1. Verify Power and Ground:
* Battery Voltage: Even if you think your battery is okay, double-check its voltage with a multimeter. It needs to be at least 12.6 volts when not cranking, and above 10 volts while cranking. Low voltage is a common cause of starting problems. Clean the battery terminals thoroughly; corrosion significantly hinders current flow.
* Battery Cables: Inspect the positive (+) and negative (-) battery cables for corrosion, looseness, or damage. Clean them thoroughly and ensure they're securely connected to both the battery and the starter. Loose or corroded connections can prevent sufficient current from reaching the starter.
* Ground Connections: A poor ground connection is a frequent culprit. Trace the negative cable from the battery. Make sure it's well-grounded to the engine block (and ultimately the chassis). Clean any grounding points along the path. Consider adding an auxiliary ground strap from the engine block directly to the chassis for a better connection.
2. Check the Starter Solenoid:
* Listen Carefully: When you turn the key, you hear the solenoid *click*. This indicates it's receiving power but isn't engaging the starter motor. The click may be faint, so listen closely.
* Solenoid Wire Connections: Inspect the wires connected to the starter solenoid. Look for corrosion, loose connections, or broken wires.
3. Test the Starter Relay:
Although you say the relay is spinning, there's a chance the relay itself is faulty. You can test this in several ways:
* Visual Inspection: Check the relay for any signs of damage.
* Relay Swap (Best Method): If you have a spare relay (a similar one from another system in the truck, maybe a headlight relay), temporarily swap it to see if that fixes the problem. This is the easiest and most conclusive way to test the relay.
* Multimeter Test (Advanced): You can test the relay's internal contacts with a multimeter, but this requires understanding how relays work and using a multimeter safely.
4. Ignition Switch:
* Check the Ignition Switch: A failing ignition switch may not be providing the correct voltage or signal to the starter solenoid, even if the relay is clicking. This is a less common problem but should be considered if other tests fail.
5. Neutral Safety Switch (Manual Transmission):
* Gear Position: Ensure the transmission is fully in neutral. The neutral safety switch prevents the starter from engaging if the vehicle is not in neutral. A faulty switch can prevent the starter from engaging even when the shifter is in neutral. You can sometimes test this by using a multimeter to check for continuity between the wires at the switch.
6. Wiring Issues:
* Inspect Wiring Harness: Check the wiring harness leading to the starter for any breaks, chafing, or damage. A broken wire anywhere in this circuit will prevent the starter from engaging.
Important Note: If you're not comfortable working with automotive electrical systems, take your F-150 to a qualified mechanic. Improperly working on electrical systems can lead to damage or injury.
If you've checked all these things and the starter still isn't working, it's possible there is an issue with the new starter itself (though less likely), even if it's spinning the relay. A defective starter isn't unheard of. Getting a professional diagnosis at this point is recommended.
