The mower deck drive belt slipped off the idler pulley on my 42'' Murray riding mower deck because the idler pulley was worn out. I need to know how to loop the belt back around the pulleys after I replace the idler pulley.

Safety and Preparation Serious injury can occur if the engine or mower blades accidentally start while you are working with lawn tractors. To prevent this, drive the Murray lawn tractor onto a flat work surface and disengage the mowing deck cutting blades. Engage the parking brake to keep the mower from moving; chock the wheels with a wooden block or brick. Adjust the mowing deck to the lowest position. Turn off the engine and remove the key from the ignition. While not necessary, you may even wish to disconnect the spark plug wire from the spark plug. Mowing Deck Removal You must remove the mowing deck on a Murray lawn tractor to gain access to the belts. Remove the adjustment rod hairpins, followed by the adjustment rod pins from the front left and right sides of the mowing deck. At the rear on the left and right sides, pull out the retaining hairpins and slide the mowing deck suspension links from the adjuster plates to separate the rear mowing deck from the frame. Remove the blade control rod stud, including the hairpin and washer, from the top of the blade control lever shift linkage at the left rear of the deck. Remove the front hanger rod, including the hairpin and flat washer that hold the rod in place. Loosen the belt guide retaining bolts with a socket wrench and remove the engine stack pulley belt so you can slide the mower deck out from under the tractor Transmission Drive Belt Installation Slide the new transmission drive belt between the engine stack pulley belt guides, over the belt guide plate and under the engine stack pulley. Fit the transmission drive belt around the top engine stack pulley. Push the free end of the transmission drive belt up through the transmission shifter hole in the lawn tractor floorboard. Slip the transmission drive belt loop over the transmission shifter. Pull the transmission drive belt down through the transmission shifter hole. Fit the transmission drive belt around the transmission drive pulley and tighten the transmission drive pulley belt guide retaining bolts to hold it in place. Fit the transmission drive belt flat side around the idler pulley. Slide the idler pulley onto the idler pulley stud while holding the belt in place. Tighten the transmission idler pulley retaining nut with a socket wrench to secure the idler pulley to the idler pulley stud. Mowing Deck Drive Belt Installation Pull back on the right side mowing deck mandrel pulley spring-tensioned belt retainer. Guide the new mowing deck drive belt around the right side mandrel pulley and release the belt retainer. Pull the drive belt spring-tensioned retainer away from the mowing deck idler pulley, guiding the deck drive belt around the idler pulley. Fit the mowing deck drive belt around the left side mandrel pulley and under the rear mowing deck cross link rod between the right and left side mandrel pulley. Slide the mowing deck under the right side lawn tractor frame, reconnecting the deck to the frame in reverse order of removal. Engage and disengage the mower deck while driving the mower to test the belt for proper operation.

465306X8B - Exploded drive belt parts view.

The deck has an idler pulley that keeps tension on the belt. The idler pulley is mounted on a pivoting bracket, that has a spring which provides the tension. You have to move the bracket so as to take the tension off the belt, thereby allowing you to slip the belt off the other drive pulleys

This is likely due to a worn and slipping belt. Replacing the belt should solve the problem

If I understand the issue correctly, the belt that drives the machine's transmission squeals. When does it squeal? When engaging the clutch? When in motion?
Typically it's the condition of the belt. It's likely slipping due to stretch, wear or from an oil leak causing it to slip, or simply due to a loose idler pulley, worn idler pulley bearing, broken spring. Look for all that stuff. Look at the drive belt and inspect for wear, cracks, fraying, missing rubber, etc. Replace it if any of those conditions are found.

Question how old is the mower? Drive belt may need to be replaced due to being streched . Also is the clutch fully engaging , are there any worn pulleys?

It sounds to me like the traction drive belt slipped off of a pulley or broke. The easiest way to get to the traction belt is to remove the mower deck. There is no easy way to replace the belt, however. It runs from the transaxle at the rear to the engine at the front, around idlers. Let me know if you would like assistance in replacing the belt and I can walk you through it, Daniel.

recheck ur drive belt either wrong belt or spring/idler/tentioner pulley needs attention adjustment replacement also check free wheel actuator that not partially activated

There is an electrical clutch that drives the belts. This clutch may be worn. Some will have adjustments that will allow it to tighten them up and not slip. Now when your tractor is running and you turn on your blades there is power sent to this clutch, it is mounted on the engine shaft and has flexible metal plate that is pulled into a pulley buy an electro magnet this plate is worn or could be full of grass/dirt. You need a good strong light, with the tractor OFF, get under and look at this clutch, look for grass/dirt or if the metal has changed colors meaning it has gotten very hot. If it has gotten very hot it is time for a new clutch... Sorry if this is the case. Good hunting.

There's a couple of things that could be causing this. First, make sure the idler pulleys and blade pulleys spin freely. Second make sure the belt is behind the belt keepers on the two idler pulleys. There is only one on the left idler, but TWO on the right idler. I suspect that is your problem. Anyway, once the belt is on, engage the blades, with engine OFF, and inspect the belt routing, making sure it's not over or under something it shouldn't be.