I've fixed two of them.
The gray pot metal gear is likely spinning on the shaft. The solution is to make that gear grab the shaft again.

Tools needed:
long nose pliers
toothpick or similar sized drill bit
scotch tape
locktite blue

Pull the instrument cluster
remove the center instruments (spedo and odometer)
remove the odometer component.
you'll see metal pin going through the numbers
put the scotch tape over the numbers to keep them aligned and from falling out.
There is a small metal disk on the side oposite the plastic drive gears. Using the long nose pliers, pull this off carefully by pushing the pin through it.
Just as you get the metal pin to out of this disk, follow the pin with the toothpick or drill bit. You don't have to completely remove the pin, just push it out about 1/2"
Now, the pin should stick out past the pot metal gear about 1/2".
Scuff up the pin (shaft) and put a drop of locktite blue on that scuffed up area.
push the pin back into the assembly.
Put the metal disk back on the shaft like it was before.
I spin the odometer using the white plastic gear for about 5 minutes to spread the loctite around.

You're done.

I hope this was helpful..
clint

__________________
81 300D N/A 130K miles (Nelly)
87 300D Turbo 80K miles (The Scalded Dog...because it runs like a scalded dog)
95 e320 Wagon 104K
2003 Sprinter 47K miles.
1999 Tandem Bicycle
One beautiful low miles wife (who likes diesels) (that's my wife holding the sign)

My son and I fixing Nelly's odometer