Car will not turn over

For intermittent cranking problems on Ford products, a common cause is a corroded battery cable. The corrosion takes places right down by the starter, but it's hidden under the insulation. The clue is you will still hear the single, rather loud clunk of the starter relay when you turn the ignition switch to "crank". To verify this, use a test light to check for voltage at the starter motor. Ford used a number of different circuit designs. Without looking up the one used on your model, start by watching that voltage at the starter. It could be 0 volts or 12 volts before the ignition switch is turned on. What's important is what you find when a helper turns the ignition switch to "crank". The problem is you have to measure this voltage while the no-crank is acting up. If you find 0 volts, or only a few volts, (very dim test light), wiggle the cable. If the starter suddenly starts working, suspect that cable.

If you don't hear the relay clunk when you turn the ignition switch, the neutral safety switch becomes a good suspect. Try shifting to "neutral", then try cranking the engine.