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Does the belt come right off or go a while before this happens?
Does the belt squeal?
With the belt off you will need to see if any of the pulleys are not turning freely.
If all turn freely (none locked up)
The next thing to check for is to make sure that all of the pulleys are aligned, if one of the pulleys are not correctly aligned the belt will come off.
Also it could be the tensioner is bad or not tight enough.
Did your son replace the belt or did a shop do this?
To check the alignment you will just look at them in a straight line and see if the belt does not go in a straight line or if you see one pulley out farther than the other.
I believe the tensioner on this model is a manual adjustment (not automatic like the newer models.) If is too tight it could cause it to prematurely break and/ or squeal. Too loose will make a squeal and throw the belt also.
If your son is not mechanically inclined a shop should charge somewhere around a half hour to do a check over to see the cause of the belt issue.
When you say they break (like a cut in the belt) does this happen 'straight across' the belt or along the 'length' of the belt?
"Straight across such as if you were to just cut the belt to make it so it is not round. "
The shop who did the first two should be responsible for the replacement of the belts as they did not diagnose the issue correctly or may have incorrectly installed the belts.
The squeal indicates that a pulley is not spinning correctly or the belt / tensioner is faulty or not correctly installed. To test tensionsers all that you do is pull on the belt to see if their is too much slack or not (this is after the belt is adjusted)
Being it is just cut straight across this indicates that something is putting too much pressure on the belt and causing it to do that.
If this is happening on just one of the belts then you will be able to isolate the issue to those pulleys or anything that may be interferring the belt. If it is happening with both belts then you can narrow it down to just the components within both belts.
If the pulleys are aligned correctly I would suspect the tensioner is the issue. Also make sure that the tensioner pulley spins freely. You may also need to put a belt on run the engine and take a mechanic stethoscope or a long prybar (put your ear on the bar then put it around areas to pinpoint where the noise is coming from to find the source of issue.) Whatever is causing the noise is most likely the cause of the belt going bad!
I would suspect that the tensioner is the cause of the belt breaking as it could very well be putting too much pressure on the belt or if the bearings are bad on the pulley of the tensioner it could be heating up can causing the belt to break. When it was replaced was it replaced with a new or used one? If it was used I've ran across used parts that were faulty.
Being I cannot see the belt, hear the noise, or feel the pulleys it makes it a little tougher to diagnose it.
You need to focus on the pulley's that are off of the alternator belt.
As stated before the noise is most likely the cause of why the belt is breaking! If the pulleys spin freely you need to also make sure their is no play in them.
What is happening with the noise is that is a noise due to too much friction and after enough time it will generate a large amount of heat and will heat the belt up well enough that it will stretch and cause the cut you are seeing. The belt can only take so much tension before it will just snap.