2018 Kenworth T680 — Serpentine Belt Routing & Length

Quick answer

• Belt length and rib count: Depends on engine and accessory layout. Most 2018 T680s use an 8- or 10-rib Micro-V belt typically in the ~120–130 inch range. Confirm by VIN/engine serial number (ESN) or reading the number printed on your existing belt.
• OEM serpentine belt PN: Varies by engine (PACCAR MX-13/MX-11 vs. Cummins X15) and options (A/C, alternator amperage, auxiliary drives). Obtain the exact PN using the truck VIN (Kenworth dealer) or ESN (PACCAR/Cummins). Do not order until confirmed.
• Aftermarket cross-refs (belt): Match by rib-count and length. Look up by VIN/ESN in: Gates FleetRunner Micro-V HD, Dayco Gold Label Poly-V, or Continental/ContiTech Elite/Extreme Duty Micro-V catalogs.
• Automatic tensioner PN: Application-specific (varies by engine and bracket). Common quality replacements come from Litens, Gates DriveAlign HD, and Dayco. Confirm by VIN/ESN and mounting style.
• Idler pulley PN: Application-specific. Your truck may use one or more smooth/grooved idlers of different diameters. Confirm by VIN/ESN and pulley position.

Note on engine options: 2018 Kenworth T680s were commonly built with PACCAR MX-13 (and MX-11) or Cummins X15. Each has unique bracketry, pulley diameters, and sometimes alternate belt paths. Always confirm by VIN/ESN or the underhood VECI/drive label.

Belt routing

Because we can’t include an image here, use this text routing order as your guide. Before removing anything, take a clear photo of your current belt path and note which pulleys are grooved (belt ribs ride in) versus smooth (belt back rides on).

Typical routing order (clockwise path example):

1. Start at the crankshaft pulley/harmonic balancer.
2. From the crank, route the belt up to the A/C compressor.
3. From the A/C compressor, go over an idler pulley.
4. From that idler, wrap the alternator.
5. From the alternator, route down around the power steering pump.
6. From the power steering pump, go across the water pump.
7. From the water pump, go through the automatic tensioner and back to the crank.

Important: The exact pulley layout can differ by engine and accessory package. Use your underhood routing decal if present, and your photo, to confirm orientation. The belt ribs must ride only on grooved pulleys; the smooth pulley faces contact the belt’s back.

How to replace the serpentine belt

Tools: serpentine belt tool or a long breaker bar. Most HD tensioners accept a 1/2-inch square drive; some use 3/8-inch—verify before you start. A pulley holder or long-handled wrench can help with access.

1. Photograph the existing routing from multiple angles. Note ribbed vs. smooth pulleys.
2. Locate the automatic tensioner. Fit your serpentine belt tool or breaker bar into the square drive or onto the hex boss.
3. Rotate the tensioner in the direction that relieves belt tension. Hold it fully released.
4. Slide the belt off the easiest accessible smooth pulley last (often an idler). Slowly release the tensioner back to its stop.
5. Compare the old and new belts for rib count and length. A correct belt will match rib count and be within a small tolerance for length.
6. Route the new belt per your photo and the order above, keeping the belt centered in each grooved pulley.
7. Leave one easy-access smooth pulley for last. Rotate the tensioner to full relief and slip the belt on. Release the tensioner slowly, ensuring the belt seats fully.
8. Visually confirm all ribs are engaged and the belt backs are on smooth pulleys only. If possible, crank the engine briefly and shut it off to recheck tracking. Re-seat if needed.

Tip: If the tensioner or idlers feel gritty, wobbly, or the tensioner indicator is out of range, replace them along with the belt.

Replacement interval

• Inspect at every PM service. In severe duty (high heat/contaminants/stop-start), inspect more frequently.
• Typical replacement: 100,000–150,000 miles or 12–24 months, whichever comes first, or sooner if any wear indicators appear.
• Replace the automatic tensioner and idlers at least every second belt service, or immediately if noisy, loose, or the tensioner travel indicator is out of spec.
• Always follow OEM recommendations for your specific engine (PACCAR or Cummins) and vocation.

Signs of a failing belt or tensioner

• Audible: Cold-start chirp, persistent squeal under load, or intermittent chirps during accessory engagement (A/C on, steering at lock).
• Visual belt wear: Rib cracking, glazing/shiny ribs, missing rib chunks, frayed edges, contamination (oil/coolant), or glazing transfer on smooth pulleys.
• Tracking issues: Belt riding off-center, rib walk, or dusting (excess black rubber near pulleys).
• Tensioner/idler problems: Tensioner arm flutter at idle, grinding/rough pulley bearings, wobble, or an indicator out of the normal window.
• System symptoms: Battery/charging light on, dimming lights, heavy steering, engine temp creeping up, or A/C performance dropping at idle.

If you don’t have a clear underhood decal or are unsure of the exact parts for your 2018 T680, use your VIN (Kenworth) or ESN (PACCAR/Cummins) at a parts counter to confirm belt length, rib count, and the correct tensioner and idlers before ordering.

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