1. Identify the 4WD System Type: 1991 Silverados used either a part-time or a shift-on-the-fly system. Knowing which one you have is crucial. The owner's manual should clarify, or you can often find this information online by searching for your specific trim and drivetrain.
2. Check the Simple Things First:
* Transfer Case Lever/Switch: Ensure the transfer case lever (if applicable) or the 4WD switch is correctly positioned. Is it in 4Hi or 4Lo? Does the indicator light illuminate? A faulty indicator light doesn't necessarily mean a problem with the 4WD, but it's a good starting point.
* Low Fluid Levels: Check the transfer case fluid level. Low fluid can prevent engagement. Consult your owner's manual for the correct fluid type and procedure. Also check the front differential fluid level.
* Fuse/Relay: Check the relevant fuses and relays in the fuse box. Your owner's manual will show the fuse/relay diagram. Some systems use a relay to activate the 4WD system.
3. More Advanced Troubleshooting (May Require Mechanical Knowledge):
* Transfer Case Issues: If the fluid levels are fine, the problem could be within the transfer case itself. This might include:
* Broken linkage: Inspect the linkage connecting the shift lever to the transfer case.
* Internal failure: This is more complex and requires professional diagnosis. Symptoms could include grinding noises or a complete lack of engagement regardless of the lever position.
* Actuator motor (if equipped): Some systems use an electric motor to engage the 4WD. This motor can fail.
* Front Axle Issues: The issue might be in the front differential or axle components. This could include:
* Low or contaminated differential fluid: Check the fluid level and condition.
* Broken or damaged axle components: This might require a visual inspection of the axle shafts, CV joints (constant velocity joints), and other components.
* Engagement issues: The front axle might not be engaging properly, even if the transfer case is working correctly.
* Electrical Problems: If it's an electrical system, a wiring harness issue, faulty sensor, or a problem with the 4WD control module might be the root cause. This typically requires a multimeter to check voltage and continuity.
4. When to Seek Professional Help:
If you've checked the simple things and are still not engaging 4WD, it's time to consult a mechanic. Diagnosing and repairing transfer case or axle problems often requires specialized tools and expertise. Attempting repairs beyond your skill level could lead to further damage and increased costs.
Important Considerations:
* Owner's Manual: Your owner's manual is an invaluable resource. It contains specific instructions and diagrams for your vehicle's 4WD system.
* Safety: Working under a vehicle can be dangerous. Use jack stands and chocks to ensure the vehicle is securely supported.
By systematically checking each component, you can narrow down the cause of the 4WD problem. Remember that tackling more complex issues requires mechanical aptitude and the right tools. If unsure, leave it to a professional.
