If the speedo works but not the odo, he likely needs an odo gear.
If a W126 speedo doesn't work, he's got an electrical problem, either with the sending unit, the wiring, or in the instrument cluster.

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'79 300SD W116
'86 190E 2.3-16V W201
'92 300D 2.5 W124
'99 E320 4MATIC S210