GMC 2001 Sierra - Manual (Page 373)

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How to Check Lubricant
1500 Series shown, 2500 LD Series similar.
2500 HD and 3500 Series located on rear axle cover.
The proper level for the 1500 Series is from 5/8 inch to
1 5/8 inch (15 mm to 40 mm) below the filler plug.
The proper level for the 2500/3500 Series is from
0 to 3/8 inch (0 to 10 mm) below the filler plug.
Add only enough fluid to reach the proper level.
What to Use
Refer to the Maintenance Schedule to determine what
kind of lubricant to use. See “Recommended Fluids and
Lubricants” in the Index.
Four - Wheel Drive
Most lubricant checks in this section also apply to
four
- wheel - drive vehicles. However, they have two
additional systems that need lubrication.
Transfer Case
When to Check Lubricant
Refer to the Maintenance Schedule to determine how
often to check the lubricant. See “Periodic Maintenance
Inspections” in the Index.
How to Check Lubricant
Manual Transfer Case