. I've just paid out for a new alternator and still the lights on. The battery is kicking out 12.6 volts which is a little high, while the alternator is going up to 18V, varying, which points to the signal wire. I had a quick look and it seems solid, so before I order a fuel filter, so I can change it when I look at the blue wire though to the alternator, can anyone advise where the blue wire exits at the other end? I can then make a bypass to check.

Thanks

Not sure it is this wire as with mine broken, everything worked fine with about 13.7 across the battery with the engine running, it just brings on the light, and limits some of the high consuming items. Mine felt solid as well, but the plastic loom covering had cut through the insulation as the wire exits, and the inner had rotted away, but the wire was not actually detached just open circuit.

Good luck

Thanks Paul,

The output voltage varies on mine and was the same with the old alternator. I thought it would be regulator problems, so bought the whole unit. I've tried a diagnostics test and it just points to power problems. I guess its the wire, because lack of the signal will ( I guess) cause the voltage to fluctuate. I like to do everything together, so thought I could check the wire at the other end, but don't know where it goes.

Well, I should have read these threads before I splashed out on an alternator. Overcharging... The books say alternator. Then read the threads above and checked the blue wire. Couldn't believe it, it was like the images above. kinked and almost split in two. Too short a link and it was a right pain to fix. I wanted it done so went for a solid double crimp. Too much fuel around for soldering.i
It should be the first thing to check. We live and learn.

Cheers!