Replaced Mower Deck Drive Belt. Belt runs from Engine pulley to Mower deck Pulley. Purchased new belt rom JD dealer. Belt is slack, Is there suppodes to be a tensioner for this belt.

if not level or not proper adjusted it will or pulley -bearing or tensioner problem -bent blades

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.

Hope this helps. Serpentine Blade Drive Belt
1 Lower the mower deck to its lowest position.
2 Route the serpentine blade drive belt (part number 6109) onto the V-pulleys located on the mower deck. One double pulley is located at the front of the deck near the center. The other two pulleys are located near each side of the deck. Route the belt around the outside -- the side toward the outer edge of the mower deck -- of each pulley. For the double pulley near the front of the deck, route the belt around the lower pulley (part number 6058) only. If pulleys are fitted with belt guards, work the belt in between the pulley and shield.
3 Push the idler pulley toward the center of the deck with your hand. The idler pulley (part number 6059) is the fourth pulley on the deck and is located near the rear of the deck. It is the only movable pulley on the deck. Wrap the belt around the side of the pulley nearest the edge of the deck. Release the idler pulley to place tension on the belt. No adjustment is necessary because the serpentine idler spring (part number 6065), located on the idler pulley, adjusts the belt to the correct tension.

sounds like one of the pulleys is frozen up checkall pulleys and replace the bad ones

If it is the drive belt do the following place your mower on a level surface set parking brake, remove mower deck, remove belt from stationary idler and clutch idler, pull belt slack towards the rear of tractor , then remove belt upwards from transaxle pulley by deflecting the belt keepers, then pull the belt towards the front of mower and remove downwards from engine pulley Install new belt in reverse procedure

To install the drive belt you will have to remove the drive belt first.
Follow the procedure: ---
Park the tractor on a level surface. Set the parking brake and remove the key from the ignition. Study the belt replacement guide decal located underneath the left foot board for proper belt routing instructions.
Remove the mowing deck by first moving the attachment clutch lever to the disengaged position (toward you). Move the attachment lift lever all the way forward to lower the mower housing to its down position.
Crawl beneath the tractor and roll the drive belt off engine pulley (it hangs directly beneath the engine). Remove small retainer clip that holds the clutch spring to the pulley bolt located at the center of the mowing deck. Remove the large retainer springs (two of them), then slide the belt collar away from the mower housing and off the housing bracket.
Disconnect the anti-sway bar rods from the chassis brackets by removing the retaining cotter pins. Disconnect front links from the mower deck by removing retaining cotter pins. Disconnect the suspension arms from the rear mower deck brackets by removing the two cotter pins.
Move the attachment lift lever all the way to the forward position to raise the mower suspension arms to the full up position. Slide the mower deck out from beneath the tractor.
Crawl beneath the tractor and roll the drive belt off of the stationary idler pulley and the clutching idler pulley. They are both located immediately adjacent to the engine pulley. Be sure the belt is already away from the engine pulley, as well.
Push the drive belt slack toward the rear of tractor. Remove the drive belt from the Trans axle pulley by taking it over the trans axle pulley while compressing the movable belt keepers surrounding it. Remove the old belt from the tractor by pushing it clear of the center span keeper (located beneath the Trans axle pulley).
Install a new drive belt by following the reverse of the instructions for removal. Test the tractor and mower for normal operation.
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Based on experience with MTD and AYP products: 1) remove mower to deck belt, 2) remove deck, 3) have helper press brake pedal down which puts slack in drive belt, 4) pull drive belt first from front engine pulley or rear transmission pulley then remove entirely. Reinstall in reverse order.

CAVEATS 1) If a pulley is grooved then the narrow part of the belt goes into it, if the pulley is flat then most likely the back of the belt goes against it in a tensioning function. 2) There are often guides or keepers which hold the belt in place, these can be hard to see and if the belt is not routed inside them it may well not work or hop out under load.

Is it a triple blade mower deck? Is it the drive belt to the deck or the belt from the deck pulley to the triple blades?

You need to take the cover off that encloses the 3 pulleys on the deck.
Take note of the direction and routing.
You will find and spring loaded idler pulley on the slack side of the belt or the unloaded side of the belt.
Pry back the idler on it's pivot point and slip the belt off. Place new belt in same configuration as the old.

jd lt 166 has 5 different decks
3 of which have the same belt routing
1st the belt needs to be adjusted correctly to stop it from "slacking off"
then on most decks the belt is routed in between the 2 small idler pulleys and then to the engine pulley
so let me know the deck i.e. 36", 42 ", 46", 48"
different belt sizes also
kendo

I am so far from expert...me answering this surprises me. However, I can...JD Small Engines has plenty of diagrams for the 316. Here's how a old single lady changes the belt on her 85 316. There is a bolt running from side to side of a black metal box...on the very front at the bottom of the mower below the grill. If you pull that out, just the front of the mower deck separates from the mower..slip the belt in and around the rear central pulley. Put the pin back in with the belt now around the back pulley and outside the part where the pin goes back in. Get a pneumatic tire jack and jack up the right (your left as you face the front of the machine) front pulley..try to catch the pulley armature or bolt so you don't torque the pulley. This releases the tensioner...thread the belt on the right pulley and reach up under the grill and put the belt over the front engine pulley....now all that's left is to get the belt over the left pulley. At this point, I remember having to either lower the deck to the ground or raise it up all the way...to the ground, I think as it wasn't already in the correct position. Once you start fussing with the left pulley, you will see it is spring loaded and easy to force the belt over if the right pulley is still on the jack. Really important to get the belt inside all the belt guides and lowering the deck before you frustrate yourself with getting the belt over the last pulley. When all is done, release the jack and everything tightens right up.