Any other reason besides a faulty spring tensioner why my belt would keep coming off?

several possible reasons
build up of worn belt in the teeth of the gears
incorrect belt being used (wrong pitch belt being used -not genuine)
shafts being driven are not free to turn--seizing on the bearings, incorrect placement of rocker shafts /pushrods---valve springs bottoming out locking the shaft
incorrect tensioner actioner ( hydraulic tensioner not tensioning properly allowing the belt to flap in operation)
idler pulley problems ( bearings or misalignment of all pulleys

have the regulator checked as the required voltage is 14.5-14.8 volts dc
check for drive belt condition and tension
with a voltage of 12 volts and insufficient charge the battery will drain down as more is going out than being provided by the alternator
besides that if the regulator is faulty it may be a reason for draining the battery

2004 Ford Freestar:
Accessory belt drive removal and installation:

1.Put the vehicle in neutral, and put it on hoist.
2.Rotate the accessory drive belt tensioner counterclockwise.
3.Push the spring clip towards the belt tensioner to lock the accessory drive belt tensioner in place and remove the belt.
4.To install the accessory drive belt, on the engine, rotate the accessory drive belt tensioner counterclockwise till the spring clip releases.
5.Install the belt.

The three most common reasons are - the belt is stretched and the tensioner can not keep it tight - the tensioner spring is damaged and its not keeping the belt tight - or one of the pulleys is not aligned with the others.

drop the deck and take a long look at the drive belt. somewhere along its path you will see a spring loaded arm with a pulley on the end. that arm is the tensioner, if it is not keeping adequate tension on the transmission end of the belt then either you are using the wrong belt, the spring has been taken off and put back on wrong, or the spring has just lost its springiness. You are going to have to figure out which problem and remedy it. Mower parts are common and are sold at small engine repair shops which you can find in the phone book or with a google search.

one other possibility is that someone has removed the drive belt and put it on incorrectly. there are generally guides close in around the front pulley to keep the belt from jumping off, and it has happened before that someone has unknowingly put a new belt on over instead of under the guide. you have to look close to see this.

If your belt kept slipping off I would check your tensioner pulley. It is possible that the spring mechanism inside of it has broke and the belt is not getting tight enough. That could also be the reason the pulleys look misaligned. I have seen worn out tensioners that sit funny just because they are worn out. Take the belt off and check all of your pulleys make sure the bearings all spin fine and put your wrench on the tensioner and move it, see if it springs back.

Removal & Installation

1. Rotate the drive belt tensioner clockwise and remove the drive belt.
   WARNING Never suddenly let go of the tensioned idler pulley. The force of the spring pressure suddenly released may damage the idler pulley mechanism. Always release the spring pressure gradually.
   
   
   

   Fig. Rotate the belt tensioner to relieve tension on the belt-4.6L engine shown, others similar
   

To install:

1. Route the belt over the pulleys making sure all the grooves in the pulleys and the belt line up correctly. Refer to the accessory belt routing section of this manual.
2. Rotate belt tensioner clockwise and slip drive belt over idler pulley on tensioner.

1. Rotate the drive belt tensioner clockwise and remove the drive belt.
   WARNING Never suddenly let go of the tensioned idler pulley. The force of the spring pressure suddenly released may damage the idler pulley mechanism. Always release the spring pressure gradually.
   
   
   

   Fig. Rotate the belt tensioner to relieve tension on the belt-4.6L engine shown, others similar
   

To install:

1. Route the belt over the pulleys making sure all the grooves in the pulleys and the belt line up correctly. Refer to the accessory belt routing diagrams above.
2. Rotate belt tensioner clockwise and slip drive belt over idler pulley on tensioner.

If for any reason, the tensioner is crooked, replace it!!! Also, install the belt with the writing facing you like you are trying to read a book. This WILL (a trick I learned at Mercedes) prevent even a new belt from squeeling!!

If the belt has got oil soaked into it could be one reason why it is coming off. Check the belt tensioner. The spring may have become weak or the tensioner bearing is bad and the pulley is wobbling.

2.7L & 3.5L Engines same Tools & Procedure Rate Me Please 1/2 in. breaker bar & Special Tool 7198 for conventional V-belts and Poly-V belts

1. REMOVAL
   AIR CONDITIONING BELT
   
   NOTE: The A/C drive belt self tensioner is NOT a dynamic tensioner. After adjustment the tensioner bracket bolts are tightened. The torsion spring is no longer responsible for tensioning the A/C belt. The torsion spring is only used for initial belt tensioning.
   1. Remove generator/power steering belt to gain access to A/C belt. Refer to GENERATOR/POWER STEERING BELT for procedure.
   
   
   1. Loosen BUT DO NOT REMOVE tensioner locking bolt and pivot bolt (Fig. 2).
   2. Insert 1/2" drive breaker bar into square opening on belt tensioner. Rotate tensioner counterclockwise until belt can be removed from pulleys (Fig. 2).
   3. Slowly rotate tensioner clockwise to relieve spring load.
   CLEANING
   Clean all foreign debris from belt pulley grooves. The belt pulleys must be free of oil, grease, and coolants before installing the drive belt.
   
   INSPECTION
   Belt replacement under any or all of the following conditions is required:
   
   • Excessive wear
   • Frayed cords
   • Severe glazing

• Poly-V Belt system may develop minor cracks across the ribbed side (due to reverse bending). These minor cracks are considered normal and acceptable. Parallel cracks are not (Fig. 3).
  
  NOTE: Do not use any type of belt dressing or restorer on Poly-V Belts. INSTALLATION
  
  AIR CONDITIONING BELT - NEW
  
  WARNING: DO NOT REMOVE A/C BELT TENSIONER LOCKING BOLT WITHOUT FIRST HOLDING THE TENSIONER WITH A 1/2" DRIVE BREAKER BAR, AS PERSONAL INJURY COULD RESULT.

NOTE: Verify that torsion spring position is in the new belt position before installing a NEW belt (Fig. 4). If torsion spring is not in the new belt position, follow steps 1-7 to change the tensioner spring from the USED belt position to the NEW belt position.

1. Insert a 1/2" drive breaker bar into the square opening on the tensioner. Hold counterclockwise pressure on tensioner while removing the locking bolt (Fig. 2).
2. Carefully release spring load of the torsion spring on the tensioner.
3. Remove pivot bolt, tensioner, and spring from front timing cover.
4. Insert spring arm into the NEW belt position on the tensioner (Fig. 4).
5. Install torsion spring, tensioner, and pivot bolt.
6. Install pivot bolt. Tighten only finger tight, at this time.
7. Using a 1/2" drive breaker bar, apply counterclockwise pressure until locking bolt can be installed.
   1. Install the NEW belt by rotating the tensioner counterclockwise until the NEW belt can be installed on pulleys. See (Fig. 5) for belt routing.
   2. Release tensioner and remove breaker bar. Belt is automatically adjusted with the tensioner's torsion spring.
   3. Tighten tensioner locking bolt and pivot bolt to 28 Nm (250 in. lbs.) (Fig. 2).
   4. Install generator/power steering belt.
   AIR CONDITIONING BELT - USED
   
   NOTE: A "used belt" is considered having more than 500 miles of service use. NOTE: Perform steps 1-7 to change the tensioner spring from the NEW belt position, to the USED belt position.
   1. Insert a 1/2" drive breaker bar into the square opening on tensioner. Hold counterclockwise pressure on tensioner, while removing tensioner locking bolt (Fig. 2).
   2. Carefully release the tension of spring on tensioner.
   3. Remove pivot bolt, tensioner, and spring from front cover.
   4. Insert spring into the USED belt position on tensioner bracket (Fig. 4).
   5. Install spring, tensioner bracket, and pivot bolt.
   6. Install pivot bolt. Tighten only finger tight, at this time.
   7. Using a 1/2" drive breaker bar, apply counterclockwise pressure until locking bolt can be installed.
   8. Install the USED belt by rotating tensioner counterclockwise until belt can be installed on pulleys. See (Fig. 5) for belt routing.
   9. Release tensioner and remove breaker bar. Belt is automatically adjusted with the tensioner's torsion spring.
   10. Tighten tensioner locking bolt and pivot bolt to 28 Nm (250 in. lbs.) (Fig. 2).
   11. Install generator/power steering belt.

I Hope This is Of Help To YOU IF So PLEASE HELP ME RATE ME it Helps Thank You Very Much and Happy Motoring !!!!!!!!

the only reason for the belt coming off the pulley is the something in the belt path is off aligment.
put the belt back on and chek the path where the belt pass. water pump, alternator, AC compressor, Power steeling unit, etc....... or even the pulley tensioner itself.
chance are there is a belt in the wrong pully slot.