Intermittent Battery Charge/Warning Light

My battery warning light comes on and off, sometimes several times within the space of a minute. I had this problem 6 months ago and thought I'd fixed it by cleaning an oxidised connector behind the warning light panel - however today the problem has returned. I have read, digested and tested but am starting to doubt my understanding of how things work here so here are some results of a test I have just carried out. I hope someone can suggest what the problem might be, and what to do next.

Defender 300TDI

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Car is stationery. I first disconnected the charge light wire ( brown/yellow ) from the alternator leaving it free hanging. Start the engine. The battery warning light ( BWL ) went into a continous flicker. The voltage on the charge light wire ( not attached to the alternator ) reads a rapidly variable 5 to 7 volts. The alternator output reads 12.8 volts i.e. this is the battery voltage - no charge is being generated.

I then connect the charge light wire to the charge light terminal on the alternator. The battery warning light lights strong and continous, but there is still no alternator charge i.e. still reads 12.8v on the alternator output and across the battery terminals.

I then used a length of wire to momentarily connect the alternator output terminal to the charge light terminal. I saw a spark and the engine reved slightly as the alternator kicked in. The BWL went out. The alternator output read 14.4v and the battery was charging. Hurray ..... or perhaps not ......

About a minute later with no intervention the BTW goes on again and the alternator again stops charging.
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Thats all folks. Your thoughts and suggestions appreciated.

Cheers

Dave

You have a dodgy connection somewhere in the field wire to your alternator.
Without the wire connected to the ALT you should see full battery voltage at all times and not variable as you state. This is a classic symptom of a dry/corroded joint.
When you short the ALT output (which is effectively a direct connection to the battery) the exciter windings get full voltage and "excite" the ALT windings which start to charge the battery. As soon as you remove the voltage from the exciter field windings the ALT stops charging hence your warning light comes on.
A common side effect of connecting the wire direct to full battery voltage is to temporarily break down any corrosion in the dodgy connection which is why your warning light stays off for a short while until the joint breaks down again.

A classic example of this is if you put your dipped head lights on and one of them is dimmer than the other. If you flash your main beam often the dim dipped light will come up at full brightness for a while. The chief culprit for this is a dodgy earth connector either on the back of the lamp unit or very near to the lamp housing. The full beam draws much more current and breaks down the earth connection corrosion so that the dipped beam will work for a while at full brightness.

Cheers Shifty. I thought it would be the worse case scenario !

Any suggestions on the best place to start looking ? My schematic of the Landy electrics shows a resistor and a diode in the warning light circuit so I guess the joints here are likely culprits. Anyone know where they are ?

Thanks

Dave

Sorry mate but you'll just have to work your way back from the ALT checking and cleaning all the connectors.