1. The Obvious First Steps:
* Shift to 2WD: Make absolutely sure you've tried shifting the transfer case lever to 2WD. Seems obvious, but it's the first thing to check! Sometimes the lever feels like it's in 2WD, but isn't fully engaged.
* Check the Indicator Light: Is the 4WD indicator light on the dash illuminated? If it's flashing, that's a strong indicator of a problem. A constantly lit light might mean a sensor is stuck, while a flashing light usually signifies an electrical fault or sensor issue.
* Terrain: Are you on a slippery surface? The system might be engaging itself due to wheel slippage, trying to maintain traction. Try driving on a dry, level surface.
2. Electrical System Checks (Most Likely Culprit):
* Transfer Case Motor: The electric motor that engages and disengages the 4WD system can fail. This is a common point of failure. It could be seized, jammed, or have a burned-out winding. You might need a multimeter to check for power at the motor connector when you try to shift.
* Transfer Case Control Module (TCM): This module controls the motor. A faulty TCM can prevent the system from disengaging. This usually requires diagnostic testing with a scan tool capable of reading OBDII codes (if your Blazer has OBDII, which it might, depending on its trim). Many auto parts stores can do this for free.
* Wiring and Connectors: Check the wiring harness leading to the transfer case motor and the TCM for any damage, corrosion, or loose connections. A broken wire or corroded connector can disrupt the electrical signals. Pay close attention to connectors near the transfer case – they’re exposed to the elements.
* Shift Motor Solenoid: This is a part of the transfer case actuator. If it's malfunctioning it could be preventing the shift.
3. Mechanical Issues (Less Likely, but Possible):
* Binding in the Transfer Case: It's less common, but internal mechanical binding within the transfer case itself could prevent disengagement. This usually requires professional diagnosis and potentially a costly repair or replacement of the transfer case. You'd likely hear unusual noises if this were the case.
4. Diagnostic Steps:
* OBDII Scanner: A scan tool that reads codes for the Blazer's system can reveal error codes related to the 4WD system, pointing you towards a specific component. A professional mechanic will have the equipment to do this properly.
* Professional Mechanic: If you're not comfortable with electrical diagnostics or mechanical work, taking it to a mechanic is the best approach. They have the tools and expertise to diagnose the problem accurately and perform the necessary repairs.
Important Considerations:
* Driving in 4WD on dry pavement can damage the drivetrain. Don't continue driving in 4WD if you don't need it. The constant engagement puts extra stress on components.
* Safety: If you're unsure about the cause, don't attempt complex repairs yourself. Working on a vehicle's drivetrain can be dangerous if not done correctly.
The most likely cause is an electrical problem within the 4WD system (the motor, TCM, or wiring). Start with the simple checks and then proceed to the more involved electrical troubleshooting if needed. Don't hesitate to seek professional help if you're not comfortable with automotive diagnostics and repair.
