Top (alternator) serpentine belt

My top serpentine belt keeps on jumping off the tensioner especially if I happen to drive in very wet conditions. I have replaced the tensioner and belt and also had the pulleys aligned but it keeps happening. Is there such a thing as a guide or shield for the tensioner pulley or even a pulley with a flange to keep the belt on? Any ideas will be greatly appreciated. Thanks.

My guess is you have a bum pulley. If the pulley bearing is bad it will look good but walk out of alignment when running. One way to tell is to take off belt then rap the pulley with a mallet on its face. A mild rap not a whack. With a bad bearing it will not take much to nudge the pulley out of alignment and you will be able to see the misalignment. In my experience the steel pullies go out of line after several years of use.

Also check by turning them by hand, do any bind or squeak even a little? those would be up for replacement too.

Thank you Albertj. I should also mention that I have had 3 new tensioners over the last 18 months or so. After a while they snap at the casting and I was assuming that they were breaking because I was constantly putting some strain on them when putting the belt back on. The belt still comes off a few days after putting the new tensioner on. It is driving me mad, my wife keeps on telling me to sell the car but it is by far the best ride I have ever owned and I am very reluctant to get rid of it as everything else is in great shape.

I understand your frustration. I had a similar issue with belt misalignment this year. Once I figured it out, everything has been fine ever since. Don't give up - it's probably something simple, not worth selling the car over.

Have you replaced any of the accessories? If possible, check the diameters of the pulleys are correct. Have you replaced the belt? Check the length is correct. Check the routing of the belt is correct. If the belt is too tight because of one of these things, it can cause the problems you're seeing. The tensioner should not wear out that quickly. I'm still running my original at 253k miles.