by the dutch guy » Tue Aug 08, 2017 8:53 pm
alternator light comes from the alternator. if it really gave up then the warning will not work also as the light is a integral part of the alternator, at least it was with older "dumb" alternators.
with older alternators the light does work by a balance. When you turn on the key, 12 volts is supplied to the alternator's field windings through the idiot light. This gives an indication that the alternator's field circuit is intact, and it also provides the current to start the alternator charging. In fact, the alternator may fail to charge if that light bulb is burned out. When the engine starts and gets up to a high enough rpm for the alternator to start putting out voltage, there is a diode inside that takes some of the output voltage and featured back to the terminal that goes to the idiot light. The net result of this is that when the engine is running and up to proper rpm, the idiot light is receiving 12 volts at both ends, which means that the total voltage drop across the light is the zero volts and the light doesn't light.
i do believe the mk6 has one of those new fangled smarty pants alternators that work differently and regulate the voltage down when the battery is charged.
in any case: alternator needs a rebuild/replacement.
might also want to checkt the battery, if that thing gave up completly it will hold no charge and the alternator will have to do all the work. but considering the abs indicated a low voltage during driving i put my money on the alternator. but let the garage do a load test to see if the battery is still up to par. if its bad it can kill alternators real fast.
Mk6 SWB TDCI 125