Runing problems with my MKIII

I am obviousley knew to this forum, but i have major problems with my supra. I have a 1989 toyota supra and i am not getting feul into the cylendars. I have feul to the injectors and the feul pump cycles just as it should but the injectors never open. I have power to the injectors but i dont have the negative pulse that i need for the injectors to open and close. I have already replaced the ECU because i thought that is what controls the negative pulse to the injectors but it did not change a thing. What could it be?