I've got a 86' El Camino that had idiot lights in the dash and I got a dash cluster from another 86' with gauges including a tach. All of the gauges and clock works and the tach goes to 0 at key on but when I start the engine the tach goes to 6000 rpm and stays there. I don't have a tach filter but the service manual says that's for radio interference. The car was originally a v-6 with external coil but I'm using an HEI distributor (in a 500 Cadillac engine) that has a tach connection at the coil. I've checked power and ground at the tach and the wiring from the tach connector to the tach. All good. I saw a thread here that talked about a jumper wire that may be needed that converts from 6 cylinder to v-8 but mine doesn't have that. There doesn't seem to be any provision to switch from 6 to 8. I did check between pins on the circuit board and and what diodes and resisters I could. All seems OK, no opens. Any ideas?