1999 Ford Expedition truck will not start, just click click

Starter Drive Will Not Disengage
Starter motor loose on mountings. Tighten starter attach bolts
Worn drive end bushing
Damaged engine flywheel teeth. Replace flywheel and starter pinion gear for damage
Drive yolk return spring broken or missing. Replace return spring
Faulty ignition switch. Replace ignition switch
Insufficient clearance between winding leads to solenoid terminal and main contact in solenoid. Replace starter solenoid
Starter clutch not disengaging. Replace starter clutch
Ignition starter switch. Replace ignition switch contacts sticking

SYSTEM TESTS
ENGINE DOES NOT CRANK OR RELAY CLICKS
Check battery condition. Battery voltage should be 12 volts or more. Load test battery at approximately one-half cold cranking amperage rating. See load tester manufacturer's instructions. If battery voltage is less than 12 volts or loaded battery voltage is less than 9.6 volts, service battery or charging system as necessary. If battery is okay, go to next step.
Measure voltage between positive battery post and negative battery cable connection at engine block. If battery voltage exists, go to next step. If battery voltage does not exist, perform STARTER GROUND CIRCUIT TEST under ON-VEHICLE TESTING.
Measure voltage between positive battery post and starter motor case. If battery voltage exists, go to next step. If battery voltage does not exist, clean starter motor mounting flange and ensure starter motor is properly installed.
Turn ignition off. Measure voltage between starter solenoid terminal "B" and ground. See Fig. 3. If battery voltage exists, go to next step. If battery voltage does not exist, perform VOLTAGE DROP TEST under ON-VEHICLE TESTING.

UNUSUAL STARTER NOISE
Verify starter is installed properly. Ensure bolts are tight. Repair as necessary and check system for normal operation. If starter is mounted properly, go to next step.
Connect remote starter switch between starter solenoid terminal "B" (positive battery cable connection) and terminal "S" (Yellow/Light Blue wire). See Fig. 3. Engage starter motor and verify noise is coming from starter motor. If noise is coming from starter motor, go to next step. If noise is not coming from starter motor, diagnose engine mechanical concern. See appropriate engine article in ENGINES.
Remove starter motor. See STARTER MOTOR under REMOVAL & INSTALLATION. Inspect flywheel ring gear for wear or damage. If ring gear is okay, replace starter motor. If ring gear is worn or damaged, replace ring gear and inspect starter drive gear. If drive gear is also damaged, replace starter motor.