When you last changed the fluid, did you change it all, or only 3 liters? The system holds 7.3 liters. I actually put 7.4 into mine, but I did drain the tranny cooler too. You need to change all the fluid to rule it out as the cause of hard shifting. Hard to do ifyou only change 3 liters. As stated previously, changing the spring in the valve body doesn't fix hard shifting. It only fixes slipping between 2-3 or 3-4.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280