1987 Nissan Maxima injecturs

Electrical problem
1987 Nissan Maxima 6 cyl Front Wheel Drive Automatic

this is my problem a year ago my car's engine was repaired completely since this time my car doesn't start I have already checked for the eccs computer in another similar car an it runs perfectly, with a digital multimeter I found that the injectors negative wire doesn't have the negative ground pulse to open it the injectors, while the positive works good. Also i've checked for the continuity in the harness and it's in good condition, the matter is it can run with a little drink of gas if you put it trough the intake manifuld that means the distribuitor is sending the right signals to the eccs cause if not the spark plugs couldn't have the electrical signal.
i hope you can hel me wiyh this thank you in advace.
Alain