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Battery warning light

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The first thing I would be checking Andy is all the wiring  connections and get a drop test on the batteries  ,it could be something as simple as a bad earth before you work backwards towards the Alternators and charging systems which is will be the more expensive end of the problem



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Yes if the connections are ok, and the belts are not slipping, then the alternator is more likely at fault.

However if the battery is unable to take any charge, ie Kaput (not my normal technical term for a duff battery lol) then this could cause similar symptoms.

If it is the alternator, then it could be any of the diodes, or it could be the regulator.

Some diode faults (but not all) lead to the alternator drawing current from the battery when the engine is off, sometimes as much as 4 amps, so you can also check for this too.

Incidentally, another fault finding tip, NOT relating to this post, but if for some reason the ignition bulb doesn't light, because it had blown, then the alternator may not give any output, as it relies on the small current through the bulb to excite the windings of the alternator, so when the alternator starts to charge, the voltage on the exciter winding increases, and the bulb goes out, as the voltage each side of the bulb is more or less the same.

I just felt that might help for future diagnostics.

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