The sender should read 0 (low) to 90 (high) ohms. You can check it at the tank connector. It could be a dirty contact in the connector, or the instrument cluster as well.
The car should be shut off. It would not run anyway if you disconnected the connector, because it is also the power for the pump. You are probing the tank side of the connector, and looking for a reading in ohms. 0 is empty and 90 is full. If it is over 90, or high for the amount of gas in the tank, the float is probably not hooked to the potientiometer arm properly, or the arm is bent. You stated that it worked properly before the mechanic touched it, and did show some motion since, so I would assume that the arm is not connected properly. You can check until you are blue in the face, but you are eventually going to have to pull the tank. Make sure the tank is near empty when you pull it.Not much experience with DMM's. found some info from an old cache'd post-
Here's a test you can do w/o dropping tank:
Borrow a DVM if you haven't got one, and set to Ohms.
Unplug rear gas tank 3 Wire Harness connector, and touch one probe to Cavity A(Black wire) & the other to Cavity B(Purple wire).
If reading is Zero, there's an open in intank Float Rheostat Circuit = Drop tank & look for open/faulty Rheostat Ground spade connection
But, first remove harness loom, to examine whether Purple is cut/broken under der.......
do I do this with KOER? and am I probing the tank or body side of the harness- or tank harness-to-body harness? Please bear with me here.