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Sundstrand 15 U type tranny leak...

1905 Views 41 Replies 6 Participants Last post by  MrBorland, 
M
I bought a non-running John Deere 300 sometime ago and just finished rebuilding the engine. The engine runs fine now, so I hooked up the driveshaft, started it up, and immediately saw tranny fluid is leaking pretty heavily out of the the upper fitting of the suction tube (next to the filter), and I'm looking into why it could be leaking so bad. If anyone's got a guess, please pass it on. FWIW, the check valve adjuster is backed all the way out, but the tractor doesn't move when the lever is moved forward or reverse.

The manual indicates this is a 7/8-14 "O-ring" type of fitting - does this indicate there should be a separate O-ring on the fitting? The fitting's plenty tight, but a missing O-ring could certainly cause a leak.

If I'm reading the circuit diagram right, should be part of the suction flow to the charge pump. If the tractor doesn't move when the shifter is moved, and the suction tube leaks, could the charge pump be creating back pressure?

I haven't yet removed the fender deck to look at the check valves. They were rebuilt, but I suppose the plungers could be stuck in the down position, which would explain why the tractor doesn't move. Would this also explain the leak? Does running the engine/tranny with the check valve plungers depressed create an over-pressure situation which would cause a leak?

If any of you Sundstand experts have any ideas about where to start looking, let me know...
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Quick and dirty diagnosis for charge pump issues of a Sundstrand series 15 hydro. If the hydraulic implement lifts do not work, the charge pump drive pin is broken, Replace the drive pin.

With the pin broken, the charge pump cannot pump fluid. Fluid can only move to the outside if driven by pressure. The supply side components (tube from the reservoir to the transmission and the filter0 are under negative pressure (vacuum) and unlikely to result in a fluid leak unless an O-ring is missing or a fitting is cracked. An air leak to the interior is much more likely.
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Another name for the check valves is cross-port relief valves. When one is closed, the other is open to the low pressure side of the hydro. The above diagram shows both as being open in an active system, which isn't going to normally happen.

Pumps are designed to flow fluid against pressure. To get fluid flow backwards through through the pump means that the pump is broken and there is a secondary method of moving fluid. Possible, but not likely.
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When both check valves are depressed, the fluid follows then path of least resistance back to the reservoir. There is always internal leakage and there is only 0.913 cu-in of fluid in the motor.

Drop the driveshaft for a look see.
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Have you corrected the leak mentioned at the start of the thread? Does the transmission work?
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Check the shaft on the side of the transmission that controls direction and speed. There is a similar shaft on the other side that must move in harmony. It may only appear as a stub shaft that is flush with the housing, but nevertheless, it should still turn.

Failure to turn indicates that the spring pins locking the shafts together have worked their way out. This is a low cost but high labour time repair since the hydro has to be disassembled. However, this is separate from the implement lift since it only affects the hydro.

Back to the charge pump. The guts of the charge pump cannot be installed wrong. However, if the charge pump cover is installed wrong, the pump parts will conform to the cover and neither the implement lift nor the hydro will function.
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Pressures should be zero in the hydro until such time as the charge pump and implement lift system is working. The charge pump ensures that the hydro has fluid to work with.
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If the implement hydraulics are working, then you might have a reason to to do pressure checks on he hydro.

Charge pump is first on the diagnostic list for a reason. Follow the path of the fluid, reservoir, filter, charge pump charge relief valves, hydro charge relief valves, and last is hydro.
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Any hydraulic pump that is starved for fluid will freely spin. It will not cause the engine to bog. It will only bog if the pump is working against pressure that it can get relief from.
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