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Minding my P's & Q's
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Ya mean check the check valves in the one I just put back together? Yeah that intellectual curiosity might bug me for a while but I think I'll get over it. Maybe not as right now I'm wondering if the pressure leak is from wear on the check valve or wear where it contacts the bore in the center section. That's kinda making me want to take the assumed good one out of the donor and try it in the original.

I could have done without that thought. The suspect one on the original, which is now completely put back together, is the one on the motion control lever side of the pump. I could get that out pretty easily by removing the fender and gas tank (again) and not even loose any oil.


Probably a lot more than anybody really wants to know about a 20 yr old trans. I'm thinking probably more than I wanted to know about it.

You are wrong hydraulic oil breath. (Channeling Ed McMahon on the Johnny Carson tonight show)

I for one am very interested in what you have done. I own a GT with the same trans. And actively looking for another one or two. Either as a parts donor and/or good working machine.

You have done a great job on this. And we appreciate cha.
The older tractors are the better tractors.

You are right, you can get that one out as you described. Without removing the trans from the tractor. I had mine out at one time and found the surface of the valve was a little rough. Spun it in a drill locked in a vise and smoothed it with 800 and 1200 grit cloth backed emery cloth and a hard block backer. Made a little difference. But found the real problem I was trying to solve was the whole transaxle was loose in the tractor and shifting position as I drove forward or back.

Will try pushing mine with the bypass in drive position. Already know if I park it nose down on the hill it will creep very slowly forward with a load in a cart, unless I put the parking brake on. Don't know about reverse though.
 

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Big and Wide
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Discussion Starter #102
Give me a little while to come up with a question for Carnac. I'd been casually looking for a parts machine for a while. Got lucky in that one popped up real cheap right when this fail happened. Seems to have become my backup tractor.

So I asked Hydro Gear support if replacing the check valves was a routine (wear and tear) fix or does it generally require doing something with the bore in the center section. Their answer was check the bore when checking the valve and attached a copy of the service manual (that I already have) that doesn't say anything about how to check the bores.

I recall your post about that and as I recall the pics of your valves didn't look significantly different from mine. It's probably a very small amount of wear that makes a difference. Somehow I think I'm about to know even more about these axles than I wanted to.
 

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Minding my P's & Q's
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Put the deck and bagger on mine today. Time to start picking up leaves.
Can push it forward a foot, more if I wanted to. Can not push it at all in reverse.
 

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Somehow I think I'm about to know even more about these axles than I wanted to.
Never happen!

What you don't (or can't) apply to a specific transmission/final drive provides the basis for understanding other aspects of hydraulics and gear reduction systems, and so many GT owners would love to have a better understanding of those two subjects.

You're doing an awesome service for the members! :thThumbsU
 

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:ditto:

I have two of these, and another similar one. Would really like to know more about the drifting when parked.

Mike
 

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Big and Wide
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Discussion Starter #106
I park my tractor in the basement. To avoid exhaust gas getting in the house my routine is to put the bypass valve in the transport position and push the thing out the door. After doing that today I put it back in the drive position and tried to push it. Was a bit surprised when I could roll it both directions. Fired it up, drove it around to the garage, backed it in, tried again and could only roll it in one direction. My working theory is that rolling the tractor (engine off) spins the pump enough to drain the pistons. Net result being to relieve the hydraulic pressure that keeps you from being able to roll it after shutting it off.

Yanked the seat and fenders because no way yer get gonna get to the check valve with them on.
 

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Big and Wide
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Discussion Starter #107
This is where I really wanted to type that I switched the "reverse" check valves between tractors and ended up with one I could push in both directions and one I couldn't push in either direction. One problem with that is still waiting for a filter to show up, so no oil in the "donor" yet. The other problem is apparently I screwed up and put the valves back in the opposite sides when I had the original one torn apart on the bench.

I'd generally be annoyed by doing something like that but as it seems to be working as is, not so much. I also allow for the possibility that some other idiot switched them in the "donor". FWIW: had both out of the original while the center section was off the housing. When poking around the donor the other day, took them out and replaced them one at a time.

This [re]raises the question of why they are different part numbers and btw, the one with the groove and dimple machined in to it is twice the price of the other one. Without whipping out the micrometer I don't have, they look otherwise pretty identical to me. See also: in the older HG diagram in the original Sears owners manual it shows the same part number ball (not poppet) in both sides.

At this point my plan is to wrestle with my OCD to let go of this. I fixed what was broken and both tractors should run, drive and cut in both directions and good enough. When one of you guys get around to cracking yours open maybe get back to me on which one is where. Yes, I kinda do wish I had swapped them anyway.
 

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Big and Wide
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Discussion Starter #108
The only part of the HG service manual that talks about these valves. I think it explains the reason for switching from a ball to a poppet but not why two different ones.

Charge Check Valves

Remove the check valve plug with a 1/4" internal hex wrench.

Remove the check valve spring and check ball (or poppet) from the center section.

CAUTION: Do not allow the check ball to fall into the closed loop passages in the center section. Removal may be difficult but can be accomplished with a magnet, or by removing the plug from the end of the center section. Do not allow contaminants to be introduced to the system.

Inspect the check balls (or poppets) and mating seats in the center section for damage or foreign material. Position the transmission so that the check valve port will be in the upright position (as shown) and install the check ball (or poppet), spring and plug (with o-ring) into the center section. Make sure the plug stem is properly positioned into the poppet or damage and failure will occur. Be certain the check ball does not fall into the closed loop passage.

Torque the plug to 15-20 ft.lbs (20-27 Nm).

Turn the unit over and repeat the procedure for the other side.
 

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I also allow for the possibility that some other idiot switched them in the "donor".
Nice! Beavis and Butthead fan?

Mike
 

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Too many great quotes. The movie was hilarious. My favorite part was when their hippy freak teacher started whining about due process, and got a rifle butt to the stomach, and his hair went flying (skip the first 55 seconds):


No, I don't know why...

Mike
 

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My working theory is that rolling the tractor (engine off) spins the pump enough to drain the pistons. Net result being to relieve the hydraulic pressure that keeps you from being able to roll it after shutting it off.
The rear wheels drive the motor section of the hydro. The by-pass valve, or freewheeling valve, allows the fluid being pumped from one side of the motor a clear passage to the other side, otherwise it would go to the pump section where the swash plate is centered and will not easily allow free flow of the fluid.

Why can hydros 'slip' with the engine off? Hydraulic pumps and motors have clearances to allow a limited internal leakage of fluid for the purpose of lubricating the pistons and cylinder valve plates when in motion. Most hydros contain less than a cubic inch of fluid in the motor cylinders which will leak out and back to the reservoir through these clearance with a relatively low physical effort over time. Park a hydro tractor with the engine off on the slope near the top of a hill without applying the parking brake and you can expect to find it at the bottom of the hill in a relatively short time frame, depending on the wear in the hydro. It only takes a few inches of rotation of the rear tires, coupled with the gear reduction in the final drive, to rotate the motor section one revolution. By the time the rear tires have rotated 1/4-1/2 revolution, there isn't much fluid left in the hydro motor to provide any braking action.

Actually, this will occur with the engine running as well. It just takes a little more time.
 

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OK Bob, what (if anything) can be done about this, or is this just something we have to live with as hydros age?

Mike
 

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Is there a 'pinhole' in the center of the left hand check valve piston? If so, it might allow for 'slow leakdown' or movement when in N. I suspect it and the bored out center might be there to 'cushion' the check valve closing or coming off its seat.
I have not thought it through, but moving from F to R or R to F might be a 'thumping' condition, or even movement from a drive range to N might cause the check valve to come closed quickly. The shape of the piston, the bored center, and perhaps pinhole might afford a cushion in closing, resulting in less clunky operation and longer life. The 'clunky' might be small to the operator, but high pressure stuff can be pretty hard on the parts, causing spikes in pressure, that may be un-noticeable to the operator.
An example, perhaps hokey, would be to pull the lever from full-boat forward into N. Fluid flowing will want to keep flowing, and a check valve slamming shut will cause a hydraulic shock. More or less. If Bob the Tudor knows more about that topic, I welcome correction and criticism. I am just a learner, not a pro.
tom
 

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Big and Wide
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Discussion Starter #116
The other day when I had the donor on the bench I spent a little time spinning the input pulley by hand and playing with the motion control lever to figure out which check valve was forward and which was reverse. It didn't take much oil leaking out (check valve removed) while doing that for the motors to loose enough fluid that I couldn't do it any more. Not hard to imagine pushing the thing a few feet in bypass mode to do same thing and see Tudor's explanation above. See also the picture of the center section face a few page back.

The other filter showed up yesterday so I just buttoned up the "back up", filled it, purged and it took for a test drive. Just for grins I switched the check valves before doing that. After I shut it off (and it before it bled down) I tried the roll it with the bypass valve in drive position test. I was still able to roll it only one direction but it was the opposite direction. My conclusion here is that one of the check valves is worn enough to not seat 100%. I'm still trying to go with drives both directions and well enough to let this slide. If it loses too much speed dragging a load up hill, I'll just move the lever a bit past the "cut" position toward the "full speed" position and not worry about it.

I also think I worked out my parking problem.
 

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Big and Wide
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Discussion Starter #117
A final thought on the economics of this 10 week adventure:

Fixing my 1200 hr, bought it new, primary was about $210.
Brake shaft, motor shaft, gasket, oil & filter.

Turning the 800 hr basket case donor in to a back up I trust was about $265.
Got it cheap, fed it (2) front tires, (2) ignition coils, ameter, oil & filter, flywheel fan, de-bozo-ed the alternator magnets, drive belt tensioner and wiring. Still needs an air filter, a few missing bolts on the engine cover and a battery hold down. The back tires are dry rotted but holding air. It had recent new belts, battery and blades when I got it.

Knowing I'll never have to worry about getting it stuck in the mud hole I shouldn't have driven through or how I'm gonna cut the lawn if the hydro ever gives up like they have a reputation for doing: maybe not priceless but certainly worth the less than 1/5 th the cost of buying a new one and still have those concerns.

Almost makes me wanna go get one of them stuck.
 

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There's only one thing better than having a duplicate tractor to back you up in the event of a failure: an identical one (so you can trade parts, model-specific attachments, and if nothing else, have another one to look at when fixing something that's broken).

The only thing better than that is getting the identical backup for cheap money.

Congrats on checking all the boxes!!!

Mike
 

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OK Bob, what (if anything) can be done about this, or is this just something we have to live with as hydros age?

Mike
Unfortunately, the options are to live with the leakage, or have the hydro seize from lack of lubrication.

Is there a 'pinhole' in the center of the left hand check valve piston? If so, it might allow for 'slow leakdown' or movement when in N. I suspect it and the bored out center might be there to 'cushion' the check valve closing or coming off its seat.
I have not thought it through, but moving from F to R or R to F might be a 'thumping' condition, or even movement from a drive range to N might cause the check valve to come closed quickly. The shape of the piston, the bored center, and perhaps pinhole might afford a cushion in closing, resulting in less clunky operation and longer life. The 'clunky' might be small to the operator, but high pressure stuff can be pretty hard on the parts, causing spikes in pressure, that may be un-noticeable to the operator.

An example, perhaps hokey, would be to pull the lever from full-boat forward into N. Fluid flowing will want to keep flowing, and a check valve slamming shut will cause a hydraulic shock. More or less. If Bob the Tudor knows more about that topic, I welcome correction and criticism. I am just a learner, not a pro.
tom
I'm not a pro either, but I find myself constantly challenged to learn a little more so that I can explain it to others.

You have a good grasp of what's going on. Where you slip up is in assuming that there are check valves in the main hydro circuit. There are a couple in the charge pump circuit where it connects to the hydro circuit, but they are only open to the low pressure intake side of the hydro pump.

Optionally, it may have been relief valves that you were thinking of, but relief valves only open when the pressure exceeds the spring compression holding them closed. They don't even necessarily slam open, but will pop only as much as is necessary to relieve the pressure in the system. Since fluid is non-compressible, in an enclosed volume it only takes a drop of fluid escaping to lower the pressure from 3000 psi to zero.

Slamming the drive control through F-N-R is only going to create pressure spikes in accordance with the traction available for the tires. An extremely well ballasted tractor on the proper traction surface may be able to drive the pressure high enough to pop the relief, but that is not going to be a regular occurrence. Your neck will object to the whiplash effect.
 

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Big and Wide
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Discussion Starter #120
Yard work season was over by the time I got this adventure wrapped up so both tractors just got a quick test drive then parked for the winter. A couple days ago I pulled them out for the annual oil change and lube job and first cut of the year. The axle I repaired is back in the original tractor and it's working just fine dragging the tow mowers around.

A couple weeks ago I started using the "donor" to clean up the woods and I was really impressed with how well this dirt cheap basket case was running. At least for the few short runs I gave it, then it started back firing and I had to keep the choke on about 1/2 way to keep it running normal. Fed it plugs and fuel / air filters on GP but no help. Tweaked the governor while doing the oil changes and it seems to be fixed. Gonna use it for my other than grass cutting yard work this summer and see how what goes wrong with it next.

With any luck it will only be the dry rotted back tires going flat so I can pull the trigger on replacing them with AG tires. Has been on the list and even more so since getting the thing almost completely stuck in the soggy spring ground. The recoil starter on the dirt cheap craigs score chipper has been a constant problem but I just fixed that too. Ok, not exactly "fixed". After multiple issues I yanked the whole thing from the clutch out and replaced it with an old pulley.
 

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