I purchased a 2002 mustang gt the person put a used engine wiring harness and used ccrm harness before I purchased The fuel pump worked fine before he did this and I ran hot wire to it to make sure it still works Here is my problem I am not getting fuel spray gas in throttle body it starts for a few seconds but stops I can hot hear fuel pump on any at all unless I run hot wire and ground to it then it comes on can hear it but no start I have checked inertia switch it has 12 volts on both wires Checked wires at connector going to fuel pump at back of tank two wires have 5 volts on them when I turn switch to startI I connect hot wire and ground red and black to red and black on top of fuel pump pump runs but have NO fuel pressure at engine and still no start Anyone have any ideas what is wrong

 

Why did the original owner replace the harnesses? Didn't they test the car after the swap and before they handed it over to you or, how did you drive it if it wouldn't start?

Aside from that, is it certain that the harness used are for the same year car and with the same transmission (auto/manual)? You could have an open circuit somewhere that the PCM/ECM is expecting to see a signal on that is not seeing it. I'm pretty sure the Fuel pump should have 12V, not 5V but I'm not an electrical expert so, you'll have to wait for someone to chime in with direct knowledge/experience on that.

 

Is it possible that the fuel pump pressure is regulated by "pulse-width-modulation? If so, a simple volt meter would integrate that waveform into an apparent lower voltage. You would need to look with an Oscilloscope if that's the case.