~15yo riding mower/tractor tranny likely dying

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My ~15 year old riding mower/tractor - Sears DYS4500 54" (917.28690) - has been having a harder and harder time getting moving lately. I likely overwhelmed it pulling a dethatcher with too much weight on it, but regardless, it's had a pretty long life so far so I'm not too surprised. Last time I went to mow my lawn, pushing the go pedal made noise but no pull at all at first; after a little while it finally consented to move the tractor around.

I pretty recently replaced the drive belt so I'd be really surprised if it's that, especially as it did eventually drive.

I looked up the transmission part and it's a K46BT.

The entire "tractor" isn't new obviously, but the engine seems to run pretty well and it mows really well. I've been mowing our small lawn (maybe 2000 sq ft) weekly, but in the later spring I also mow our orchard area (an acre) a few times here and there, and my wife uses it to pull a 10 cu ft trailer when she does major weeding or pruning of the various decorative shrubs.

The questions:
1 - reading the manual I see there's a "purge" operation. I doubt it needs purging as it's not new, but would it hurt to do this anyways? Could it help?
2 - is the K46BT serviceable, can I possibly fix it?
3 - this part claims to be a K46BT, for $580. Good price? Same on amz is $800...

 

[Roadworthy]

Your transaxle may be low or out of fluid. There should be a filler cap on top but no place to check the level. You could try adding a pint or so of 85W90 or 85W140 gear oil. I drilled a hole in mine and added a hydraulic jack plug just below the case split line on mine to check fluid level.

 

[stevem5215]

Yep, take the trans axle out and pull the vent plug and dump it into a 5 gallon bucket. Reload it with 5w-50 synthetic and it (might) work again.

 

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What's the easiest way to get at the vent, or to remove the transaxle entirely? ... before I just start undoing whatever bolts I see lol

 

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Looks like a couple bolts inside each rear wheel is about all that holds it, so I'll pull the wheels, belt, connecting rods and the bolts.

I'm not too hopeful as when I disengaged the drive to move it there's shuddering likely from the gearbox, I'll probably be splitting it to look inside...

 

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... progress

 

[LS Tractor Owner]

I have a 23 y/o Craftsman DGS6500 garden tractor. The trans is "non-serviceable" as yours. I basically removed all the sheet metal covering the transmission. The service manual that I found online showed that when filling it, you need to rotate the tires so the oil gets to all the nooks and crannies. Drained it, and refilled with the synthetic 5w-50 previously mentioned.

I swear that the tractor could do wheelies now! I definitely got a few more years out of it!

 

[ROUSTABOUT]

Looks like a couple bolts inside each rear wheel is about all that holds it, so I'll pull the wheels, belt, connecting rods and the bolts.

I'm not too hopeful as when I disengaged the drive to move it there's shuddering likely from the gearbox, I'll probably be splitting it to look inside...

It will do that if low on fluid. Get a suction gun, looks like an old timey grease gun, and use it to get old fluid out, instead of removing rear axle. Some had a filter, so get one if yours does. Mine is 22 years old and abused regularly. I hack down briar thickets with it. Yours should be fine, just fluid.

 

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I pulled the tranny and am draining the "oil"... It's dirty rusty milkshake...

I've got some T6 5-40; in the spirit of continued abuse I'll put that in it instead of buying 5-50. I figure if I fill it and it's still crunchy (when trying to spin the axles in the same direction together with the drive disengaged), I'm just taking it apart anyways.

 

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After refill, I used a drill to drive the input shaft and got it purged and then it seems to drive the axles properly without extra sounds (at least not that I can hear over the drill). With the drive disengaged, I can rotate the axles by hand with what sounds like "normal" gear noises.

Last questions:
1 - the filler plug came detached from its cover (first pic; bigger black thing is the cover and isn't attached to anything any more, it used to be attached to the plug). I assume the filler is also a breather, so it's probably important to have a proper cap? Should I get a new plug & cap? If so, where - would a local lawnmower fixer have something like that?

2 - what's the other flat black plastic thing (second pic)? It looks kinda cracked; should it be replaced?

Thanks!