Not sure about the 300D and 300SD. In the 190D, at the bottom of the sender unit are 2 metal contact points. The float will come down and make the connection to turn on the low level warning light. There may be a metallic disk at the bottom of the float. Normally they are cover in muck and grime. See how the float comes down and what it touches, clean those and it should be OK.

If you can trace the wire back to the connector and short the 2 wires at the connector then you can see the light comes on without actually installing the sending unit.

Good luck.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.