Hi all. I have a 300TDI Discovery fitted with ZF automatic transmission. The engine RPM will drop tremendously i.e. from 3000RPM to 1800RPM when the gear shift from 3rd to 4th. The RPM then slowly creep up from 1800RPM to more than 2500RPM then only I can accelerate again. To over come it, I usually manually shift it to 3rd, picks up enough speed and RPM then only manually shift it to 4th.

Please advise what is wrong. Thank you in advance.

 

Might be torque converter or direct drive clutch or its control valve. In 3rd the trans and engine are locked at 1:1 ratio, in 4th the torque converter is supposed to disengage, sounds like it isn't initially.