W123, Suspension & Driveline

I have a newly acquired 1980 300TD and the first thing I wanted to do was change the transmission fluid. The stick was reading 2 or 2 ½ inches above full at temperature before I did anything, but outside of some issues with the 3rd to 2nd gear kick down, it was well behaved. The gasket was hard and the fluid was very old which is not good news, but the car has suffered fair neglect and I was not too dismayed. The issue of immediate concern was that I retrieved only 6 qts. of fluid from the system, including torque converter. At present I have replaced the fluid to the level I found it at, but am not driving the car. I might guess that the dipstick is either a mismatched junkyard replacement, or maybe the dip tube has been modified or maybe the AT fluid was overfilled.