Well it can NOT work if the cluster is NOT grounded. By ungrounding the cluster, you are creating some sort of false situation.
1--Treat the entire gauge(s) as a "system" from front to rear. If you believe the temp gauge is working OK you can eliminate up to the output of the IVR
2--Check that power to the cluster PC board is actually there. You should get 12V that is, "same as battery" at the supply terminal of the IVR. Check the harness connector pins to the board.
3--The IVR fits into contact fingers in the PC board. These contact fingers may or may not be making contact with the board traces. Solder jumpers across if necessary
4--The IVR MUST be grounded, it has 3 connections, 12V in, ground, and regulated output. Connect a ground pigtail from the ground board traces / mounting screws to the column brace, etc
6--The gauge studs/ nuts may not be making contact. Replace the nuts with "real" nuts, and loosen/ tighten them a few times to "scrub" them clean
7--To eliminate wiring harness and connection problems, ground the gauge right at the stud, and obviously if the gauge deflects quickly to "full," you have a wiring problem. Once again, this could be right at the board harness connector pins.
8--To check either temp, fuel, (or oil if equipped) get yourself some resistors. For example, 1/2 scale is 23--25 ohms, so buy a pack of four -- 100 ohm 1/2 watt from Rad Shack, and wire all four in parallel, which gives you 25 ohms.
Empty is 70--75 ohms, full is 11-13 ohms.
9--Don't forget that the gauge itself can be bad. For example, if you "rig" your test 25 ohms resistor, and connect right at the temp gauge stud, and the temp gauge works, but then move it to the fuel gauge stud and it does not, if you are sure the studs are making contact, then you have a bad gauge unit.
