I have a 1999 Plymouth Grand Voyager Espresso with a 3.8 liter V-6 engine with about 135,000 miles. The serpentine belt comes off when ever it is driven in heavy rain that has water on the road. I have replaced the belt but have the same result. I am wondering if the idler pulley is worn or if the tensioner assembly could have gone bad. Any help would be appreciated. I am tempted to replace both is see if this will take care of the problem but it is very inconvenient to have the belt come off because you lose power steering, water pump (thus the engine overheats) and alternator as a result.
Assistance is appreciated.
Bradley Willyerd

Yes, replace the tensioner and idler pulley assembly. That will solve the problem.

Read this article:

Chrysler Minivan Belt Comes off (http://www.ricksfreeautorepairadvice.com/diagnostics/by-symptom/caravan-throwing-belts%E2%80%94solution)

I replaced the tensioner and all seems well.