My John Deere 4620 loses its hyds when i hold down the clutch

my john deere 4620 loses its hyds Problems

My 4620 jd loses its hydraulics when I press the clutch. I know if it lasts too long it is the nature of the beast. but it has reached the point that in a matter of seconds they are gone.

Its main hydraulic pump is driven from the front of the engine. But it’s getting its oil from the transmission pump that would stop spinning if the clutch is released.

The hydraulic pump should be able to “suck in” some oil on its own. Check the hydraulic filters, they may be clogged. Or the main pump is weakening.

I’m pretty sure the drive pump, which powers the main hydraulic pump, stops rotating when the clutch pedal is depressed. So if you are losing hydraulics after stepping on the pedal, the main hydraulic pump has to suck up its own oil.

The foreline pump cannot suck a drop of oil. It has to be charged. Once the entire return is underway, it is captured and sent back to the front. When the pedal is depressed, the cooler oil drains into the foreline pump and takes the place of the transmission pump oil that does not arrive now. The transmission pump may not have the volume it used to have, so there is never enough oil in the front. Leak in charging system. Drive pump filter relief valves. High pressure leak to crankcase.

The JD system is a bit complicated, but here is the summary. The foreline pump cannot suck a drop of oil and has twice the volume of the drive pump that feeds it. so to make it work they “capture” all the return oil and send it back to the front. All valves maintain pressure (center closed) and the front piston pump maintains pressure at 2200 psi all the time, but if no oil is being used there is no flow so the pump shuts off and just maintains pressure. When this happens, the oil that the transmission pump is sending goes through the piston pump and through the cooler and back into the crankcase. The benefit of this is that the large pump only pumps oil when you really need it. The problems with this is, if the valves do not maintain the oil pressure (2200) they leak and cause the pump to send oil all the time.

So I think what is happening is that the pump is using all the oil that is coming up ahead and nothing is actually going to the refrigerator. It is making a circle.

Here are the main things to check:

  1. scv valve leak
  2. steering valve leak
  3. rockshaft leak
  4. pressure control valve leak The little line on the back of this valve runs through the steering, so don’t blame the steering valve unless you are sure this valve is not losing pressure. Normally if this valve breaks the rock shaft and the scvs will not work quickly.

Since each valve maintains 2200 psi in neutral there should be no flow through these valves. A high pressure leak will cause heat. You can usually feel the heat from the return line. when you first turn it on you can keep moving from valve to valve and holding the return to see which valve is leaking.

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