I disremember exactly how it works, but it seems to me that if you ground the wire going to the sender, the guage should max out. If that doesn't happen, the first thing I would check is the continuity of the wire all the way back to the guage plug, and look at the traces on the cluster. If all that checks out good, you probably need a new guage. You could try PM'ing Tools R Us to confirm all this, he haz knowledge.
-Spike