1977 Tachometer into a 1990 Cluster

Well I put the 77's gas where the battery idiot light used to be. I put the tach where the big gas gauge used to be. I hooked it all up and
everything worked for a few days. Then the tach started randomly jumping around and finally just stopped around 4K. I checked all the wiring and it seems to be correct - ground, 12V, and Hz at the coil wire. These gauges were a little nasty out of the 77 so I re-painted them, the gauge may have been on it's way out no way to know.... I can't get the gauge to do a thing so either I fried it or it just died? Thanks for any input....
GMC Gremlin
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