check the battery earth has a good connection between it, the engine and the chassis, as there was a problem with the earth previously the day I replaced the starter motor and the starter wouldn't turn due to not enough current passing through the system, I gave it a quick fix then but it may need looking at again. also check the voltage with the engine running to check if the alternator is providing power it should be about 14.6v, and also check the resistance of the wiring between the alternator and small fusebox/battery as this can get quite high with age/corrosion.
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