96 Ford Aerostar won't start!

I have a 96 Ford Aerostar 4.0L that will not start. I get a spark, but it will not turnover. I have replaced its dead battery, its dead starter, positive and negative cables, and cyphalid(sp?). I cannot figure out what would be causing it to not start. The belts used to jump a little when turning it on. Now they don't move at all and it just clicks once. I still get a spark. Need help!!

Are you saying the starter won't crank? If so, check the starter circuit. Did you try cranking in neutral if automatic. Check the circuits at the starter relay, voltage to load/switch side of relay is hot all the time. Voltage to the coil side of relay, goes hot with the key in the crank position, is wired through neutral switch.

If your saying, it cranks good and spark at all plugs, use a gage and check fuel pressure.

Thank you much. It ended up being the starter relay.