fuel gauge problem, help

I have a 93 s-10 pickup with a fuel gauge that does not read correctly. I went ahead and replaced the sending unit, i had no luck on fixing the problem. I went ahead an changed the instrument cluster but still dont get a reading at all. I dropped the gas tank and checked the sending unit to see if anything was stuck but nothing was in the way. I checked the old sender and the new one while I had it out of the tank and they both work fine. The gauge works fine then. but when I put it back in the tank and try it I get no reading. I disconnected the battery to reset everything but still the tank reads empty when its not. Anyone have any ideas what it can be. Frusted and tired of dropping the tank.

Only a guess, but I had an old Dodge years ago that lost the ground and I had to run a ground wire to the tank. Would not think that this would be the case, but if you are getting no reading at all either power or ground would be at fault. I assume you have checked all fuses.

I also have a 93 S-10(Blazer) and have a set of GM service manuals. If the gauge is reading empty when full, it states that the circuit is grounded. Here is what it says.

Fill the tank (already done)

Disconnect the lead wire at the fuel tank. Place ignition switch in run.The guage should read past the full mark. If the gage reads past the full mark replace the sender. If it remains on empty find the ground in the circuit between the gage and fuel tank.
Seems confusing but that is how it reads.
If it is reading full all the time there are other ways to test, but all comes back to sender, open or grounded circuits, or defective parts.

If you want to check the sender outside the tank, here is what they say to do.
Test with OHM meter - empty should read 0-3 ohms.
half full - 44 ohms, full - between 88-92 ohms.
Hope this helps.

Thanks for the help jsgold, but I checked the ohms on the unit inside the tank and on the old one as well.They both work fine. Tested them outside the tank and they both work fine as well. Ground is good too. When I was looking at the autozone site and I saw a fuel pump switch, that happens to be in actuality an oil pressure switch, could this be the reason why I still cant get a reading? Why would it say fuel pressure when in fact its an oil pressure switch. On the instrument panel I have noticed the oil gauge moves as I press on the gas pedal and it didn't do that before, it always stayed pretty much stationary; could this be the problem why I still can't get a reading for the fuel gauge. Instrument cluster works fine as well. Are those two things tied together somehow. Any thoughts.

there is a tsb on fullsize gm trucks about this problem wich requires the dealer to reflash the computer. i dont know if it would be the same case for your truck but it may work? also its usually easier to pull the bed off than to drop the tank

With my 98 astro, the fuel pump circuit is run through an oil pressure switch which cuts off the fuel pump if you lose oil pressure i.e. in case of accident or oil pressure problem. Some kind of safety circuit. It has no effect on the sending unit or fuel gauge.