My friend has a 77 240D. His fuel gauge
correctly shows full when the tank is full.
BUT, when the tank is 3/4 full, it drops
down to empty. Last weekend, we pulled the
sender out and cleaned it as new. I even
pushed the float up and down to see if it
would register on the gauge;it did perfectly.
I then put it in a bucket of water to see if
the float would work;it worked perfectly again. Let it dry off, did my first test
again (moving float by hand), did perfectly...again. Guess what, it is still
showing full when full, but empty when the
tank is 3/4 full!! I am assuming it is not
gauge because if works by moving float by
hand and in the "water" test. HELP!!!!

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Did you plug in the sender when out of the tank and move it by hand to see what would happen? It probably is the sender.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

YES. TWICE!! The gauge worked fine when
moving the float up and down the cylinder
by hand...as well as in the "water".

The float must not be acting properly while inside of the tank. I would say to replace it.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM