Knowing when it's time for a new belt
Ideally, you aren't waiting until you're stranded on the shoulder of the interstate with a snapped belt to think about a 6.7 cummins serpentine belt replacement. You want to be proactive. Usually, I start taking a close look at the belt every time I change the oil once the truck hits about 50,000 miles.
What are you looking for? Most modern belts are made of EPDM (ethylene propylene diene monomer), which doesn't crack as obviously as the old neoprene belts did. Instead of deep cracks, these belts just wear down thin, sort of like the tread on a tire. If the grooves look shallow or if you see any fraying along the edges, it's time. Of course, if you hear that high-pitched squealing on cold mornings, that's the belt's way of screaming for help. Sometimes it's just stretched out, and other times the tensioner is getting weak, but either way, the belt usually needs to come off.
The gear you'll need
You don't need a massive rolling tool chest for this, but having the right stuff makes it way less frustrating. The most important tool is a 1/2-inch drive breaker bar or a long-handled ratchet. The belt tensioner on the 6.7 Cummins has a square 1/2-inch hole built right into it.
I'd also recommend a "belt grabber" tool or just a long piece of stiff wire with a hook on the end. Since the 6.7 is a deep engine and the fan shroud takes up a lot of space, getting your hands down to the lower pulleys is a pain. A long pry bar or even a piece of PVC pipe can also help you nudge the belt onto the pulleys you can't quite reach. And, honestly, grab a flashlight. Even in broad daylight, it gets dark down near the crank pulley.
Before you pull the old belt off
Here is the most important tip I can give you: find your belt routing diagram. Most trucks have a sticker on the radiator support or the underside of the hood. If yours is missing or faded, take a picture of the belt with your phone before you touch anything.
It seems simple enough when the belt is on there, but once that long, snake-like piece of rubber is sitting on your driveway, trying to remember if it goes over or under the water pump pulley feels like solving a Rubik's cube in the dark. The 6.7 Cummins has a fairly complex path, weaving around the alternator, the AC compressor, the idler pulleys, and the water pump. Save yourself the stress and make sure you know exactly how it's supposed to look.
Releasing the tension
Once you're ready to start the 6.7 cummins serpentine belt replacement, find the tensioner. It's located on the passenger side of the engine, somewhat low. Insert your 1/2-inch breaker bar into the square hole on the tensioner arm.
You're going to want to rotate the breaker bar toward the driver's side (counter-clockwise) to compress the spring and release the tension. This spring is strong—it has to be to keep that belt tight under the torque of a Cummins—so give it a good, steady push. Once the tension is off, use your other hand to slip the belt off the topmost pulley (usually the alternator is the easiest one to reach). Slowly let the tensioner back down. Don't just let it snap back, or you might crack the housing.
Fishing the old belt out
Now comes the fun part: snake-charming the old belt out of the engine bay. You'll need to work it around the fan blades. This can be a bit of a juggle. You have to thread the belt around each blade one by one until it's free. It's greasy and dirty, so don't wear your favorite shirt. Once the old belt is out, don't just toss it in the trash immediately. Lay it out next to your new belt to make sure they are roughly the same length. A new belt will be slightly shorter because it hasn't been stretched out for 60,000 miles, but it should be very close.
Inspecting the pulleys
While the belt is off, it's a great idea to give all the pulleys a quick spin by hand. They should spin smoothly and quietly. If you hear a grinding noise or feel any "crunchiness" in the bearings of the idler pulley or the tensioner, replace them now. There is nothing worse than finishing a 6.7 cummins serpentine belt replacement only to have an idler pulley seize up two weeks later and ruin your brand-new belt.
Routing the new belt
This is where the patience comes in. Take your new belt and start from the bottom. I usually find it easiest to loop it over the crank pulley first, then work my way up. Use that long wire hook or "belt grabber" to feed the belt down past the fan shroud.
The trickiest part is usually the AC compressor and the water pump. Make sure the "ribbed" side of the belt is seated perfectly in the "ribbed" grooves of the pulleys. If the belt is off by even one groove, it'll shred itself within minutes of starting the truck. Check and double-check that the belt is centered on every single pulley.
Finishing the job
Once the belt is routed around everything except for one easy-to-reach pulley (again, the alternator is usually the best candidate), grab your breaker bar again. Pull back on the tensioner to create the slack you need, and slip the belt over that final pulley.
Slowly release the tensioner and let it take up the slack. Before you pull your tools out, do one last visual inspection. Look down at the crank pulley and the AC compressor to make sure the belt hasn't slipped off the edge during the process.
The first start
I always like to have someone else start the truck while I watch the belt from a safe distance (keep your hands and loose clothing far away!). You're looking for any wobbling or strange jumping. If the belt chirps for a second and then settles into a smooth hum, you're golden.
If it's squealing like crazy, shut it down immediately. Usually, that means the belt isn't seated perfectly in the grooves or you've accidentally routed it the wrong way. If everything looks good, let it run for a minute, then shut it off and check the tension one last time.
Wrapping up a 6.7 cummins serpentine belt replacement feels pretty good. It's a simple piece of rubber, but it's the lifeline of your engine's accessories. Keeping a spare belt behind the seat of your truck isn't a bad idea either—if you're ever out in the middle of nowhere and the belt snaps, you'll be glad you have the part and the know-how to swap it out. Plus, now that you've done it once, the next time will be twice as fast.