injectors not pulsing 95 3.9 auto

[bobm7007]

does anyone know what would cause the injectors not to fire?what triggers the injectors to pulse? I have tried another ecu and still no injector pulse. the 95 has a distributor, no cam sensor, no crank sensor. is there a relay for the injectors? does the ecu get a signal from the ignition system like the old bocsh systems did with the tach input. i am baffled

[Elemental]

The computer needs to see at least 50 rpm on the tach signal(off the coil) before it turns on the injectors.