Vstech - my experience on my 81 300SD was different. The red light glowed faintly all the time and brighter with higher RPM. I replaced the voltage regulator(?), the easy removable thing that fits into the alternator. Did not help. Turned out that the alternator had gone.

So I am wondering if his problem is alternator.

PS - dear old Sears checked my system before I made the repair and said both my voltage regulator and alternator were fine.