Tach not working !!!
 

I'm trying to figure out why my tach is not working correctly. When I start the engine up 351w EFI 87 F350. Tach acts like it wants to work as it goes up to around 600rpm and stays, then when I push gas peddle down tach goes to 0 and lower. Back ground : replaced 351w carb engine with 351w Efi changed all wiring under hood plus all wiring in cab, I kept original gauge cluster in truck but I do have a replacement from doner 351w EFI truck if there is a difference in them, all other sensors gauges seem to be working correctly. Any help would be great!