If the tach grounds out when the engine is revved, I'd suspect the tach itself. Check the connections to the tach (any corrosion
at all should be cleaned up--some 400-600 wet/dry sandpaper) and then hit it with some CRC & try it again. You can hotwire the tach to see if the guage is working. I think you should be able to take your lead to the block and put it to ground (engine NOT running, batteries
to off) and the tach should peg at max. At least then you'll know that the guage is good.