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sako243



Member Since: 08 Jul 2014
Location: Wales
Posts: 1250
Wales 1994 Defender 110 300 Tdi CSW Alpine White
The battery light, certainly on older vehicles can only come on by one method. One side of the light is attached to battery positive via the ignition switch / a switched live circuit. The other goes to the alternator field windings.

The reason it comes on when you turn the ignition on is 12V is applied to one side. The other side being connected to the alternator field windings is a short to ground. Once the engine fires up the laws of magic pixie dust and magnetics comes in and it is no longer a short but is being used to power the field winding and turn mechanical energy into more electrical.

So either you have a shorted out high side switch or a dodgy alternator, or something else causing a short to ground on the low side.

If the light comes on when the engine is off then try removing the small yellow wire off the back of the alternator. If the light goes out then you've got a dodgy alternator. If it doesn't then either you have a short somewhere along that wire (chaffing?) which you could buzz out with a multimeter or your ignition switch is playing up. Ed
82 Hotspur Sandringham 6x6
95 Defender 110 300Tdi
Post #683869 3rd Feb 2018 5:38pm
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JonJop



Member Since: 23 Jan 2018
Location: St Agnes
Posts: 63
United Kingdom 1996 Defender 90 300 Tdi HT Arles Blue
Thabks for this - makes complete sense - some more investigative work is needed it seems...

The main thing is that it seems as though it’s charging. I’ve used it around 15 times since picking it up a couple of weeks ago and I imagine, by now, it would have drained the battery completely if it wasn’t charging.
Post #683872 3rd Feb 2018 5:42pm
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sako243



Member Since: 08 Jul 2014
Location: Wales
Posts: 1250
Wales 1994 Defender 110 300 Tdi CSW Alpine White
It might be one of the diodes in the diode pack has failed but not the others. Ed
82 Hotspur Sandringham 6x6
95 Defender 110 300Tdi
Post #683873 3rd Feb 2018 5:45pm
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sako243



Member Since: 08 Jul 2014
Location: Wales
Posts: 1250
Wales 1994 Defender 110 300 Tdi CSW Alpine White
As a simple test you could remove the wire anyway and measure the resistance to ground, if there's anything it'll be a bit suspicious. Ed
82 Hotspur Sandringham 6x6
95 Defender 110 300Tdi
Post #683875 3rd Feb 2018 5:46pm
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