I just did the lift pump in my 1990 240, and it looked like the sender hasn't ever been removed before.
I used brake cleaner to melt the underbody sealant that was gluing the lock ring shut.
Once it was melted, I scraped and cleaned what I could out from the lock ring groove etc, so that none would get in the tank when the sender unit came out.
Then I used 2 big flat blade screwdrivers crossed over each other for leverage, with the blade of each sitting in opposing notches in the lock ring (ie at 3 and 9 o'clock). The shaft of screwdriver in your right hand goes behind the the shaft of the screwdriver in your left hand, and the blade of the screwdriver in your right hand goes into a notch on the left hand side of the lock ring (ie 9 o'clock… although you might not be able to use 3 o'clock and 9 o'clock if there's pipes etc in the way.) Just so long as the blades are as close to being on the opposite sides of the lock ring as you can get them.
Then I just twisted anti clockwise. Was far easier than I expected it to be!
There's a high chance of slipping with this method though, so be very careful! You don't want a screwdriver through the top of the tank, or your flesh!
(I searched various phrases on Google for 20 minutes trying to find a diagram, but came up with nothing. Hopefully my description isn't too ambiguous)
