742 low charge pressure

[mike_belben]

Ive got a 742 that sat for a long time without oil awaiting an engine swap. i had no prior history with machine. Put another engine in, new fluids and filter, runs good. Initially had zero drive but loader was great. Charge pressure was zero. Hidden bronze filter was perfectly clean and new, but the cold bypass valve seat was all rusty so i lapped it with compound, cleaned out real good, refilled and got charge pressure pressure to come up to 17-20 psi and machine can be driven. Whiney noisy chattery drive when turning but pretty tolerable in straight line. I then pulled charge relief valve which was also all gummed up. cleaned, reinstalled and expected huge improvement but got exactly the same result. Loader still works great, fast and smooth and quiet but it seems to hit relief pressure pretty easy and yet the motor doesnt lug and i dont hear a typical bypass whine as i expect. Im kinda stunned that the primary charge relief was that bad looking and that cleaning it up to nice again didnt make charge pressure jump to any more than 20psi. The loader will lift the bucket right up good and fast, and i can spin all 4 tires in dirt against a tree without much noise. I highly doubt the gear pump is bad but i cant think of any other relief valves in there that woukd cause this. Basically my cold bypass valve is providing the entire charge pressure, and the charge relief is apparently adding none. Anyone got a picture of the spring that is supposed to go behind the charge relief? I also pulled the replenishing valves on passenger side only because easy access, looked okay to me. The loader spool valve has a small leak, could that be related? Your thoughts?

 

[Tazza]

I was thinking your pumps may be worn causing low charge pressure, but ifyou are spinning the wheels, it's doing ok.
The charge pressure is controlled by a spring and poppet, they also useshims to increase the pressure to get the 100 od PSI that you want. Can you add a small washer to the charge relief and see if it brings your pressure up?
As for it hitting relief pressure quickly, hook up a gauge to your front aux coupler and see what pressure you get, it should be around 2,400 PSI. The relief on the control block should have a screw to increase the pressure till you get there.

 

[mike_belben]

I was thinking your pumps may be worn causing low charge pressure, but ifyou are spinning the wheels, it's doing ok.
The charge pressure is controlled by a spring and poppet, they also useshims to increase the pressure to get the 100 od PSI that you want. Can you add a small washer to the charge relief and see if it brings your pressure up?
As for it hitting relief pressure quickly, hook up a gauge to your front aux coupler and see what pressure you get, it should be around 2,400 PSI. The relief on the control block should have a screw to increase the pressure till you get there.

im thinking the charge relief spring might not be the right one. It was fairly soft and longer than i expected. Had to compress it pretty hard to catch first thread and i dont think id be capable of getting a shim in there, was barely able to install the spring alone. I know some has monkey'd around in there because my cold bypass spring has actually been clipped of a coil or two.. Probably another incorrect spring. Actually, i should verify.. Charge relief is in the hydrostatic pump body up above the high pressure valves right behind the control bars right? and cold bypass is behind the forwardmost hose in the aluminum block, correct? im taking pressure reading from the pressure switch port on the metal pipe tee just before it enters the pump Any idea what gpm the charge pump should put out? I think my flow gauge goes to 10 gpm

 

[Tazza]

im thinking the charge relief spring might not be the right one. It was fairly soft and longer than i expected. Had to compress it pretty hard to catch first thread and i dont think id be capable of getting a shim in there, was barely able to install the spring alone. I know some has monkey'd around in there because my cold bypass spring has actually been clipped of a coil or two.. Probably another incorrect spring. Actually, i should verify.. Charge relief is in the hydrostatic pump body up above the high pressure valves right behind the control bars right? and cold bypass is behind the forwardmost hose in the aluminum block, correct? im taking pressure reading from the pressure switch port on the metal pipe tee just before it enters the pump Any idea what gpm the charge pump should put out? I think my flow gauge goes to 10 gpm

I can't remember where the charge pressure is controlled in that pump, i thought it was the aluminium block, but i may be wrong.
The charge pressure is gegerated by the oil from the main pump going back to the tank. The vane pump is a 10 or 12GPM pump i believe.