1. Verify Switch Replacement:
* Correct Part: Double-check you installed the *correct* sensor and dash switch for your specific year and model Expedition. There can be variations.
* Proper Installation: Ensure both the sensor and switch are securely seated and connected. A loose connection can cause intermittent or no function. Inspect for bent pins or damaged connectors.
2. Beyond the Switch and Sensor:
The dash switch and sensor are just part of a larger 4WD system. Problems could lie elsewhere:
* Transfer Case Motor/Actuator: This motor engages the 4WD system. It might be faulty, sticking, or have low voltage. Listen carefully for any unusual noises (grinding, whirring) when attempting to engage 4WD.
* Transfer Case Linkage (if applicable): Some systems use a linkage to connect the actuator to the transfer case. Check for proper movement and any binding.
* Vacuum System (if applicable): Some older 4WD systems use vacuum to shift. Check for leaks in the vacuum lines going to the transfer case.
* Low Voltage/Wiring: A poor electrical connection anywhere in the 4WD system can prevent engagement. Check all wiring for damage, corrosion, or loose connections. A voltage drop test might be necessary.
* Blend Door Actuator: While less likely, a faulty blend door actuator could cause the 4x4 to not engage.
3. Diagnostic Tests:
* Check the Transfer Case: With the vehicle safely parked, try to manually engage the transfer case lever (if equipped). If you can engage it manually, the problem lies before the transfer case (likely electrical or vacuum). If not, the problem is within or after the transfer case.
* OBD-II Scanner: Use an OBD-II scanner to check for any diagnostic trouble codes (DTCs) related to the 4WD system. These codes provide valuable clues.
* Wiring Diagram: Obtain a wiring diagram for your Expedition's 4WD system. This will help trace the electrical circuits and identify potential problems.
* Professional Diagnosis: If you're not comfortable with electrical diagnostics, a qualified mechanic with experience in 4WD systems can perform a thorough inspection and diagnosis.
4. Specific to 1998 Expedition:
The 1998 Expedition's 4WD system is relatively straightforward compared to more modern electronic systems, but still requires attention to detail. Refer to a repair manual specific to your year and model to better understand the system's components.
Safety Precautions:
* Never attempt to force 4WD engagement. This could damage the system.
* Work on a level surface with the vehicle securely parked. Engage the parking brake.
* If unsure about any repair step, consult a professional mechanic.
By systematically checking these areas, you should be able to pinpoint the cause of the 4WD malfunction. Remember to always consult a repair manual specific to your 1998 Ford Expedition for detailed information and diagrams.
