Gas Guage help


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My buddy just swapped out his automatic 93 dx coupe. The dx model has no tach and he didnt want an aftermarket tach. He also did an auto/manuel swap. So we went to a salvage yard and bought a 5th gen ex cluster. It has the tach/speedometer/temp. & gas. So we install it fine. I have did it myself with a 95 dx to an si guage cluster in my coupe (r.i.p. due to hydroplaining). It was just plug and play. So anyway we installed his new ex cluster. We used his old speedometer for milage purposes. We start it up. The tach works fine. The temp. guage works fine. But the gas goes up to the top. But the thing is he had like 1/8 of a tank of gas. So we finish putting the dash back together and drive to put gas in it. The speedo works fine too. We put the gas in thinking maybe if we put a full tank in there then it will lower itself back down. But it didnt. Does anyone have any troubleshooting ideas to help fix our problem. I know we can just go buy the new temp/ gas guage but if we can fix it that would be better. Thanks in advance.