1. Identify the 4WD System:
* Determine the type of transfer case: Your Silverado likely has either a NP203, NP205, or a similar transfer case. Knowing the specific model helps pinpoint potential problems. Check your owner's manual or a parts diagram online (using your VIN for accuracy).
* Understand the engagement method: Is it a floor-mounted shifter, or a column-mounted shifter?
2. Gather Information:
* When did the problem start? Did it happen suddenly, or gradually?
* What are the symptoms? Does it not engage at all? Does it engage but bind or make noise? Does it only work intermittently? Does it work in some 4WD settings (e.g., 4HI but not 4LO)?
* What were the conditions when the problem occurred? Was it on pavement, off-road, in mud or snow? This can sometimes help pinpoint the issue (e.g., a problem might only show up under load).
3. Visual Inspection:
* Check the transfer case shifter linkage: Make sure the linkage is properly connected and moves freely. Look for any broken or bent components. A disconnected or loose linkage is a common cause of 4WD problems.
* Inspect the driveshafts: Look for any damage, binding, or unusual wear. Check the U-joints for play.
* Check the front axle: Inspect the axle hubs for damage or binding. If your truck has manual locking hubs, ensure they are properly locked in when engaging 4WD.
* Check fluid levels: Check the transfer case fluid level (using the dipstick if present). Low fluid can cause all sorts of problems. Note the fluid's condition – milky or burnt fluid points to a serious issue.
4. Testing and Diagnostics:
* Try engaging 4WD on a level surface: Attempt to shift into 4HI (high-range 4WD) and then 4LO (low-range 4WD) with the truck stationary. Listen for any unusual noises (grinding, whining, clunking).
* Test 4WD on a loose surface: (Gravel, dirt, etc., in a safe area). If it engages, see if the front wheels are turning. If they're not, it's likely a problem with the transfer case, front axle, or hubs.
* Listen for noises: Drive slowly and listen carefully for unusual noises coming from the front end or the transfer case. Grinding or whining noises are common indicators of problems. A loud clunk when engaging or disengaging points towards worn components within the transfer case or driveline.
* Check the front differential: If your front axle is engaged but the wheels still aren't turning, there might be an issue with the front differential itself. This requires more advanced diagnostics.
5. Possible Causes and Solutions (based on your findings):
* Transfer Case Issues: Low fluid, worn gears, damaged internal components, linkage problems. This often requires a professional repair or transfer case rebuild/replacement.
* Front Axle Issues: Binding, broken parts, lack of lubrication.
* Driveshaft Issues: Worn U-joints, bent or damaged shafts.
* Hub Issues (if applicable): Manual locking hubs might not be engaging correctly.
* Electrical Issues (Less likely in a 1990 model): Although primarily mechanical, a rare electrical problem could affect some 4WD components.
Important Considerations:
* Safety First: Always test 4WD in a safe, controlled environment. Avoid sudden acceleration or harsh maneuvers while troubleshooting.
* Professional Help: If you're not comfortable working on your vehicle, take it to a qualified mechanic specializing in older trucks. Diagnosing and repairing 4WD systems can be complex.
* Repair Manuals: A Haynes or Chilton repair manual specific to your year and model Silverado will be invaluable. It provides detailed diagrams and troubleshooting steps.
This detailed guide should help you diagnose your 4WD problem. Remember to systematically check each component and be patient. If you're unsure about any step, consult a professional.
