I changed the deck belt on mower, but when I engage the blades, the belt comes off.

ed, be sure there is nothing wrapped around the blades. lower the deck and be sure the belt is around all of the pullys. If the belt comes off one of the pulleys or the pulley on the engine it may feel normal but it wont engage the blades.

With the mower turned off and the deck at its lowest setting, check the top of the deck for any debris like rocks or sticks. Then watch the top of the deck while you move the engage lever. If nothing is moving, you either need to adjust the cable, reattach the cable, or install a new cable. If you see the lever with the pulley on it moving, when the engage lever is locked on, check the tightness of the belt. If it's loose, that is your problem and it could be a stretched belt.

check your pulleys to make sure they haven't split as this does happen. Adjust the belt tensioner to tighten the belt when you engage the clutch to drive the mower. You should be able to see the belt tensioner by dropping the cutting deck to its lowest point and looking underneath the mower to see which pulley actually puts the tension on the belt to make it drive. there is no need to start the mower to find which pulley needs to be adjusted, it should be obvious when you look at the belt and the pulleys.
Hope this helps you

when you actuate the "engage blade" lever, two things may occur,
First, a mechanical action, a spring moves to put tension onto the belt.
Second; at the end of its run, actuate a switch (PTF switch) to engage the clutch that moves the pulleys that consequentially move the blades.
if the tensioner is not moving , check if spring has come lose, undone or broken, (more likely in your case) or check the PTF switch.
any other assistance, contact me, good luck.

If you need further help, reach me via phone at https://www.6ya.com/expert/gerardo_79638d1eac6a0a1f

You either have a bad bearing on one of the pulleys or one is loose and moving up and down drawing the belt when it is engaged. I would pull the mower deck off and go through it .

The belts may be loose.With the mower off,engage the blades and check the tension.Next would be to get under the mower and check for debris in and around the lever that engages the blades.Sometimes trash will get hung up in there and the blades will disengage.The next step would be to inspect the pulleys for debris and to make sure the spindles are not bent.While at the pulleys make sure there are no burrs or cacks in the pulleys,as that will cause the blades to disengage.

Hello,
My name is Dane and I am going to help you solve your problem. You said that the "Pulley" is loose when the blades are disengaged. I hope you meant that the belt is loose, because the pulley should never be loose. If it is loose, you should remove the pulley and make sure the KEY inside the center hole is still ok. Sometimes when the pulley gets loose it chews up the KEY and allows the pulley to turn on the crank. If the KEY is ok, just reinstall the pulley and tighten it.
I believe that your belt is probably just stretched and worn. The mower deck belt gets it's tension from lowering the mower deck. the lower the deck the tighter the belt. Just replace the belt. I hope I've helped! Thank you! Dane

http://weborder.husqvarna.com/order_static/doc/HO/HO2005/HO2005_5321938-34.pdf
This is the owners manual if you do not have one.
There is a brake on the pulley, make sure that is not hanging up.
Please let me know what you find.

This belt is easist to install with the deck all the way down. Make sure there are no burrs on any of the pulleys, (will eat up your new belt in short order), also make sure that the 2 drive pulleys for the blades turn freely, (may have grass impacted around them, you may need to remove the 2 guards on top of the deck to clean, (if they do not spin eaisly). Triple check to make sure the belt is properly routed, one mistake and your belt is broken as soon as you engage. With the engine OFF, engage blades to check tension. If the belt is still loose, you likely have it routed incorrectly.