What is the purpose of having a Hydraulic Oil Filter and a Transmission Oil Filter?

[rScotty]

Hydraulic filter is a suction filter protecting basic hydraulic system and general transmission. Hydro filter is a pressure filter, able to filter out much finer particles that can damage the precision high pressure piston pump and motor of the hydro. Two completely different cleanliness levels required by the gear pumps and gerotor of the hydro charge, versus pristine cleanliness of high pressure piston pump and motor. Notice how much heavier built and tighter torque for the pressure filter of the hydro vs suction filter of the hydraulic system.

That's a nice compact explanation from MHarryE. I'll add my my own way of looking at it and hope it helps as much as his does.

The main filter is a suction filter that filters all the trans/hydraulic oil for the whole system. After that filter, some of the oil goes to the main pump/steering pump combo where fluid is pressurized and sent onwards to be used everywhere for everything in the hydraulic system except the HST. Oil to be used for the HST is also drawn through that same main suction filter, but then goes into the HST charge pump and from there under charge pump pressure to an even finer HST filter before continuing on into the workings of the HST.

So one filter is purely suction and fairly coarse, while the other one is much finer & uses HST charge pump pressure to super clean the HST oil. That HST filter is built a bit heavier to handle the pressure.

Regardless of where it is used, all of the oil ends up back in the common sump.

Some of the confusion about the two filters is the fault of the Service Manual where the initial hydraulic schematic and the text for that schematic don't match up, making their attempted explanation of charge pump oil flow confusing.

But as you read farther into the transmission section the subsequent artist's flow drawings are correct - and easier to follow than the hydraulic schematic anyway.
enjoy,
rScotty




rScotty