High-Probability Causes:
* Dead Battery: This is the most obvious and easiest to check. Test the battery voltage with a multimeter. It should read around 12.6V when not charging. If it's low, charge or replace it.
* Bad Battery Cables/Connections: Corroded, loose, or broken battery cables (positive and negative) can prevent current from flowing. Inspect them thoroughly and clean the terminals.
* Faulty Ignition Switch: The switch itself could be internally failed, preventing power from reaching other components. This is a less likely cause initially, as other symptoms (intermittent starting, etc.) usually precede complete failure.
* Blown Fuse(s): Check the fuses related to the ignition system. Your owner's manual will show fuse box locations and diagrams. Look for blown fuses (broken filament).
* Faulty Neutral Safety Switch (NSS): This switch prevents the engine from starting unless the transmission is in park or neutral. A faulty NSS won't allow power to the ignition system. This is especially likely if the truck won't crank *at all*.
* Loose or Corroded Wiring: Look for loose connections or corrosion in the wiring harness leading to and from the ignition switch. Pay close attention to areas prone to moisture or vibration.
Less-Likely Causes (but still possible):
* Faulty Alternator: While the alternator charges the battery, a completely failed alternator might not allow enough power to the ignition system. This is more likely to present with dimming lights and other electrical issues *before* a complete power loss.
* Starter Solenoid: A failed starter solenoid prevents the starter motor from engaging, even if the ignition switch is receiving power. You might hear a click but no cranking.
* Main Power Relay: Some Ford vehicles use a main power relay that might be faulty, cutting off power to various systems, including the ignition.
* Ignition Switch Wiring Harness: Damage to the wiring harness near the ignition switch itself is possible but less common unless there's physical damage to the harness.
* Body Control Module (BCM) Issues (Less likely on a 1990): While less prevalent on older models, a failing BCM could interrupt power distribution, but this is usually accompanied by multiple electrical problems.
Troubleshooting Steps:
1. Check the battery and cables first. This is the simplest and often the solution.
2. Consult your owner's manual for fuse box locations and diagrams. Replace any blown fuses.
3. Visually inspect all wiring related to the ignition switch and battery. Look for corrosion, breaks, or loose connections.
4. Use a multimeter to check for voltage at various points. Start at the battery, then trace the positive wire to the ignition switch and see if voltage is present. This will help pinpoint where the power is lost.
5. Test the NSS (Neutral Safety Switch): There are several ways to test this; an online search for your specific truck will yield information on testing procedures.
If you've checked these and still have no power, it's best to consult a qualified mechanic. They have the tools and experience to diagnose more complex electrical problems. Trying to troubleshoot advanced electrical systems without proper knowledge can be dangerous.
