Help me diagnose a snapped serpentine belt!

About 2 months ago I took my truck through some deep mud and eventually developed a loud whining sound from the engine bay. As a result I've replaced all the idler pulley (including the tensioner pulley with a pressed in NSK bearing) as well as the alternator. I have an 08 DCLB with the 4.0 1GR-FE. Shortly after replacing all this, my belt shredded itself to bits. I've got a new belt but I'm trying to figure out what would have caused this failure to begin with. Here are some observations/thoughts that have crossed my mind:

- After removing the shredded belt, I checked all idlers and accessories and the pulleys spin freely; nothing is bound up or dragging
- The brand new upper pulleys have a tiny bit of play in them but it seems to be due to the fact that the aftermarket idler pulleys use a machined sleeve as opposed to issues with the bearings. I imagine all sleeved aftermarket pulleys exhibit this issue.
- The old pulleys and the new ones have rubber deposits on them.
- The only drive accessory I've swapped out was the alternator and as far as I can tell the pulley depth isn't adjustable; the pulley is butted right up against the body of the alternator which in turn is directly fixed to the engine block. The replacement alternator is rebuilt with an OEM Denso core.
- At no point did the belt/pulleys squeak

So with all that I'm not sure what this issue could be. The fact that the pulleys had rubber deposits on them indicates the belt is overheating but I'm not sure what the cause could be. Any ideas on things I could try to diagnose the root cause would be appreciated.

 

You were one rib off on one of the pulleys and it killed the belt. Not hard to do.

After installing a new belt inspect the routing on the pulleys. Then tap the starter to rotate the engine a little, and see if the belt is still riding on the pulleys correctly.

 

Also, double check all the pulleys and idlers. Dragging, noise, end play, radial play.....any looseness is trouble.

Clean all pulleys. The rubber transfer to the pulley indicates belt slip.

 

OP, ^^^^^ good responses from the above. Also, check the AC compressor pulley bearing. And did you use an OEM or quality belt or some cheap one?

 

My buddies serpentine belt crapped out when his AC compressor seized$$$$$$ . The AC compressor cycles in the defrost position .

 

Everything spins freely with no play. The only exception are the two aftermarket upper idler pulleys which use and internal sleeve so there is a tiny bit of play between the sleeve and the inner race of the bearing.

I also notice that my fan always spins so I likely need a new fan clutch. Not sure if this would contribute to belt damage...

Belt was an OEM Bando

My guess at this point is that I may have skipped a groove on my last belt install.

 

OP, it's super easy to do, particularly if that skipped groove is on the bottom of a pulley like the crank pulley. I did this a few years ago on my 4Runner and made it a mile away from home before it let go. It sounded like a bomb went off and the shredded belt was would up in my fan. It also busted a water bypass hose. Damage was minimal but I definitely learned my lesson.

 

If you can wrangle a straight edge in.......place it on the face of the pulleys. Use the Crank pulley as base 0. This will check alignment of the pulleys. The straight edge should make contact on 2 points of each pulley with the crank pulley as base 0.

The AC compressor may be a bit difficult due to the clutch. But you should be able to get an idea of the alignment.

Alignment being off is a bit of a long shot.



I'd consider dumping the aftermarket idlers with the play for idlers with bearings. No bushings.

 

Yikes. Mine didn't make any noise at all when it went; just suddenly no power steering. I also got away with no damage to any hoses etc. But I'll be keeping an eye on this belt for the next couple months.