2024 GMC Sierra 2500HD Serpentine Belt Routing & Replacement Guide

As an ASE-certified master technician, I regularly service heavy-duty trucks, and the 2024 GMC Sierra 2500HD is a frequent visitor to the bay. Whether your Sierra is equipped with the 6.6L L8T Gas V8 or the legendary 6.6L L5P Duramax Turbo-Diesel, maintaining the serpentine belt drive system is critical. A failed belt on these workhorses means an immediate loss of alternator charging, power steering, water pump circulation, and air conditioning—often leaving you stranded with a heavy load.

Below is the comprehensive serpentine belt reference guide for the 2024 GMC Sierra 2500HD, focusing primarily on the 6.6L engines. Always verify exact part numbers with your VIN, as dual-alternator setups or specific accessory packages will change the required belt length.

Quick answer

Here are the essential specifications and part numbers for the primary drive belt system (standard single-alternator setup):

• Belt Length: 120.3 to 120.5 inches (depending on engine and alternator package)
• Rib Count: 6-rib
• OEM Belt PN: GM/ACDelco 12678521 (Duramax Primary) / 12699276 (Gas V8)
• Aftermarket Cross-References: Gates K061203, Dayco 5061205, Continental Elite 4061205
• Auto-Tensioner PN: GM/ACDelco 12695646
• Idler Pulley PN: GM/ACDelco 12678539

Note: Models with dual alternators or dual A/C compressors will utilize a secondary belt and different primary lengths. Always cross-reference your exact VIN with your parts supplier.

Belt routing

Because under-hood configurations can be complex, always take a clear photograph of your existing belt before you remove it. This will save you a massive headache if you forget the routing path halfway through the job.

For the standard V8 accessory drive, the belt follows a serpentine path with the grooved side gripping the accessory pulleys and the smooth side riding against the idler and tensioner pulleys. Here is the text-based routing sequence, starting from the bottom and working clockwise:

1. Start by looping the belt around the bottom of the Crankshaft Pulley.
2. Route the belt straight up and over the top of the A/C Compressor.
3. Bring the smooth side of the belt down and under the smooth Idler Pulley.
4. Pull the grooved side up and around the Alternator.
5. Route it down and around the Power Steering Pump (if hydraulically equipped, otherwise standard routing block).
6. Wrap the belt around the Water Pump pulley.
7. Finally, slip the belt over the Auto-Tensioner Pulley while the tensioner is depressed.

How to replace the serpentine belt

Replacing the belt on a 2024 GMC Sierra 2500HD is a straightforward maintenance task, though the ride height of the HD trucks might require a step stool.

Tools needed: A dedicated serpentine belt tool or a long-handled 3/8" or 1/2" drive breaker bar (depending on your specific tensioner square drive).

1. Locate the tensioner: Find the automatic belt tensioner on the front of the engine block.
2. Release the tension: Insert your 3/8" breaker bar or serpentine belt tool into the square slot on the tensioner arm. Rotate the tensioner arm (usually clockwise, though check the tensioner body for directional arrows) to relieve the tension on the belt.
3. Remove the old belt: Slide the belt off the easiest accessible pulley—usually the idler or the alternator—while holding the tensioner open. Carefully release the tensioner arm. Extract the old belt from the engine bay.
4. Route the new belt: Follow the routing path described above, or refer to the photo you took. Leave the tensioner pulley for last.
5. Install: Rotate the tensioner arm again, slip the new belt over the tensioner pulley, and slowly release the arm so it applies pressure to the belt.
6. Verify: Double-check that all ribs of the belt are seated perfectly inside the grooves of every ribbed pulley. A misaligned belt will shred immediately upon startup.

Replacement interval

For optimal reliability—especially if you use your 2500HD for towing, plowing, or heavy hauling—you should inspect the serpentine belt every 30,000 miles.

Replacement is generally recommended between 60,000 and 90,000 miles. I strongly advise replacing the automatic tensioner and the idler pulley at the same time you replace the belt. The bearings in these pulleys experience the exact same mileage and wear as the belt, and a seized idler pulley will instantly snap a brand-new belt.

Signs of a failing belt or tensioner

Don't wait for a total failure on the highway. Look and listen for these warning signs:

• Squealing or Chirping: A high-pitched squeal on cold startups or when accelerating indicates the belt is slipping. This can be due to a stretched belt, worn ribs, or a weak tensioner spring.
• Visible Cracks or Fraying: Inspect the ribbed side of the belt. If you see more than three cracks per inch, or if chunks of the ribs are missing, replacement is mandatory. Fraying edges indicate a misaligned pulley.
• Bouncing Tensioner Arm: While the engine is idling, watch the auto-tensioner arm. It should remain relatively still. If it is violently jumping or vibrating, the internal spring or damping mechanism has failed.
• Heavy Steering or Dimming Lights: If the belt is slipping badly, the power steering pump and alternator will underperform, leading to heavy steering feel or electrical voltage drops.

If you aren't comfortable doing this job yourself, or if you suspect your tensioner assembly has failed and requires professional torque specs to replace, don't risk your engine. Find a trusted shop on Repairius.