Why does the fan belt come off 1997 Dodge Caravans when it rains, how do you fix?

Is the belt loose?

This is what I found on your problem.

NO: 07-02-99
SUBJECT: Serpentine Belt Slips Off Idler Pulley
DATE: June 14, 1999
OVERVIEW:
This bulletin involves replacing the bracket that the idler pulley attaches to.

NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 3.3L OR 3.8L ENGINE.

SYMPTOM/CONDITION:
Loss of power steering assist and occurs when driving through deep snow or standing water. Snow or rain can enter the engine compartment from underneath the vehicle and force the serpentine belt off of the idler pulley.

DIAGNOSIS:
Visually inspect the serpentine belt and idler pulley to determine if mis-alignment exists (If the serpentine belt is off the pulleys install the serpentine belt). Start the engine and let it run for one minute before checking alignment). The belt should be approximately 2mm (0.08 in.) From the inboard edge of the idler pulley (Figure 1) when alignment is correct and this bulletin does not apply. If the belt and pulley appear as in Figure 2, perform the Repair Procedure.

PARTS REQUIRED:
Qty Part No. Description
1 04612412 Bracket, Engine Mount
AR (1) 04861322 Belt, Serpentine

REPAIR PROCEDURE:
THIS REPAIR IS COMPATIBLE WITH DAIMLERCHRYSLER'S MOBILE SERVICE PROGRAM AND DOES NOT REQUIRE HOISTS OR OTHER FULL SERVICE FACILITY SPECIAL EQUIPMENT.

Remove the serpentine belt from around the idler pulley by rotating the tensioner clockwise (Figure 3).
Remove the purge duty cycle solenoid and wiring harness from the right side engine mount.
Remove the two right side engine mount insulator vertical fasteners and loosen the horizontal fastener. DO NOT remove the large nut in the center of the core of the insulator (Figure 4).
Remove the load on the engine mount by carefully supporting the engine with a floor jack.
Remove the vertical and horizontal fasteners from the engine side bracket. Remove the engine mount assembly.
Remove the idler pulley from the engine mounting bracket. (Figure 3).
Remove the engine mounting bracket
Install a new engine mounting bracket. Torque the fasteners to 40-68 Nm (30-50 ft lbs.).
Install the idler pulley to the engine mounting bracket. Torque the fastener to 40-68 Nm (30-50 ft lbs.)
Properly route and position a serpentine belt onto all pulleys except the idler pulley (Use a new belt if there is any evidence of damage to the original one).
Rotate the belt tensioner clockwise until the belt can be installed onto the idler pulley. Slowly release the belt tensioner.
Install the engine mount; torque the assembly in the following order, engine mount to rail fasteners 68 Nm (50 ft. Lbs.), Vertical fastener 102 Nm (75 ft. Lbs.), And horizontal fastener 150Nm (111 ft. Lbs.).
Install the purge duty cycle solenoid to the engine mount.
Start the vehicle and let it run for one minute then turn the engine off.
Verify the serpentine belt is properly aligned (Figure 1).
If the serpentine belt has moved toward the outboard edge of the idler pulley, further diagnosis will be required