I had the same thing happen to my 240D about 100,000 miles ago. I pryed the plastic cap off the alternator connector, found a loose wire, soldered it back, replaced cap. It's worked perfectly since.
One more alternator suggestion from someone who has put quite a number of them on my 123 cars. You are money ahead in the long run if you buy Bosch authorized rebuilts. I'll be surprised if you can't buy them from the Parts Shop here.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in