[Luce]

No code and the tach plummeting faster than the engine can rev down sounds like loss of crank trigger or loss of power. Not sure what you have, but if you're running a trigger wheel and dizzy, maybe the module the pick up plugs into or the pickup itself. If it's the 36-1 ford trigger, once it misses a tooth, it's stuck for the rest of the 360 degrees and quits transmitting the timing info the the ECU.