I have a 1991 300TE that wants to hang in 3rd gear, doesn't want to
shift out till over 40mph. Also, on the highway, it will continually shift
down into 3rd at speeds right around 60mph. If I drive it at 65-75, I
have no problem. Do these have some kind of bowden cable or adjustment
to correct this. Trans has no slipping or other issues so I don't feel
it is a serious issue requiring rebuild.

any insight?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

Check vacuum to trans.

Tap comes off intake manifold by cylinders 5 and 6.
Line crosses over to passenger side, and connects to vacuum modulator.

Low/no vacuum signal would tell trans the engine is working hard and that could cause the trans to delay upshifts.

Vacuum modulator on the trans body can also fail, check to see if it holds vacuum.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles