* Transfer Case Issues: This is the most likely culprit. Problems within the transfer case itself could prevent 4 High from engaging. This could include:
* Shift linkage: The linkage connecting the shifter to the transfer case might be bent, broken, or improperly adjusted, preventing the 4 High gear from selecting properly.
* Transfer case chain/gears: Wear, damage, or a broken chain inside the transfer case could prevent 4 High from engaging.
* Transfer case actuator (if equipped): Some systems use an actuator to engage the transfer case gears. A faulty actuator, its wiring, or the associated shift motor could be the problem.
* Low fluid level or contaminated fluid: Low or dirty transfer case fluid can cause poor shifting or complete failure to engage.
* Front Axle Issues: While less likely if 4 Low works, there could be a problem with the front axle engagement:
* Front axle disconnect (if equipped): Some Cherokees had automatic front axle disconnects. A malfunction in this system could prevent the front axle from engaging even if the transfer case is in 4 High.
* Front axle hubs (manual locking hubs): If your Cherokee has manual locking hubs, they might not be fully engaged. Make sure they're locked in properly. (Check the owner's manual if you're unsure how to do this correctly).
* Front driveshaft problems: Although less likely, a problem with the front driveshaft (like a broken U-joint) could prevent 4 High from engaging effectively, but 4 Low might still work due to the lower speeds and torque involved.
* Vacuum System Problems (if applicable): Some 4x4 systems on older Jeeps use a vacuum system to engage the front axle. Leaks in the vacuum lines can prevent proper engagement.
* Electrical Problems (if applicable): Depending on your Jeep's specific system, there could be electrical components involved in engaging 4 High. A faulty sensor, switch, or wiring harness could be at fault.
Troubleshooting Steps:
1. Check the transfer case fluid: Low or dirty fluid is a common problem.
2. Inspect the shift linkage: Ensure it's properly connected and moves freely.
3. Check the front axle hubs (if applicable): Make sure they're locked in properly.
4. Listen for unusual noises: While driving in 4 Low, listen for any grinding, clicking, or whining sounds that could indicate transfer case or axle problems.
5. Inspect vacuum lines (if applicable): Check for leaks or damage.
6. Consult a repair manual: A service manual specific to your 1988 Jeep Cherokee will provide detailed diagrams and troubleshooting steps.
If you're not comfortable diagnosing this yourself, it's best to take your Jeep to a mechanic experienced with older Jeeps. A proper diagnosis will save you time and potential further damage.
