* Transfer Case Issues: This is the most likely problem. The transfer case is responsible for distributing power to the front and rear axles. Problems can include:
* Low Transfer Case Fluid: Check the fluid level and condition. Low fluid or contaminated fluid will prevent proper engagement.
* Transfer Case Motor Problems: The motor that engages the 4WD system might be failing, or its electrical connections could be faulty. This could explain the switching sound but lack of engagement.
* Internal Transfer Case Problems: Gears, chains, or other internal components might be worn, broken, or binding, preventing the system from properly engaging. This often requires a rebuild or replacement.
* Shift Linkage: The linkage that connects the shift lever to the transfer case might be broken, bent, or disconnected.
* Front Axle Issues: Even if the transfer case is engaging, problems in the front axle can prevent 4WD from working:
* Front Axle Disconnect (if equipped): Some Blazers have a front axle disconnect system. This system might be malfunctioning, preventing the front axle from engaging even if the transfer case is working.
* Front Axle Hubs (manual locking hubs): If your Blazer has manual locking hubs, ensure they are properly engaged. If they are automatic, they may be faulty.
* CV Axles or U-Joints: Damaged CV axles or universal joints (U-joints) in the front axle will prevent power from reaching the wheels.
* Electrical Problems:
* Wiring: Damaged or corroded wiring between the transfer case motor, the 4WD switch, and the control module can interrupt the electrical signal.
* 4WD Switch: The 4WD switch itself could be faulty.
* Control Module (BCM/PCM): A problem with the Body Control Module (BCM) or Powertrain Control Module (PCM) can prevent the system from engaging correctly. A diagnostic scan is needed to check this.
Troubleshooting Steps:
1. Check Transfer Case Fluid: This is the first and easiest thing to do. Check the level and condition of the fluid.
2. Inspect Linkage: Visually inspect the linkage connecting the shift lever to the transfer case for any damage or disconnections.
3. Check Front Axle Hubs (if applicable): Ensure the front axle hubs are properly locked if they are manual hubs. If automatic, they may need inspection or replacement.
4. Listen Carefully: While attempting to engage 4WD, listen closely for unusual noises coming from the transfer case or front axle. Grinding, whining, or clicking sounds indicate a problem.
5. Diagnostic Scan: Take your Blazer to an auto parts store or mechanic for a diagnostic scan. This can read trouble codes from the vehicle's computer that pinpoint the exact problem.
Important Note: A blinking 4WD light is a clear indication of a problem. Continuing to try to engage 4WD when the system is malfunctioning can cause further damage. It's best to diagnose and repair the problem before attempting to use 4WD again.
