1. The Shift Linkage:
* Loose or Damaged Linkage: Inspect the linkage connecting the transfer case shift lever to the transfer case itself. Look for bent, broken, or loose components. Worn bushings are common culprits. Movement should be smooth and positive with no binding.
* Incorrect Shift Lever Adjustment: The shift linkage might need adjustment to ensure the lever is correctly engaging the 4-High position. This often involves adjusting connecting rods or cables.
* Binding or Sticking Linkage: Dirt, debris, or corrosion can cause the linkage to bind or stick. Clean and lubricate all linkage components thoroughly.
2. The Transfer Case:
* Internal Transfer Case Issues: A problem *inside* the transfer case is the most likely culprit if the linkage seems fine. This could involve:
* Shift Fork or Rail Problems: The internal components that mechanically shift the transfer case may be worn, broken, or binding. This requires disassembly and inspection of the transfer case itself.
* Low Transfer Case Fluid: Check the transfer case fluid level (usually requires removing a fill plug) and inspect for condition (it should be clean and the correct type). Low fluid can cause shifting problems. If low, a leak needs to be investigated.
* Internal Damage: More severe damage, such as stripped gears or broken components within the transfer case, is a possibility, and usually requires professional repair or replacement.
3. Electrical Issues (Less Likely for just 4-High):
While less likely to affect only 4-High on a 1994 S10 Blazer (which is predominantly mechanically shifted), there's a small chance of:
* Electrical Solenoid Problems (if applicable): Some later models may have electrical components involved even in mechanical systems. Inspect any related solenoids for proper operation and wiring integrity. This is less probable given the problem only affects 4-High.
4. Vacuum System (Less Likely):
While not common on 1994 S10's 4x4 engagement, ensure there aren't any vacuum leaks impacting the 4x4 system. A vacuum leak would likely cause issues in both High and Low range.
Troubleshooting Steps:
1. Visual Inspection: Thoroughly inspect the entire 4WD shift linkage from the cab to the transfer case. Look for any obvious damage, looseness, or binding.
2. Check Transfer Case Fluid: Check the fluid level and condition. A leak is a significant possibility if the fluid is low.
3. Manually Try to Shift (Advanced & Careful): If the linkage seems okay, *carefully* attempt to manually shift the transfer case into 4-High using a wrench or socket on the transfer case shift lever (only if you're comfortable working on vehicles). This can help isolate if the problem is linkage or internal.
4. Professional Help: If you're not comfortable working on the transfer case, take it to a mechanic specializing in 4x4 systems. Diagnosing a transfer case problem requires skill and specialized tools.
Remember safety first! Working under a vehicle requires proper jack stands and safety precautions. Improper repair attempts can be dangerous.
