MTD Transaxle Chatter

Hello,

I have a White Outdoor model: 13AP60TP090 riding mower. The transaxle started to make some chatter the other day while driving. I was trying to climb a hill and it would start to chatter and the wheels weren't spinning so I parked it immediately. I recently put on a new drive belt, and it rode fine for a good hour or so when I started to hear this noise. I did have to make adjustments to the belt tension so that it would stay tight enough but still not grab when not pressing the pedal. The transaxle is model:618-04632. I took it off and split the cases an found that there is a bit of wear on small pinion on the short horizontal shaft, and even less on the big gear. It seems like a robust design, meaning that there is little that can go wrong. I thought about the belt tension, but any impact it would have would be on the bevel gears or dog gear in the middle if you change directions with the pulley spinning, but those seem fine. The bushings are worn, but again, that wouldn't have much affect on the two straight-cut gears. Honestly I would be fine with a little chatter due to mower's age, but I don't understand why it stopped moving? That part could be attributed to a loose belt, so I can try adjusting it again. Let me know your thoughts!

Here's a link to the diagram:
https://www.partstree.com/parts/white-outdoor/mowers-lawn-garden-tractor/13ap60tp090-lt-5000-white-outdoor-lawn-tractor-50-deck-2008/transmission/

Also, if there are any compatible transaxles that could serve as a replacement, or for parts, let me know. It may be easiest to just get another bolt-on transaxle, but I don't know which models would work.

The shift gear, part 5 is not making full contact with the Bevel gear.
You usually find it will chatter like a Hollywood machine gun in forward but drive fine in reverse.
Wear in the bushes part 9 and the thrust washers part 7 & 8 make this worse.
The cheap fix is 2 new bushes # 9 and clutch collar # 5 then swap the two bevel gears # 4 over so the new collar gear is working against an unworn section of the bevels.
Slightly more expensive is to add a new forward bevel gear.

I usually replace the entire kit as freight from the USA is $ 60 so trying to save $ 50 is a fools errand.
I also use thinner grease in the transmission, usually a 01 grade tractor gearbox liguid grease.
It will leak out a little but it will remain fluid and continue to provide proper lubrication for a lot longer.

Replacing the grease in the tranny is a service item that never ever gets done ( that includes me ) till the box starts to show problems.

What Bert describe is the issue. It takes very little wear on those gears to cause issues. The little bit of wear on the pinion and the little bit of wear on the drive gears creates a ramp effect and under load will jump from one notch to the next.

I heard the original grease used is actually pretty good (Bentonite Grease I think). It's thick, but it still has good flow and clings very well to the surfaces. Also, I don't think there's enough wear on those teeth to actually skip.

For the benefit of others with the same problem who might chance upon this thread because the original poster has closed his mind
A grease is made from solid particles that are naturally slippery ground up to a particular size then mixed with a fluid, usually oil to allow it to flow.
It goes off by the liquid evaporating or separating out & leaking. We have all seen this when we open on olf tub of grease & see some oil sitting on top of the grease in the tub.It is also what leaks out of a grease gun when you are not using it.
The solids rarely deteriorate unless you get to extreme heat.,
If you open a box and see most of the grease is nice & new yellow brown looking but only small amount near the moving parts is black then the grease has gone off , well gone too dry to flow to be accurate.
Nice clean grease sitting between the case halves does nothing to lubricate the gears, it just occupies space.
If the grease is working properly then it will all be varying shades of grey / black.

Now to the gears.
Normally in a dog clutch the teeth are undercut so they lock in together when placed under load.
However in the case of these gears they are strait cut so when they start to wear the only thing that keeps them together is the pressure of the spring part # 18 pressing the ball part #19 into the grooves in the shaft part # 1
In use the clutch gear , part # 5 is always turning against the fork so wear happens and as the grease gets harder because it dries out not enough gets between the fork & the gear so the wear accelerates.
As written in the original response the clutch gear ( dog if you like ) crashes into the teeth on the bevel gear causing them to wear into a sawtooth shape.
Also because of the mechanical set up the whole set up is also pushing against the end bushes, so the flange on the top hat bushes part # 9 wear thinner which is why there is a replacable thrust washer in there.
Now the position of the shift fork is set by the position of the hole with the ball bearing & spring .
Over time wear will allow the entire clutch shaft to be loose between the two bushes ,# part 9 and eventually it becomes loose enough for the teeth on the clutch dog , part 5 to skip over the corresponding teeth on the bevel gear.

Thus makes a mechanical ratcheting type of sound and is easily diagnosed because it will generally happen only on the forward position or will be substantially worse forward than backwards.
I have simply put thicker thrust washers on the shaft and swapped the two bevels around on some boxes belonging to pensioners who could not afford to replace the complete assembly and most are working OK although reverse does slip a bit so they change their mowing pattern to avoid reversing as much as possible.
The ones where we replaced the entire shaft are running like new.
When pulled apart, the gears always look good to the naked eye but when properly cleaned and viewed under an illuminated x200 magnifier the wear is clearly apparent & the customers convinced that the parts needed replacing.

The tollerances on these are very tight so a very small amount of wear in a few different places add up to a failure.
Every box I have pulled apart had nice clean looking ( thus too hard ) grease in there and I replace the original bentonite grease with a lot thinner EP grease to make sure all parts are getting lubed.
The oldest one I did is now 4 years old & working quite well save the fact that a bit of grease oozes out of the axels as there are no seals fitted.
The next one I did the space between the inner & outer axel bushings ( machined into the cases ) got filled with the bentonite that was in the tranny in the first place and it does not leak.

Bert,

Thanks for the additional info. However, that's not my issue. I posted some videos. The issue I'm having is in the differential, and you can see from the videos that even without the front shaft moving, there is still noise. Also, the noise would occur in both directions, and when pushing the tractor while it's in neutral, so I think the shift gear and bevel gears are in the clear. It does look like the differential is toast, and it doesn't appear to be serviceable.

Part 1: https://www.youtube.com/watch?v=eMHO3eQ8wGk
Part 2: https://www.youtube.com/watch?v=gBxV8YkzJZc

Does anyone know of a good replacement for the 618-04632?

Now we have seen what you have seen, yes you are correct, the Diff is toast.
looks like a lot of steel in that grease on the bull gear.
AFAIK the diff is not servicable but more knowledgable than I might know where to get one fixed.
I have fixed a MTD diff using Peerless parts but it was a different one to yours.
Time to chase up some mower breakers.
Those trannies are at a premium price as the racing mower crowd like then to lock solid for racers.

Thanks for the info on grease. I have, over the years, opened up a gearbox or something like that and seen the clean white looking grease. I usually smear some of this new looking stuff over the gears and think it's good to go. Now I know.

What about welding the diff locked? What would be the downsides? Is it just more likely to tear up the lawn? I guess there could be slightly more stress on the gears. Have any of you used a locked diff for everyday mowing?

I do not own a mower with a diff.
I mow about 3 acres of roadside verge as part of my rental agreement for the workshop.
The only thing is the mowers can not turn tight and I end up doing a lot of 3 point turns & mowing in reverse.
I have 2 commercial customers who mow up to 150 residential yards a week in high season using Greenfields 34" Fastcut mowers and when I service them the diff is always locked & I often have to heat them up to unlock them.
So you will just need to learn how to mow differently.
Try to do a very tight turn on a mower without a diff and the front wheels lock back against the frame then slide.
This will end up bending the drag links.
OTOH mowing backwards is very fast and you learn to do tight mowing backwards.
So for a big yard you mow in a figure 8 as if you ha 2 yards side by side or do a spiral mow as we used to do before carchers so all the clippings were in a single pile in the middle of your lawn ( yes I am that old )