I second the voltage regulator-- although you can get some more miles out of a worn out one by smacking it, should it wear down to the point where it no longer makes contact.

Also, being a diesel you can drive forever without one- if the sun doesn't go down and you dont like radio/AC etc...

That said, replace it- cheap.

The only part to ever leave me stranded in 10 years- knock on wood.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E