Re: 656 hydro question

Posted by Owen Aaland on July 30, 2009 at 23:56:34 from (65.174.247.87):

In Reply to: 656 hydro question posted by theponyroper on July 28, 2009 at 20:14:09:

If the tractor just slips and does not stop moving the high pressure relief valves are most likely OK. A sticking valve will cause the tractor to stop and will not start moving again until the valve resets. Sometimes they will reset themselves but it usually happens when you start to remove them.

These are pilot operated valves and are controlled by the Foot-N-Inch pedal. They are a spool type valve with a tiny hole in the middle. There is light spring to keep them closed. You can open them by hand by pushing on the spool. They need to slide smoothly enough that the spring can push them back closed. The hole in the middle of the valve allows oil to flow through the spool so that the pressure is the same on both side of the spool. Since the pressure is the same, the spring is enough to keep it closed. Pressing down on the Foot-N-Inch pedal allows oil to flow out the end of the relief valve. When the oil flowing out is more than the can flow though the small hole in the spool, the pressure will drop off and the higher pressure on the hydro side of the valve will cause the spool to open dumping oil from the pump side if the hydro circuit. This is what happens each time you use the Foot-N-Inch pedal like a clutch. A sticking valve will cause the tractor not to move again with the Foot-N-Inch pedal released.

With the lines capped off the relief valves can not open unless there is a leak between the valve and the fittings where you have them capped off. Sometimes you will find a crack in one of the jumper lines that go between the valve and the center housing. The line will crack by the flare, under the nut, where it attaches to the fitting on the valve or housing making it hard to find.

The check valves are used to allow oil from the charge pump to enter the closed loop of the hydro system. Each time a relief valve is opened oil is dumped from the loop. Pressure from the charge pump forces the check valve open to allow oil to enter to keep the loop circuit full. There is also some loss of oil between the brass and steel plate on both the pump and motor side.

If the pump or motor plates get scratches or grooves in them they will allow oil to enter between and separate them under load. When this happens you usually have to drop the pressure in the closed loop by either bringing the speed lever to neutral or using the Foot-N-Inch pedal. The tractor will then pull again until you reach the pressure point where the plates lift again. This, or a cracked line, is usually the reason for failure when when the problem only occurs under heavy load.

Replies:

• Re: 656 hydro question theponyroper 06:01:24 07/31/09 (1)
  • Re: 656 hydro question Owen Aaland 15:29:23 07/31/09 (0)

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