Sundstrand U15 hydro will only run in one direction.

[Jim1934]

Newbie. Have a new to me drive train from a 1650.
With the two main check valves in one location rear axles will turn but only in one direction.
Switch their positions and it won't run in either direction.
Internals of valves are free to move but don't know about o rings.
Any suggestions? Thanks, Jim.

[J-Mech]

Hello Jim and welcome to ECC!!

Couple question and I will try to give an answer. First off, you say this is the drivetrain from a 1650. Is it still in a 1650? Or did you transplant it into another tractor/machine?
Second question: When you say it will only move in one direction, I assume you mean while under power, (when being run by the engine)? I assume you don't mean just when trying to roll the machine.
Last question: Is the trans full of the correct oil at the correct level? How old is the filter?

So, the check valves in the Sundstrand units are just that. They are a check valve. Perhaps you already know, and if you do I don't mean to over explain, that the check valves are for recharging the hydrostatic system. The charge pump has to replace oil into the drive circuit that is naturally lost through pump leakage (both for lubrication purposes and just nominal bypass). It accomplishes this via the check valves, adding oil into the return side of the circuit. When in forward, the forward check valve is closed, but the charge pump can overcome the check on the reverse side and charge the system from the low pressure side of the drive circuit. It is just the opposite in reverse. So, it is possible that one or both of the check valves is not seating, thus why you get no movement when you swap them side for side. *Typically* when one check valve is stuck open, you lose only that circuit. So, if the forward check is stuck open, you lose forward motion and vise versa. When you swap them and get nothing, I am inclined to think you have another issue, but a bad check may still be the problem. There is no test port on the U15 to check drive pressure, so you can't do a quick test. The best and easiest way to check them is to swap them out with a known working check valve. If you have another tractor, you can simply swap the check valves out with the ones from another machine. If you don't, then you will have to do some more testing. The problem still could be improper or badly worn linkage, either on the dash lever, or at the spring cushions on the external swashplate. The issue could also be internal to the hydro. Some examples of internal damage that would cause this condition would be: A broken pin on the swashplate control shaft; a bad pump or motor; a bad charge pump. I can think of some other possibilities, but no need to insert them all.

Here is my suggestion: Make sure the hydro is full of oil. Good oil, not milky oil or oil of the improper type. Make sure the filter is good. Replace it if it looks old. Replace both the oil and filter if it hasn't been done, or appears to need it. Second, check the linkage to make sure it is all functioning properly. These two steps are kind of both necessary, and maybe you can swap the order, but they are for sure the top of the list. Last, swap out the valves with known good ones and see if that doesn't make it work.

If you don't mind, maybe reply to some of my questions, and let us know what you find.

Thanks!

[Jim1934]

Thanks very much Jonathan.

Some ancient history: I removed the hydro, diff.,rear axles as a package from a Cub with 1650 painted on hood.
Could be non original. Scrapped frame and all else.There are no linkages to flow control shaft. Just rotate the shaft.
I disassembled hydro and it looked very clean. Ditto diff. Cleaned both. Nothing bound. Replaced all seals in the hydro unit proper. Could see no water in oil, but added oil so it is now a mixture of, at least, two oils. Replaced filter.
One check valve was obviously frozen. Took it apart and significant corrosion had welded the long spring and actuator in place. Scrapped it. Bought Ebay used valve. Have no way of telling if internal O rings are good. Now using Ebay valve and one original valve.

I am driving gear pump at 1000 RPM by an electric drill motor. CCW as you face normal input shaft. I know 1000 RPM is slow.

Axles free to turn when not being powered by drill motor.
No wheels on unit.

When is oil at correct level? Now It is just above the top of the hyd. motor splined output shaft.

I plan to use this package to drive the carriage on a circular sawmill. Attach a chain sprocket in place of a normal tire. Drive with 17 hp Kohler gas engine.

Agree the next step is to find two good valves. Will pursue that.

Thanks again. Jim

[J-Mech]

Jim1934 Jim

That information helps a lot.

The one check valve you did not replace is likely bad, but still other possibilities.

Oil level should be at the rear fill plug hole on the diff cover. Not sure what you were describing as full.

You say axles rotate freely when not being driven. They should rotate in opposite direction. If one wheel hub is rotated forward, the opposite wheel hub should to tate backward due to differential action. If both axles will easily rotate the same direction, there is something wrong. If the rear was in a tractor and the engine was off, assuming you have automatic check valves, the tractor will roll but not easily. There will still be significant resistance.

Double check the charge pump installation. The Sundstrand U15 unit is designed to be able to be driven either clockwise, or counter clockwise. To change direction, the charge pump can be installed 180° from its other possible mounting. Run the drill in the opposite direction to see if that makes the unit work.

Also, if you had the unit completely drained and disassembled, it is not uncommon for the main piston pump to be air locked. If the wheels were on and it installed in a tractor, I would suggest slowly pushing the tractor in the direction you are operating the hydro lever all with the engine running. Doing so helps purge the air, and is not all that uncommon to need to do after a drain and refill of the hydro.

[Jim1934]

Jonathan,

Axles rotate in opposite directions when either one is rotated by hand.
Both rotate same direction under power.

Will check oil level. Somewhere I read the splined output gear, hence drain, should be under oil.

Your assertion that charge pump is bi directional opposes what I read in service manuals. But I will try if new valves are or are not not the fix. Changing rotation has advantages when connecting gas engine.

By the way, my hydro has aux. ports for a power lift. I have been advised to loop them with a hose and have done so. Lift of no value and scrapped.

In case any reader wonders, using this package instead of building a hydraulic power source and gear reduction from individual components has been a mistake.

One more thing- haver never seen oil leak out of the tops of the auto check valves.

Shipping both valves out for repair tomorrow. Will report on results.

Thanks, Jim.

[Jim1934]

Installed rebuilt valves. Won't run in either direction.
Oil at drain hole level. Spur gear nearly covered.

[J-Mech]

Installed rebuilt valves. Won't run in either direction.
Oil at drain hole level. Spur gear nearly covered.

Did you try running the drill in the opposite direction?

I still think it's air locked.

[Jim1934]

Yes, no change.
When I ran drill in reverse I could see air bubbles rise in the reservoir.
Removed valve furthest from flow control lever, ran normal ccw, and oil came up in the empty valve hole.

[J-Mech]

You need to rotate the motor while the pump is in stroke. This can be done easily if it was installed in a tractor. On a bench, lock one wheel hub from turning, then turn the other wheel hub while the pump is in stroke and the drill running. Probably easier with two people. Be careful! Once it primes, the hub you are rotating will begin to turn at twice normal speed due to differential action.

[Jim1934]

Thanks will try today. I also suspect air lock-Tried about everything else.
Didn't know procedure for eliminating air.

[Jim1934]

Been out of it for a week. Finally did what you suggested.
Locked one "wheel" to bench, powered input with drill motor at 1000RPM,
and struggled to turn the other wheel.Took at least 50 lb ft of torque
and could not sustain that for more than a few seconds. Tried both dir. of pump input.
Had to wedge a 2x4 between two lug nuts for leverage.
No indication that hyd motor was doing anything.
Thanks for trying. Makes me wonder if problem is in the gear train in banjo housing.

[J-Mech]

Well, at this point I'm going to have to assume you reassembled the unit incorrectly when you took it apart. Not many other possibilities. I don't advocate people taking hydro's apart. Pretty complex components. You either need to disassemble it again, or hop on ebay and spend $100 on a used one. Sorry, don't mean to be rude, only stating what I see at this point. I know these units pretty well. I'm thinking you got something in the main motor or pump together wrong.

[Jim1934]

Thanks for responding. It's possible I erred. Short term memory waining.
Don't agree they are complex. Not much to them.
Losing interest. Do not have the strength to wrestle them.
All the best. Jim.

[J-Mech]

Don't agree they are complex. Not much to them.

And yet here you are.......
Should be easy to find and fix the problem then. No reason to give up on it.

[Jim1934]

Beginning to wonder about the open path (hose) between aux.port pressure- return fittings for lift.
Is that flow stolen from motor output? If I decide to work on it I will try capping both fittings.

[J-Mech]

Beginning to wonder about the open path (hose) between aux.port pressure- return fittings for lift.
Is that flow stolen from motor output? If I decide to work on it I will try capping both fittings.

No. Separate circuit. Can't rob from the supply. It is no different with a bypass hose than it is with a valve in the line. Open center circuit.

[Jim1934]

Have not given up. Please tell me the correct orientation of the charge pump.
The casting has a flat on one end. Does that flat point to swash plate actuator or the other side.
Want to eliminate this variable as output won't turn regardless of casting orientation. Manual says "right side" which is meaningless"
Thanks, Jim