2006 GMC Yukon Owner's Manual - Page 422

Four-Wheel Drive Lubricant checks in this section also apply to these vehicles. There are two additional systems that need lubrication. To get an accurate reading, the vehicle should be on a level surface. If the level is below the bottom of the filler plug hole, you'll need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. Use care not to overtighten the plug. Transfer Case When to Check Lubricant It is not necessary to regularly check transfer case fluid unless you suspect there is a leak, or you hear an unusual noise. A fluid loss could indicate a problem. Have it inspected and repaired. When to Change Lubricant Refer to the Maintenance Schedule to determine how often to change the lubricant. See Scheduled Maintenance on page 6-4. How to Check Lubricant What to Use Refer to the Maintenance Schedule to determine what kind of lubricant to use. See Recommended Fluids and Lubricants on page 6-13. Automatic Transfer Case 5-48