My friend's 320CE convertible (W124) has strange transmission performance. At "D", starts to accelerate, the transmission stays on the same gear until the engine rev high enough to about 3000 - 4000 rpm, then shift to the next gear.

So we decided to take some time this evening to drain the ATF, remove oil pan, replace the sump filter, gasket, and refill with the same amount of ATF (MB fluid). But the problem still exists.

There's an ATF dipstick, with "30" and "80" degree-C reading.
I am confused about the necessary quantity... I drained about 4L of fluid, thus I refilled with 4L at the moment.

Can someone advise me how to read the dipstick to determine if the ATF is sufficient w/in the system. Should I check the ATF level with engine running? At "P" or "N" gear?

or is it a sign of transmission failure in the near future?

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'74 W114 230.6
'87 W201 190E 2.3-16
'97 W210 E320 Avantgarde