Originally Posted by funola

On my 92 VW diesel, the charge light in the cluster is an LED, not a bulb. I am not sure how the excitation is provided to the alternator, maybe it is a power resistor. You don't really need to know all that to troubleshoot the VW charge system. As long as you get a little under 12 V at the excitation wire at the alternator when the ign key is in the on position, you are most likely getting excitation. There should be 3 wires at a VW diesel alternator, the big fat wire goes to battery + terminal (on some cars via the starter motor terminal), a small W terminal wire to drive the diesel tach, and another small wire for the excitation. Ground is via the alternator mount.