Turns over but won't start

Checked for fuel pressure and spark. Now trying to test fuel injection now. Only able to get at 4, 5 and 6 injectors in front. All three test out good on the positive wire with a voltmeter with the key on at 12 volts. But in checking the negative wire while turining over the engine I get a pulse but only between 2 and 4 volts. My next step is to check the other 3 injectors. I understand if one injector is shorted I would get poor readings on the others.