1. Identify the Transfer Case: 1991 Wranglers likely have either an AX-5 or AX-15 manual transmission. The transfer case (the box that splits power to the front and rear axles) is crucial. Common transfer cases include the NP207, NP231, and NP241. Knowing which you have will help narrow down potential issues. Your owner's manual should specify.
2. Check the Obvious:
* Shift Lever: Make sure the transfer case shift lever is moving freely and fully into the 4WD positions (usually 2HI, 4HI, Neutral, and 4LO). Is there any binding or resistance? A linkage problem can prevent proper engagement.
* Fluid Levels: Check the transfer case fluid level. Low fluid is a major cause of failure. The fluid should be a reddish color and not milky (indicating water contamination). If low, you'll need to top it off or have it replaced. (Note: This needs to be done with the Jeep on a level surface.)
* Vacuum System (If Applicable): Some 4WD systems in that era utilize a vacuum system to engage the front axle. Listen carefully when shifting into 4WD – you should hear a slight vacuum "whoosh" sound as the system engages. If you don't, there might be a leak in a vacuum line or a problem with the vacuum actuator. Inspect all vacuum lines for cracks, leaks, or disconnections.
3. More Involved Checks:
* Front Axle Engagement: With the transfer case in 4HI, jack up the front axle and rotate the tire by hand. It should be difficult to rotate if the front axle is engaged properly. If it rotates easily, the issue lies within the front axle engagement system (this can involve hubs, locking hubs, or other components depending on your specific model).
* Driveshaft: Visually inspect the driveshafts for damage or anything binding. A broken driveshaft is a clear sign of a problem.
* Transfer Case Linkage: If the shift lever seems okay, but the transfer case isn't shifting, there might be an issue with the linkage connecting the lever to the transfer case. This often involves adjusting or repairing the linkage itself.
4. Professional Help:
If you're not comfortable tackling this yourself, take your Jeep to a mechanic specializing in Jeeps or 4WD systems. They'll have the experience and tools to diagnose the problem accurately and efficiently. Simply saying "4WD isn't working" isn't enough information for them to determine the cause. The more information you can give them (e.g., noises, specific behaviors, etc.), the better they can assist.
Important Considerations:
* Never attempt to engage 4WD while moving at high speeds. This can cause serious damage to your drivetrain.
* Driving in 4WD on dry pavement can lead to premature wear and damage. Only use 4WD when necessary (off-road, snow, ice, etc.).
By following these steps, you should be able to narrow down the source of your 4WD problem. Remember safety first, and don't hesitate to seek professional assistance if needed. A detailed description of the problem (e.g., does it make any noises, etc.) will help a mechanic considerably.
