Are you new to W126 gassers by chance and/or the W126.024/.025?
These have a 3.46:1 diff and a fourth gear ratio of 1:1...which yields around 3400rpm @ 70mph cruise speeds and they start in second gear normally.
Perhaps the transmission is fine and you think there is simply an issue. If not, do not take offense. Some people just have no idea of this particular combination of MB parts.
I lost all the ATF in my SDL once. Filled it back up, went on my way. Simply lost hydraulic pressure, thus quit shifting as there was no way to apply either band or either drum for forward function.
