1999 Ford Explorer XLT 5.0 AWD - starter will not engage. I hve replaced the starter/solenoid, ingition relay, ignition switch, neutral safety switch and battery. battery cables are clean and solid. I can start the Explorer by jumping across the solenoid. Once it starts it run's fine. No check engine light. I have also connected an ODB Code Reader and it does not show any codes.

If you replaced the solenoid was there one wire to the solenoid or two? If the new solenoid had two small posts did you connect the single wire to the S (starter) or I (ignition) side? If you connected to the I side this is your problem. If you had two wires were they swapped from original locations? If not check and see that you are getting electricity from the ignition switch to the solenoid. Have someone hold the key at the start position and measure if you have 12+ volts at the S side of the solenoid.

Check your Tickler wire on the Solenoid. There is a Website, PDFOO.com which is suppose to have diagrams.

When you installed the Ignition switch, was the Ignition in the Run position? As I recall there were also some slight adjustments to make for centering the Ignition switch. The cable or rod that works the Neutral start and the Interlock depend on the length of the cable to position the Interlock for starting. The switch may be new, but if the adjustments are off it won't work.

If you have a Stickshift, there was a safety switch on the clutch linkage so that the clutch had to be pressed for the Starter to engage.

I hope this info will help you.