1. Identify the Type of 4WD System: 1990 Wranglers had different transfer cases (the part that selects 4WD). Knowing which one you have (Command-Trac, NP207, etc.) will help narrow down the potential problems. Your owner's manual should specify this.
2. Check the Basics:
* Is the Transfer Case in Neutral? Make absolutely sure the transfer case lever is not in neutral. This is the most common cause.
* Is the Transmission in Neutral or Park? You generally can't shift the transfer case unless the transmission is in neutral or park.
* Is the Vehicle Stopped? Attempting to engage 4WD while moving (especially at speed) can damage the drivetrain.
* Low Fluid Levels: Check the transfer case fluid level. Low fluid can cause shifting problems. The fluid should be the correct type for your transfer case (consult your owner's manual).
* Fluid Condition: Check the fluid's condition. If it's dark, burnt, or smells bad, it needs to be changed. Contaminated fluid can prevent proper shifting.
3. More Advanced Troubleshooting (Requires Mechanical Knowledge & Tools):
* Transfer Case Linkage: The linkage connecting the transfer case lever to the transfer case itself might be broken, bent, or disconnected. Inspect this linkage carefully.
* Shift Fork or Rail Problems: Internal components within the transfer case (shift forks and rails) can wear out or break, preventing shifting. This requires disassembly of the transfer case.
* Vacuum Actuator (If Applicable): Some systems use a vacuum actuator to engage 4WD. Check the vacuum lines for leaks or obstructions and ensure the actuator is functioning correctly.
* Electrical Issues (If Applicable): Some later models might have an electrical component involved in 4WD engagement. Check for any related fuses or electrical connections.
* Binding or Frozen Components: In the transfer case or front axle, something could be seized or frozen due to rust or lack of lubrication, preventing shifting.
4. If you're not mechanically inclined:
* Professional Mechanic: Taking your Jeep to a qualified mechanic specializing in Jeeps is highly recommended if you're not comfortable tackling these more advanced troubleshooting steps. They have the tools and expertise to diagnose and repair the problem accurately.
Safety Note: Working on a vehicle's drivetrain can be dangerous. Always use jack stands and chocks to ensure the vehicle is safely supported. If you are unsure about any step, consult a professional.
