2025 Kenworth T680 — Serpentine Belt Routing & Length

Quick answer

• Belt length: Varies by engine and accessory layout; most 2025 T680 setups fall in the ~85–115 inch (2160–2920 mm) range. Always confirm by VIN/engine serial and current accessory configuration.
• Rib count: Typically 8- or 10-rib micro-V on heavy-duty T680 applications. Verify on-vehicle before ordering.
• OEM serpentine belt PN: Varies by engine option.
  • PACCAR MX-13/MX-11: Obtain the exact PN from a Kenworth dealer using your VIN.
  • Cummins X15: Confirm via Cummins QuickServe using your engine serial number (ESN).
• Aftermarket cross-refs: Match length and rib count to the verified OEM PN. Common heavy-duty lines include Gates FleetRunner (K08xxxx/K10xxxx series), Dayco Gold Label HD, and Continental/ContiTech HD micro-V. Do not order until you verify rib count and effective length from the OEM PN.
• Auto-tensioner PN: Application-specific (bracket and arm offset differ by engine). Common suppliers: PACCAR, Cummins, Litens, Gates DriveAlign, Dayco. Verify by VIN/ESN before purchase.
• Idler pulley PN: Application-specific (smooth or grooved, diameter often 70–90 mm). Verify by VIN/ESN and by checking if your idler is smooth or ribbed.

Belt routing

Because 2025 T680s are offered with multiple engines and accessory packages, the exact routing can vary. Under the hood, look for a routing decal on the fan shroud or radiator support. Before you remove the old belt, take clear photos of the entire belt path from multiple angles. If your truck lacks a decal, use the following general routing that many T680 configurations follow:

1. Start at the crankshaft pulley (harmonic balancer) at the bottom of the engine.
2. From the crank, wrap the belt up and over the A/C compressor clutch.
3. From the A/C compressor, route to the nearest idler pulley to maintain wrap and alignment.
4. From that idler, go up and around the alternator pulley.
5. From the alternator, route down to the power steering pump pulley.
6. From the power steering pump, cross to the water pump pulley.
7. From the water pump, pass under/over the automatic tensioner (depending on the engine’s bracket) and return to the crank pulley.

Notes:

• The idler(s) may be smooth or grooved; the belt’s ribbed side must contact grooved pulleys, and the smooth back of the belt runs on smooth/idler pulleys.
• If your configuration includes a second idler or a fan clutch drive pulley in the path, maintain the same ribbed/smooth contact rules while following your photos.

How to replace the serpentine belt

• Tools: serpentine belt tool or a 3/8-inch breaker bar for the tensioner square-drive. A long-handle ratchet gives better leverage on heavy-duty tensioners. A mirror and flashlight help confirm full rib engagement.
• Preparation: Disconnect battery negative if your hands will be near the starter cable or alternator B+ post. Photograph the existing routing. Verify you have the correct belt by rib count and effective length.
• Relieve tension: Insert the 3/8-inch drive into the tensioner and rotate in the direction that unloads the belt. Many HD tensioners are strong—use controlled force and keep fingers clear.
• Remove belt: Slip the belt off the easiest top-side pulley last (often the alternator) and extract it from the bay.
• Inspect components: Spin the idler(s) and tensioner pulley. Any roughness, wobble, grease leakage, or noise means replacement. Check the tensioner arm for smooth travel and proper return.
• Install new belt: Route following your photos and the routing section above, leaving the alternator (or another easy-access upper pulley) for last. Rotate the tensioner, slip the belt on, then slowly release the tensioner.
• Verify alignment: Ensure every groove is seated on grooved pulleys and the belt back rides only on smooth pulleys. Mis-seating by one rib can shred a belt quickly.
• Start and recheck: Start the engine, observe tracking, and listen for noise. Shut down and re-check seating after 1–2 minutes of running.

Replacement interval

• Inspect at every service. Replace the belt at the first sign of wear or contamination. Typical heavy-duty intervals are 60,000–100,000 miles (or 2–3 years), but follow PACCAR/Cummins guidance for your ESN and duty cycle.
• Replace the automatic tensioner and idler(s) proactively every 100,000–150,000 miles or when replacing the belt if there is any play, noise, or misalignment.

Signs of a failing belt or tensioner

• Belt: glazing/shiny ribs, cracks across ribs, chunking/missing ribs, frayed edges, belt dust, chirp/squeal on start-up, slippage under load, oil or coolant contamination.
• Tensioner: arm flutter at idle, limited travel or binding, noise from the pulley bearing, visible misalignment, or a weak spring that allows belt slip.
• Idler pulley: rough/rumbling bearing, wobble, heat discoloration, or grease leakage.
• Accessory issues caused by slip: low alternator output, intermittent power steering assist, A/C performance swings, or higher engine temps if the water pump is slowed by slip.

Engine-option variation note

• 2025 Kenworth T680 trucks commonly use PACCAR MX-13 or MX-11 engines, and some are spec’d with Cummins X15. Brackets, pulley diameters, and routing differ between these engines and between A/C, alternator output, and cooling package options. Always verify by VIN and engine serial.

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