I found out that it wouldn't charge without putting a warning light to the D+ terminal,
The D + terminal needs a small trickle of voltage to give the alternator a kick to make it start charging, This voltage used to be supplied through the warning light which was typically 2.2 watts, On the newer VAG vehicles this was then supplied through a resistor in the dash but now may be supplied by the ecu.
Check the voltage on the D+ wire with the plug out and then again with the plug into the alternator, I would expect around 12V with the plug out and around 2V-3V with the plug in the alternator, any less than that and it will struggle to start charging or may charge if you rev its nuts off. .
If there's no voltage on that wire check the wires near the starter that have already been suggested and if there is voltage on the wire Suspect the non genuine alternator.
The one that we had was cured by fitting a Reconditioned alternator, but we have sold hundreds of Copy alternators that have worked without a problem.
H.T.H Tony.
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