I replaced the drive belt a few months back as a preventive measure. Last week a suddenly shredded the belt. I didn't think too much off it, replaced it and a few days later I heard a flapping sound and when I checked the belt was 1/3 shredded. What was interesting was that the back side of the belt had 2-3 grooves cut into it, like the drive side. They were on the radiator side of the belt and that edge was being worn off. I'd thought that maybe it had some of the thread that had been pull off but looking at it closely, it looks like somehow grooves are being cut into the back (smooth side). I carefully checked clearance around all pulleys and there's over a finger's width everywhere.

One of the idle pulleys also had two grooves worn into it so I figured it was the problem. So I replaced it and put on a new belt and 2 days later, it shredded again. And what's interesting is that the front side (grooved side) is in perfect condition but the back (flat side) has been worn away so the belt is 1/2 the original thickness. It has some evidence of grooves like whatever wore it away had grooves. The dust seems to accumulate most to the drivers side of the crank. I know it seems I'm detailing a simple prfoblem much too much but I've been working on cars for 40 years and I am totally baffled. Could a worn out tensioner move too much and allow the belt to flap out and hit some hose clamp or something. It feels tight. Obviously I checked all the various pulleys for play. The alignment looks fine. Nothing has seized and there's no screeching before it breaks so that something might be intermittently seizing. Besides the front (grooved side) doesn't show any burning that would result from that. There's a big wad of stringlike rubber from the belt that's left after whatever evil creature ate the things.

Help. I'm going to replace the tensioner (90K miles) because I dont know what else to do.

 

Something got messed up during the original procedure. You havent fixed the root issue yet. I assume you did the work initially... Retrace your steps from your original replacement to figure it out

 

if im not mistaken the only surface that comes into contact with the back of the serpintine belt is the water pump. verify it is rotating freely and that the bearing feels good. also have someone bring the rpm up while you are watching under the hood to see that the tensioner is working properly. youll get it,at least it isnt intermittent.

 

ill bet tusabes is correct. if it began after you replaced the belt than it is most likely a routing problem.sounds like your a mech, the only contact on the back of the belt should be water pump and tensioner pulleys the idlers contact the grooved side. tell us what it turns out to be.

 

I've got an excellent routing drawing with the dvd star service for the car and i've checked and checked. All the pulleys, Alt, crank, a/c, power steering, hydraulic, wp are grooved. All the idlers (2) plus a tensioner are smooth. It's odd that the belt rides on one idler on the face (grooved side) but the drawing couldn't be clearer. And anyway, if you routed the belt on the back (smooth side) for both idlers, the belt would touch itself going in opposite directions. It couldn't work. And the first belt last months, the others days.

I've carefully checked the wp and all other pulleys for problems and they are fine. When I run the rpms, everything looks fine. The only funky thing, possibly, is the tensioner. As you goose it to 3-4K rpms, it moves an close to 2 inches. I've got an e320 and it barely moves.

 

Here's the diagram

 

Actually mine is slightly different. Those two items in gold/yellow are optional and not on my car.

 

i cant imagine the back side of the belt contacting the grooved surface of a pulley, it just wont last long and i can honestly say i have never seen it that way on any car i have ever worked on. the fact that the tensioner may seem to have excessive travel means that the belt would have a loose condition and should squeal whan you crank on the power steering or throw a load on the alternator. im probably going to eat these words but the back side of the belt cant contact the grooved side of a pulley. the tensioner and water pump are the only pulleys in contact with the back side of the belt.

 

maybe just take it to the dealer and have them do it.

 

Really weird.

 

just wondering if you had found the problem.

 

No. Yesterday I replaced the tensioner and then threw a belt within an hour. It's high rpms when with damage happens and the back side of the belt (the smooth side) is simply ground away in a methodically manner. I'm at a total loss. The only thing I can think of is to insulate all projections near the crank pulley because that seems to be the area where the damage occurs.

 

in the diagram you posted which idler #3 that is in contact with the back side of the belt is grooved? im assuming that the tensioner was replaced as an assyembly and not just the pulley.so that component is ruled out. if one of the accessorys had excessive drag as you throttle up it should squeal, as you back off the tensioner should absorb the belt slack rapidly. is it possible that an incorrect idler was installed grooved instead of smooth in the wrong position?. if the belt that was originally replaced prior to this had a couple years of service on it and was not worn than it is absolutely your routing. is the tensioner in mid stroke when the belt is installed?

 

Actually the mercedes part no on the two idlers are the same. They run the grooved side against the idler and the idler itself is grooved.

I may have found it though. In closely examining the alternator pulley, I looks like somehow the face has be pushed in at a spot so the first groove has been collapsed. I couldn't get the pulley off without damaging it and taking the alternator off seems like a huge problem (it's water cooled - whoever heard of such a thing). So I've repaired it. I think the damage to the smooth side was throwing me off. I think that damage occurred when the edge of the belt would get damaged by the bad pulley and then one of the reinforcement strings of the belt who start fraying and it would snag on something and pull off the back of the belt. It seems that these strings are integrated into the belt in a spiral pattern and you can pull it like a string on a fishing reel. I can't be certain yet but drove it at high rpms for a long time and it seems fine. If that doesn't do it, then I'll replace the steering pump.

I'll keep everyone posted once I've driven it for a long while, so hopefully I can save someone else the heartache. I hope it's fixed. Thanks everyone.