Inside the Transmission -- Take Apart

INVENTgineer

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My Honda HRX with a hydrostatic cruise control transmission became slow, and I decided to take it out of the mower and disassemble a part of it in an attempt to investigate the cause. One thing I noticed was that the input (pulley) shaft to the main hydraulic plunger pump was hard to turn and felt rough, as if it was binding up. 20190519_155849_resized.jpg 20190519_155844_resized.jpg After removing the top vent / fill cover and inverting the unit over a container to drain the old oil (which was very dark gray), I removed the bottom cover (four bolts hold it on) to get at the main pump - this cover is under spring pressure, so loosening each of the bolts a little at a time is advised.

Photos of the internal pump components are included here. 20190519_155144_resized.jpg 20190519_155419_resized.jpg Once the cover is removed, I suggest turning the unit on its side, or even right (pulley) side up, to remove the piston disc - otherwise many of the parts could fall out and get into the pump housing. There are seven plungers, all contained inside a rotating disc, with each having an internal compression spring. The closed end of each plunger bears against a radial roller bearing, which is designed to tilt when the orange drive speed control lever (on the handle) is engaged. This tilt angle modifies the plunger stroke, thus building pressure and forcing oil to the drive motor. 20190519_155438_resized.jpg 20190519_155446_resized.jpg

I found the hardened steel plungers in the pump had a wear pattern, and sharp burrs, at their closed ends; in addition, the associated bores in the rotating disc were scored. 20190519_155534_resized.jpg 20190519_155523_resized.jpg

One of the plungers was stuck in its bore and would not move, and the others moved if forced, but roughly. I deburred and polished each plunger, and carefully honed each inner bore, then reassembled everything, verifying that it all worked smoothly. Then I filled the trans with fresh Honda Hydro oil, spun it up on the bench to remove any air bubbles. This is FYI, just to show some of the internal working parts of the pump; I haven't ventured into the motor (axle) side at this point.
 
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bertsmobile1

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Thanks for the photos.
Do not be surprised if the unit fails totally in the next few years.
As you would have noticed there are no seals anywhere inside the box and lots of very close fitting parts running against each other.
A very small amount of dust continually being circulated around that box will destroy all of the fine machined surfaces.

before any hydro is split, the case needs to be shiny clean to prevent dirt, grit & oxide getting inside the tranny.
I blow down, degrease, soda blast , then pressure wash before opening one up.
The work is done in the shower cubicle of the workshop after it is wetted down & the parts laid out on dampened shop towels.
If I have to take a break the whole shooting match is covered with a damp towel.

Naturally the spannar work is done wearing a freshly washed pair of overalls, rubber gloves & in my case a beanie with all my hair tucked into it.

Please do not take this as a critism of what you have done because you were "just having a look" and not a professional repair person.
This is written for the benefit of others who might read you very informative post with good pictures and be encouraged to have a go themselves.

The biggest problem with all hydros is grit that gets stuck between things like your piston and wears grooves under the piston block so the oil ends up leaking between the holes & not going through the one it was intended to.
While sealed, the grit is wear particles which is usually magnetic and gets caught by a magnet ( or 2 or 3 ) placed in there for that purpose.
However abrasive dust or corrosion products that fall into the box while it is apart is not magnetic so continues to circulate doing damage.

There is nothing inside a hydro that is rocket science and they are very much consumer repairable, if you can get the parts, but excessive care needs to be taken regarding cleaning.

Thank you for your post.
 

jp1961

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The oil fill cap/dipstick tends to loosen from vibration causing the unit to leak hydro fluid. Best to check it a couple times a mowing season.

The replacement tranny is SUPER expensive.

Regards

Jeff
 

Womble

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My Honda HRX with a hydrostatic cruise control transmission became slow, and I decided to take it out of the mower and disassemble a part of it in an attempt to investigate the cause. One thing I noticed was that the input (pulley) shaft to the main hydraulic plunger pump was hard to turn and felt rough, as if it was binding up. View attachment 43960 View attachment 43961 After removing the top vent / fill cover and inverting the unit over a container to drain the old oil (which was very dark gray), I removed the bottom cover (four bolts hold it on) to get at the main pump - this cover is under spring pressure, so loosening each of the bolts a little at a time is advised.

Photos of the internal pump components are included here. View attachment 43962 View attachment 43963 Once the cover is removed, I suggest turning the unit on its side, or even right (pulley) side up, to remove the piston disc - otherwise many of the parts could fall out and get into the pump housing. There are seven plungers, all contained inside a rotating disc, with each having an internal compression spring. The closed end of each plunger bears against a radial roller bearing, which is designed to tilt when the orange drive speed control lever (on the handle) is engaged. This tilt angle modifies the plunger stroke, thus building pressure and forcing oil to the drive motor. View attachment 43966 View attachment 43967

I found the hardened steel plungers in the pump had a wear pattern, and sharp burrs, at their closed ends; in addition, the associated bores in the rotating disc were scored. View attachment 43964 View attachment 43965

One of the plungers was stuck in its bore and would not move, and the others moved if forced, but roughly. I deburred and polished each plunger, and carefully honed each inner bore, then reassembled everything, verifying that it all worked smoothly. Then I filled the trans with fresh Honda Hydro oil, spun it up on the bench to remove any air bubbles. This is FYI, just to show some of the internal working parts of the pump; I haven't ventured into the motor (axle) side at this point.
Hi, just dismantled my drive as the drive shaft popped out the top. Did you have a washer retained by a 10mm snap ring on the pump shaft (splined shaft)?

I can’t find an exploded parts diagram for this transmission and I want to make sure I’m reassembling in the right order.
 

INVENTgineer

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It makes sense that would be the case, but I really don't recall and didn't have to get into it that far since the shaft on mine was intact. Having the shaft pop out the top may be a sign that things are really worn inside, perhaps beyond the point of repair. I would expect that a washer would be associated with a snap ring in this application. Did you find loose parts inside?
 

JBtoro

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I might suggest watching these two videos. I followed this fellow's recommendations this summer on an HMA with good success. The speed increased substantially from mediocre to where I am comfortable walking in the mid range of the orange speed lever. Be sure to spend some time bleeding the air bubbles out. It takes awhile but be patient. Like he says in the 2nd video, use Honda fluid.

 

Womble

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I might suggest watching these two videos. I followed this fellow's recommendations this summer on an HMA with good success. The speed increased substantially from mediocre to where I am comfortable walking in the mid range of the orange speed lever. Be sure to spend some time bleeding the air bubbles out. It takes awhile but be patient. Like he says in the 2nd video, use Honda fluid.

Thanks, I have watched these. I successfully stripped down the transmission. It works much like most machinery hydraulic pumps just in a much smaller scale. Mine is worn but the 10mm snap ring had dislodged allowing the shaft to be removed. I purchased replacement oil seals (these are generic and much cheaper than Honda) as well as snap ring. HST fluid was available from my Honda dealer. He did say that you can also use 10W40W engine oil but I stuck to the HST. Reassembled and bleed the system. The trick I learnt was you need to cable tie the speed control lever in the fully open position. It’s sprung so hence the cable tie. This helped get the air out of the bottom passage ways.
 
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