300d transmission swap gone bad
 

I currently installed a transmission from a sd 81 300sd w126 into my 82 300d w123. I converted the rear tailstock for the manual speedometer. It was up on stands and i put fluid in it, started it and it ran through all gears forward fine even 4th. In neutral the wheels would roll forward and in park they would try but then the transmission would engage park. I checked fluid and added some more atf to replenish it and all of a suddent it would only move the wheels forward in all gears except in park they'd roll forward til the park clicked/engaged. Well after that i let the car down and checked the fluid which was fine and the car won't pull itself at all! Please any help would be greatly appreciated. The 300sd transmission was good b/c i parted the car out a year ago and drove it before removed. Thanks guys

you might have messed up the parking pawl geometry, it's actually rather sensitive to the axial position of the pawl on its pivot. e.g., a thin washer behind the pawl on its pivot shaft made a difference whether or not it engages the sectors on thge output shaft. Could be something else too! Better take that rear cover off again.

You said it was fine -a year ago- and has been sitting around since until now. There's 3 things auto trannies hate, sitting (lets the internal servo seals get dry/hard), water (eats the glue that bonds the friction material), and brake fluid (eats the seals). It sounds like you have either something leaking by, or else you have a stuck valve in the valve body from sitting, or do not have the manual valve inside the pan engaged correctly to the shaft that sticks out for the shift linkage to connect to, or something is wrong with the linkage itself.. I would check the linkage first, then the manual valve . If the parking pawl locks it in park but not in other gears, it is fine.