2019 GMC Sierra 2500HD Serpentine Belt Routing, Length & Part Numbers

As an ASE-certified master technician, I know that replacing the serpentine belt on a heavy-duty truck like the 2019 GMC Sierra 2500HD can seem daunting, especially when you pop the hood and look at the complex accessory drive system. Whether your Sierra is equipped with the 6.0L Vortec V8 (gas) or the powerhouse 6.6L Duramax V8 (diesel, L5P), having the right part numbers and routing information on hand is critical to getting the job done efficiently and correctly.

Below, you will find the complete reference guide for the 6.6L Duramax L5P (Single Alternator), which is the most common configuration for this model year's heavy-duty lineup. If you have the 6.0L Gas or a dual-alternator Duramax, be sure to verify specific belt lengths with your parts supplier, as they will vary.

Quick Answer: Specs & Part Numbers (6.6L Duramax, Single Alternator)

• Belt Length: 117.7 inches (Standard single-alternator setup)
• Rib Count: 6-rib
• OEM Belt Part Number: ACDelco 12639277 (or GM Genuine equivalent)
• Aftermarket Cross-References:
  • Gates K061177
  • Dayco 5061178
  • Continental Elite 4061177
• Auto-Tensioner Part Number: ACDelco 38389 (Gates 38389)
• Idler Pulley Part Number: ACDelco 38006 (Gates 38006)

Note: Dual alternator models require a significantly longer main belt and have a slightly different routing path. Always confirm your RPO codes or check under the hood before purchasing.

Belt Routing

Because we cannot render an image here, it is highly recommended that you photograph the existing belt before removal. This gives you a personalized reference point if you get stuck.

For the standard 6.6L Duramax V8 with a single alternator, the belt routing follows this general text-based path:

1. Start by looping the belt down and around the bottom of the Crankshaft Pulley.
2. Bring the belt up on the left side (passenger side) around the outside of the Water Pump Pulley.
3. Continue up and over the Alternator Pulley at the top passenger side.
4. Route the belt down from the alternator and under the Idler Pulley.
5. Take it straight across to the driver's side and over the A/C Compressor Pulley.
6. Route the belt down from the A/C compressor and around the outside of the Power Steering Pump Pulley.
7. Finally, bring the belt back toward the center to slip it under/over the Auto-Tensioner Pulley (which you will be holding open with your tool).

How to Replace the Serpentine Belt

Replacing the belt on the 2019 Sierra 2500HD requires basic hand tools and a bit of leverage. On the 6.6L Duramax, space can be tight near the fan shroud, so working patiently is key.

Tools Needed:

• Serpentine belt tool or a long-handled 3/8" or 1/2" drive breaker bar (depending on the tensioner square drive slot).
• Safety glasses and gloves.

Step-by-step:

1. Locate the tensioner: The automatic drive belt tensioner is located on the front of the engine block.
2. Release the tension: Insert your breaker bar or serpentine belt tool into the square drive hole on the tensioner arm. Rotate the tensioner counter-clockwise (or away from the belt) to release the tension.
3. Remove the old belt: While holding the tensioner open, slide the belt off the easiest accessible pulley (usually the idler or alternator). Carefully release the tensioner back to its resting position. Remove the belt from the engine bay.
4. Route the new belt: Following the path described above (and referencing the photo you took), route the new belt around all the pulleys except for the final idler or tensioner pulley.
5. Apply tension and finish: Rotate the tensioner again using your breaker bar. Slip the final loop of the belt over the remaining pulley. Release the tensioner slowly, ensuring the belt is seated squarely in the grooves of all ribbed pulleys and flat against the smooth pulleys.
6. Verify: Start the engine and let it idle for a few seconds. Turn it off and inspect the belt to ensure it hasn't walked off any edges.

Replacement Interval

For a heavy-duty truck like the GMC Sierra 2500HD, the serpentine belt should be inspected every 30,000 miles. Typically, modern EPDM belts will last between 60,000 and 100,000 miles. However, if you use your truck for heavy towing, plowing, or in extreme climates, you may need to replace it on the earlier side of that spectrum.

Signs of a Failing Belt or Tensioner

Don't wait for a breakdown on the highway while hauling a trailer. Watch for these symptoms:

• Squealing or Chirping: Especially on startup or when the A/C compressor engages. This indicates a stretched belt, a weak tensioner, or a misaligned pulley.
• Visible Wear: Look for missing ribs, deep cracks, fraying edges, or a shiny, glazed appearance on the ribbed side of the belt. (Note: minor micro-cracking is normal on EPDM belts, but large chunks missing is an immediate failure hazard).
• Tensioner Bounce: While the engine is idling, look at the tensioner arm. It should remain relatively still. If it's bouncing wildly, the internal spring is worn out and the tensioner must be replaced alongside the belt.
• Accessory Issues: Loss of power steering assist, battery warning lights, or an overheating engine can all be caused by a slipping or broken belt.

If you don't feel comfortable tackling this heavy-duty maintenance job yourself, don't worry. Find a trusted shop on Repairius.