1. Check the Fuses:
* Locate the fuse box: There are usually two fuse boxes in a 1990 F-150, one under the hood and one inside the cab (often under the dash). Consult your owner's manual to locate them and identify the fuse(s) for the power windows.
* Inspect the fuses: Look for blown fuses (a broken filament inside the fuse). Replace any blown fuses with fuses of the same amperage. If the fuse blows immediately after replacement, you have a short circuit somewhere in the system (see step 5).
2. Check the Window Switch:
* Test the switch: Try the window switch in different positions. If the window works intermittently, the switch is likely faulty.
* Test with another switch: If possible, try swapping the faulty switch with a switch from another window to see if the problem follows the switch. This helps isolate whether the problem is the switch or the wiring.
3. Check the Window Motor:
* Listen for the motor: When you operate the switch, can you hear a clicking sound from the window motor (located in the door)? A clicking sound usually means the motor is receiving power but is not working correctly or is jammed. No sound indicates a lack of power or a completely failed motor.
* Manual override (if applicable): Some 1990 F-150s might have a manual crank override for the window. Try using this to see if the window moves. If it does, the problem is likely the motor or its connection.
4. Check the Wiring and Connections:
* Inspect the wiring harness: Carefully examine the wiring harness in the door for any frayed, broken, or disconnected wires. Pay close attention to the area where the wiring enters the door, as this is a common point of failure due to flexing.
* Clean connectors: Use electrical contact cleaner to clean any corroded or dirty connectors in the wiring harness.
* Check the regulator: The window regulator is the mechanism that moves the window. Sometimes, the regulator can become disconnected or jammed. This often requires removing the door panel to access and inspect.
5. Check for Short Circuits:
* If a fuse keeps blowing: A consistently blowing fuse means there's a short circuit somewhere in the system. This requires careful tracing of the wiring to find the source of the short. You might need a multimeter to check for shorts in the wiring. This can be challenging and may require professional help.
6. Replacing Parts (if necessary):
* Window motor: Replacing the window motor might be necessary if it's faulty.
* Window switch: If the switch is the culprit, you'll need a replacement.
* Window regulator: A damaged regulator will also need replacement.
* Wiring harness: In the worst-case scenario, a severely damaged section of the wiring harness might need replacement or repair.
Important Considerations:
* Door Panel Removal: To access the window motor and regulator, you will likely need to remove the door panel. There are many online guides and videos showing how to do this for a 1990 Ford F-150.
* Safety: Disconnect the battery's negative terminal before working with any electrical components.
* Professional Help: If you're not comfortable working on car electrical systems, it's best to take your truck to a qualified mechanic.
By systematically checking each component, you should be able to pinpoint the cause of the problem and fix your power window. Remember to consult a repair manual or online resources specific to your 1990 Ford F-150 for detailed diagrams and instructions.
