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Drive belt keeps coming off when I shift into reverse


indyhoov's Avatar
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10-16-11, 04:00 PM   #1  
Drive belt keeps coming off when I shift into reverse

I have a MTD brand riding mower, 16.5 hp with a 42" cut. I was using it last week and when I shifted it into reverse, the drive belt came off. This was the first time this has ever happened. I put it back on and took off in forward gear but when I shfted into reverse again the belt came off again.
I replaced the belt with a new factory belt, thinking that maybe the old belt was stretched. However, it is still doing the same thing.
The only way that I can put it into reverse and be able to drive it in reverse is to ease out on the clutch pedal VERY slowly. I've never had to "baby" it like that before.
I can put the mower into forward and use it in that gear with no problem.
I pulled the access cover off and have checked the operation of the tensioner and the belt while driving the mower and shifting from forward into reverse. The tensioner seems to be operating fine and is keeping enough tension on the belt.
Any clue as to what I'm dealing with here?

Thanks in advance for any input/help!

Mike

 
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cheese's Avatar
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10-16-11, 04:33 PM   #2  
The tensioner arm on these is flimsy and tends to bend a bit, causing the pulley to get out of alignment with the other pulleys. Check for that, and check the tensioner pulley for bad bearings.


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God bless!

 
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10-16-11, 05:12 PM   #3  
You could check under the tractor to make sure there are belt guides at the pully to prevent this from occuring. These rods only purpose is to keep the belts from falling off the pully during times they are not under tension

 
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10-16-11, 08:55 PM   #4  
If it's the rear belt to the transmission on a vari-drive remember that the shifter rod retainer holding it to the frame has to be removed from the frame to thread the belt. Any chance the belt is outside the shifter rod instead of inside of it and when going into reverse it pushes the belt out of line?

 
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10-17-11, 01:53 AM   #5  
I'll check the bearings in the tensioner pully but with the location of the belt, pulleys and tensioner, it'll be difficult to check the alignment but I'll give it a shot.

There are belt keeper pins that do keep the belt in place when there is no tension.

I've checked the routing of the belt and I have it correctly routed.

Thanks for the advice!

Mike

 
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10-17-11, 06:24 AM   #6  
Take a look at the main frame where the tranny bolts up to it (4 bolts); perhaps the frame is cracked in just the right (wrong) place and in reverse, under that torque, the frame is twisting (flexing) and allowing the belt to fall off due to misalignment.

 
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10-17-11, 09:41 AM   #7  
Is it the rear belt? If so, it shouldn't ever go "slack" or not have tension on it. The front belt does go slack, and has belt guides. Maybe I'm thinking of the wrong belt. Since you mentioned a tensioner, that tells me it's the rear belt.

I have seen a mower or two with the problem Puey61 suggests, although rarely, but it happens right where the tranny bolts go through the frame behind the rear wheels.


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10-18-11, 10:10 AM   #8  
Indyhoov, post back with the model and serial numbers of the mower...located under the seat.

 
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10-21-11, 02:20 AM   #9  
Sorry for not getting back to this, I've been a bit busy the past few days. @puey61, I'll check the numbers and post them here, probably won't be until Sunday or so.
@Cheese, makes sense that there should always be tension on that belt doesn't it? I'll check the operation of the tensioner a bit closer sometime this weekend also and post my findings.
Thanks for the advice!
Mike

 
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03-25-12, 12:19 PM   #10  
Well, I'm back and I'm still having problems with the belt coming off as described in my earlier posts. I've even tried a bit shorter belt but still no luck. I do have a question though. With the mower out of gear should there be any movement up & down in the tensioner pulley/assembly? I get about 3/8" - 1/2" up & down movement. Also, when the belt is off the pulleys, as when it pops off while I'm using it, I have probably 3/4" up & down movement. Is this normal?
Thanks for any advice!

Mike

 
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03-25-12, 12:48 PM   #11  
My MTD's have some metal surrounding the Engine pulley so the belt just cannot get off that Engine pulley unless the bolt is loose and the pulley is dropping down at least 3/4".

If the Engine driven belt is coming off that slider in the Speed control, the tensioner is 'bouncing' or 'sticking' in the loose position.

I never have understood that silly speed control sliding pulley mess.
I was going to adapt a bicycle chain drive for the engine belt, and use the Trans belt tensioner as the clutch.
But that's just a dream.:NO NO NO:

 
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03-25-12, 01:55 PM   #12  
Hello Mike,


You should have little to no up and down movement in the rear belt tension idler arm, Also if this unit has the plastic flat idler pulley make sure there's no groove worn in it. Another thing to check is the transaxle input shaft bearings, Grab the trans. pulley and see if it rocks side to side. If it does the bearings have failed and is causing the pulley to tilt.

Lastly, Most vari-drive systems are designed to run a specific belt length in order to function properly. MTD is one of them, So to assure proper system operation re-install OEM or aftermarket direct replacement belts.


Good Luck

 
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03-25-12, 06:27 PM   #13  
With it only happening in reverse, I would tend to think the transmission is rocking under the reverse pressure, tilting the transmission pulley, and letting the belt come off. Check the transmission mounting brackets, bolts, and for cracks in the frame. Either that, or the transmission pulley input shaft bearings are worn out. The belt and pulley turns the same way not matter if you are in forward or reverse, so if it was the tensioner, it would do it in forward too. The transmission pulley and transmission itself tilts one way in fwd, and the other in reverse. I think the problem is related to that.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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04-21-12, 11:20 AM   #14  
The tensioner arm on these is flimsy and tends to bend a bit, causing the pulley to get out of alignment with the other pulleys. Check for that, and check the tensioner pulley for bad bearings.
I finally have this figured out! I was replacing the battery on the mower and pulled out the battery box to clean it out and just below this is the tranny pulley with the belt I've been having problems with. I was able to check the alignment of the tranny pulley and the tensioner and found that the tensioner pulley was riding about 3/4" higher than the tranny pulley. I was able to push it down to where it they are in pretty good alignment now. You're right Cheese, the tensioner assembly is bit flimsy! I tested it out by going back and forth in my yard 5-6 times and it seemed to be fine. I'll find out for sure tomorrow when I mow my yard. If I have this problem in the future, the first thing I'll check will be the pulley alignment.
Thanks Cheese!

Mike

 
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