I have an 86 300sd. It shifts smooth but will not start out in 1st and kicksdown form 4-3 around 55mph. The kickdown switch works and will make the trans downshift. around 55mph and over it shifts form 4th to 3rd. It will shift form 3-4 if I press the gas pedal but shifts back to 3rd once I let off. Is this KD solenoid related or more likely valve body? Thanks for any help.

I can't reconcile the symptoms being specific to a bowden cable being set too tight or too loose so check that it's properly adjusted anyway and more importantly that the cable moves smoothly in its casing.

Sixto
87 300D

Also make sure your vacuum system is functioning properly, in particular pay attention to the lines going to the blue modulating thingy... Also, adjust the bowden cable so there is zero slack or tension when at rest. Also check the fluid level, warm, idling in park. How many miles on this unit? 617 or 603 engine, not that it matters, transmission function is the same for both engines.

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1987 300D Turbo 5-speed
1991 350 SD Turbo (Frankenstein)
1957 180D (Betsy)
1976 300D (best trade ever!)
1980 IH Traveller SD33T (Resting!)
1996 F-250 Powerstroke

The cable seems fine. The problem gets worse as it gets hotter. If it wasnt the modulator/ vacuum problem what else could it be?

One thing you can try is to temporarily disconnect the bowden cable. Just pop the cable socket off the throttle linkage ball. The transmission will upshift quickly as if you're accelerating with very little throttle input and won't respond much to jabbing the pedal. Use the lever to downshift if you have to. All you want to determine is whether the symptoms persist with the bowden cable out of the system.

Have you done the basics like checking transmission fluid level after a 20 mile run? What's the condition of the fluid? Is it clear or is there sediment?

Sixto
87 300D

The fluid looks and smells fine. The cable makes no difference. I have a vacuum leak in my door locking system. Would this affect the trans vacuum?

W126 have a dedicated electric pump for central locking.

Vacuum affects shift quality, not shift point. I can't see your symptoms related to a vacuum problem. My day job isn't working on MBs, though

Sixto
87 300D

When the car is cold it works fine but after its warm, the kickdown issue starts. It kicks from 4th into 3rd but if I push on the gas, it shifts back t 4th. ???? SHould I just replace the trans?