Cessna 172P fuel gauge problem

[drskyking]

For the past year fuel gauges are pinned to above full and occasionally randomly give me a correct reading. Last week my mechanic and I took out sending units and operated the float out of the plane and gauges gave correct readings. We then placed them back in tanks, cleaned the metal ground tab well, sprayed with conductive lube and again they worked 15 minutes until I turned engine on, now they are pinned to full again. Check all wires, no cracks or bare spots. My question is the grounding tab. Is it supposed to contact something under the wing skin to make a good ground? I can't see under the wing skin through the hole, just no space. Curious why no grounding wire. If I connect a jumper wire from one of the sending unit screws to ground they work again. My mechanic and I are puzzled, any ideas? I am new to this forum. Also appreciate any suggestions for other good Cessna 172 technical forums!
Robert