Vacuum is used to control the soft/hard nature of the shifting.

There is a cable from the transmission to the throttle linkage (aka Bowden cable) which controls the shift timing (how early/late the transmission will up-shift depending on throttle position). It is under the intake manifold close to the oil filter canister, real pain in the but to reach on an OM603.

I don't think that either of these is the issue in your case.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC