Two things to check......
OVP (overvoltage protection relay), right knee bolster panel under glove box, known to fail after many years. Easy to replace, always keep a spare in glove box.

Tach pickup on bell housing above flexplate. It's magnetic, clean it up.

Alan

__________________
DO IT RIGHT THE FIRST TIME, DON'T BE A HACK
Political Correctness is NOT part of my vocabulary
and finally FIGHT CRIME...SHOOT BACK

'82 240D Stick
'85 300D Auto
Drove my first MB at age 16 1960 300SL W190