2021 Kenworth T680 Serpentine Belt Routing, Length & Part Numbers

As an ASE-certified master technician, I know that keeping a heavy-duty workhorse like the 2021 Kenworth T680 on the road means staying on top of preventative maintenance. The serpentine belt (or accessory drive belt) is the lifeline of your engine's accessories. Whether your T680 is powered by the popular Cummins X15 or the PACCAR MX-13, replacing the belt and tensioner before they fail can save you from a massive roadside towing bill.

Note: Heavy-duty trucks often feature multiple engine and accessory configurations. The specifications below are based on the common single-belt main accessory drive for the Cummins X15 engine option. The PACCAR MX-13 typically uses a dual-belt system. Always verify with your VIN at the parts counter.

Quick answer

• Belt length: 85.4 inches (varies slightly by accessory package)
• Rib count: 8-rib (Heavy-Duty Micro-V)
• OEM belt PN: Cummins 3103233 (or PACCAR equivalent)
• Aftermarket cross-reference: Gates K080854HD (FleetRunner), Dayco 5080854, Continental Elite 4080854
• Auto-tensioner PN: Cummins 3691280 / Gates 38667
• Idler pulley PN: Cummins 3687223 / Gates 36344

Belt routing

Because engine bays are tightly packed and heavy-duty routing can be complex, always take a clear photograph of your existing belt routing before you remove it. If your under-hood routing sticker is missing or faded, follow this general text-based path for the main accessory drive:

1. Start by looping the belt around the bottom of the crankshaft pulley (the largest pulley at the bottom of the engine).
2. Route the belt up and over the inside of the A/C compressor pulley.
3. Bring the back (smooth side) of the belt around the first idler pulley.
4. Pull the ribbed side around the alternator pulley.
5. Route it down around the power steering pump pulley.
6. Bring it across to the water pump pulley.
7. Finally, slip the smooth side of the belt over the auto-tensioner pulley as you relieve the tension to slide it fully into place.

How to replace the serpentine belt

Replacing the belt on a Class 8 truck like the T680 requires caution, as the belt tensioners are incredibly stiff.

1. Locate the tensioner: Find the automatic belt tensioner on the front of the engine block.
2. Relieve tension: Insert a 1/2-inch drive heavy-duty serpentine belt tool (or a long-handled 1/2-inch breaker bar, depending on the square drive hole in your specific tensioner). Rotate the tensioner away from the belt to release the pressure.
3. Remove the old belt: While holding the tensioner back, slide the belt off the easiest accessible pulley (usually the alternator, water pump, or an idler). Carefully release the tensioner back to its resting position.
4. Inspect components: Before installing the new belt, spin all idler pulleys, the alternator, and the water pump by hand. They should spin smoothly with no grinding, wobble, or binding.
5. Install the new belt: Route the new belt according to your photograph or the under-hood diagram, leaving the tensioner pulley for last. Crank the tensioner open with your breaker bar, slip the belt over the final pulley, and slowly release the tensioner so it applies pressure to the belt.
6. Verify alignment: Double-check that the belt ribs are perfectly seated in the grooves of every ribbed pulley.

Replacement interval

For heavy-duty applications like the 2021 Kenworth T680, preventative maintenance is strictly scheduled. The serpentine belt should be thoroughly inspected every 25,000 miles or during your standard PM intervals. Standard replacement is recommended every 100,000 miles or 2 years, whichever comes first. When replacing the belt, it is an industry best practice to replace the automatic tensioner and idler pulleys at the same time, as their bearings wear out at a similar rate.

Signs of a failing belt or tensioner

Don't wait for the belt to snap on the highway. Look out for these warning signs:

• Squealing or chirping noises: Especially on cold startups or when the A/C compressor engages.
• Visible wear: Look for cracking (more than 3 cracks per inch of ribbing), chunking, missing ribs, or a glazed, shiny appearance on the belt surface.
• Tensioner bounce: While the engine is idling, watch the auto-tensioner. A slight vibration is normal, but if the tensioner arm is bouncing wildly, the internal spring is weak and the unit must be replaced.
• Accessory issues: A slipping belt can cause a loss of power steering assist, poor A/C performance, or low alternator voltage output.
• Rust dust: A buildup of red/brown dust around the tensioner or idler pulley bearings indicates impending bearing failure.

If you are not comfortable performing this heavy-duty maintenance yourself, don't risk a breakdown. Find a trusted shop on Repairius.