Went to mess with my fathers peterbilt today. Problem with it is it keeps throwing the front acc. belt. rolls it over onto the back belt. Back belt has no issues and runs fine? Only things i can think of are damaged pully, or some end play in the accessory drive just so slight the front belt rolls but back is ok. My dad says he had the fan hub replaced years ago and the unit may not be the proper one. If it was just a little off could the belt have worn the pully to where it would be causing this issue now? I have nothing to compare it to to see how things are lined up. I did notice when reinstalling the belt that even before turning the motor the front one is angled at a 45 like its installed improperly but its not. back one is straight on when looking from overhead and front one your staring at the side of the grooves. Both belts are verified to be axactly the same in everyway. Any ideas?

That's not generic automotive, this is diesel









Um, so the part number is the same of each belt, are they sequenced? they "match"?



When they do that stuff in the factory, the two belts should be cut next to eachother and sold next to eachother. The part number might match but there is little extra text on the belts and you can tell if they were cut next to eachother or cut apart way down the line (thus making a minor increase or decrease in size and thereby throwing off equal tension)

Set them both up and measure belt deflection on them BOTH





they should have the EXACT same deflection

I measured the belts and width was the same. deflection seemed to be equal as well. Didn't know about the sequencing but a shop did put this set on so im sure they should be.

Get some new belts?

they are new! truck was driven home from shop after putting them on and thats been it. the belts themselves are not the issue. its something somewhere else.

the double pulley rotary stuff does this, the 2 belts never match....

The belts are probably junk, I deal with two belt crap all the time. They have to be the same part number, and they have to be sequenced. And they go TIGHT.





If one of them is flying off then you should have a really obvious problem like:



-Wobbly idler pulley

-Wobbly accessory pulley (alternator or a/c, whatever the accessory is)

-Wobbly fan pulley (which is really noticeable)

-Massive crankwalk

Like if that walked back over onto the other belt and you set it back and left it, then the "walker" is now stretched and can't be re-used, eh?

like if your alt pulley is wobbly the alt can get loose and you can throw a belt! even at people!