* Transfer Case Actuator/Motor: This is the most common culprit. The actuator is an electric motor that shifts the transfer case between 2WD, 4WD High, and 4WD Low. If it's malfunctioning, it might not be able to disengage 4WD Low. This could be due to a faulty motor, a bad wiring harness, or a low voltage supply.
* Transfer Case Linkage (if applicable): Older systems might use a linkage instead of an electric actuator. This linkage could be bent, broken, or binding, preventing the transfer case from shifting.
* Transfer Case Internal Problem: While less common, there's a chance of an internal mechanical problem within the transfer case itself, preventing it from shifting out of 4WD Low. This could involve broken shift forks, worn parts, or low transfer case fluid.
* Vacuum System (if applicable): Some older 4WD systems use a vacuum system to engage and disengage 4WD. A leak in the vacuum lines or a faulty vacuum pump could prevent shifting.
* Fuse or Relay: A blown fuse or a faulty relay related to the 4WD system can also cause this problem.
Troubleshooting Steps (Proceed with caution):
1. Check the obvious: Make sure the transfer case shift lever is in the 2WD position. Sometimes it can feel like it's in the right position, but isn't fully engaged.
2. Check the fuses and relays: Consult your owner's manual to locate the fuses and relays associated with the 4WD system. Check them for damage or blown elements. Replace any faulty ones.
3. Listen for noises: While attempting to shift out of 4WD Low, listen carefully for any unusual noises coming from the transfer case area. Grinding or whining sounds indicate a potential internal problem.
4. Inspect the actuator/linkage: If you have an electric actuator, visually inspect it for any signs of damage. If it's a linkage system, check for bends, breaks, or binding.
5. Check the transfer case fluid: Low or contaminated fluid can cause shifting problems. Check the fluid level and condition. Replace the fluid if necessary. (Consult your owner's manual for proper procedure).
6. Consult a repair manual: A repair manual specific to your 1999 GMC Suburban will provide detailed diagrams and troubleshooting steps for your 4WD system.
Important Considerations:
* Driving in 4WD Low on dry pavement can damage your drivetrain. Do not drive long distances in 4WD Low on paved surfaces. This can cause excessive wear and tear on your components and potentially damage your axles, transfer case, and even your transmission.
* Professional diagnosis: If you're not comfortable working on your vehicle's drivetrain, it's best to take it to a qualified mechanic for diagnosis and repair. This will prevent potential further damage and ensure the repair is done correctly.
By systematically checking these areas, you should be able to narrow down the cause of your problem. Remember safety first and don't hesitate to seek professional help if needed.
