How do you relieve the tension on the serpentine belt, in order to replace the belt. 1991 Caravan.

Locate the tensioner arm. It'll have a square hole in it for you to put in a ½ ratchet drive. Press the tensioner arm using the ½ drive so as to remove the tension. Two people would make this an easier task.

Sure looks like a 15MM socket head on my tensioner, I never found a square hole, but mine was a '93....

There is no 1/2 inch square hole on the tensioner, that would be too easy. On further investigation, I found this reply, which is correct for the 3.3 and 3.8 engines. You can use a 15 mm wrench on the bolt that holds the idler pulley to the tensioner, and pull forward in order to release the tension on the belt.
Or you can buy a special tool for $31, which does the same thing.

I went out to my van last night to make sure I was right (because I'm cocky), and rpalmer was right. Its a 15mm, not 17.. so I edited my post....
I hate to be off 2mm.... sorry folks!

Slantsixness