The two studs on the back of the tach for power and coil trigger, do they have the proper insulation spacers? Sounds like something is grounding out. I'd put the cluster on the bench and test each stud with a continuity tester and make sure there isn't one or more of the studs incorrectly being grounded out. Something is shorting out somewhere. If the proper washers are there then somewhere either the power wire or possibly the coil trigger wire is shorted to ground somewhere beyond the stud underneath the housing.