Hi Cal,
The Mk3 OEM assimilator uses a single transistor as a crude comparitor, comparing the battery voltage to the rectified and filtered output voltages (there are two output lines) of the alternator with respect to ground. When the battery voltage exceeds this alternator voltage, the transistor is on and the warning light glows. when the converse is true, the transistor is cut off, and the light extinguishes. If your battery is fully charged, say 12.5-13 volts, then the fact that the light turns off above idle shows the alternator to be working correctly. BTW, a feeble battery and a feeble alternator can also give the same result. At idle, the alternator is providing 10 Hz or so of signal to the assimilator circuit. At 3000 rpm, this frequency rises to 50 Hz, which may be a bit high to be properly filtered by degraded filtering components; the assimilator is, after all, 40 years old. This would cause a false indication of low alternator output, and show up as a flickering lamp at speed. A new assimilator, either OEM or aftermarket, would settle the issue for 50 bucks or so. Many aftermarket units avoid the battery reference entirely, and simply compare the filtered alternator output to a stable voltage reference, say 14.5 vdc, and avoid the weak-battery ambiguity.
Sorry for the verbosity; I've got way too much time on my hands today.
