If the belt is too loose, it's likely because:
* The tensioner is failing: This is the most common reason. A failing tensioner might not be applying enough force, or it could be seized. This requires replacing the tensioner, not adjusting it.
* The belt is worn or damaged: A worn or cracked belt won't grip properly, even with a properly functioning tensioner. Replace the belt.
* A pulley is seized or damaged: A pulley that's seized or damaged will increase the tension needed, potentially leading to a loose-seeming belt. These need to be addressed individually.
To address a loose serpentine belt on a 2003 S-10:
1. Inspect the belt: Check for cracks, fraying, glazing, or any other signs of wear. Replace the belt if damaged.
2. Inspect the tensioner: Look for any signs of damage or leakage. Listen for unusual noises when the engine is running. If it's suspected to be failing, it needs replacement. There's no adjustment; it's a complete replacement part.
3. Inspect the pulleys: Check for any seized or damaged pulleys. These will need to be replaced or repaired as needed.
Replacing the tensioner (or belt) is usually not a difficult job, but it requires some basic mechanical skills. You'll likely need some basic tools, including possibly a socket wrench, ratchet, and possibly a serpentine belt tensioner tool depending on the design, although some tensioners can be manipulated by hand using a pry bar or similar tool with caution. Consult a repair manual specific to your 2003 S-10 for detailed instructions, diagrams, and torque specifications. Improper tension can damage the belt and other components.
In short: Don't try to "adjust" the tensioner; if the belt is too loose, the problem lies in a worn or failing component (belt, tensioner, or pulley) that needs replacing.
