Why Charge Pressure?

[marrt]

Most of you probably know this...but I didn't....

I've noticed PT talking about measuring charge pump pressure to get an indication of the "health" of the variable displacement pump (VDP). As most know, a charge pump provides "make up oil" to VDP to avoid cavitation in a closed loop system. So, why does charge pump pressure give an indication of the VDP's health? Because, if the pressure is below a certain spec, for your specific pump, then oil is bypassing in the pump due to wear. Just like low oil pressure in a vehicle can suggest worn main bearings. Duh!

That's so obvious but I never made the connection...I'm embarrassed to admit.

 

[MossRoad]

Makes sense.

 

[ponytug]

I'll bite.

How do you measure the pressure, and where, and under what conditions? e.g. cold, warm, hot oil?

Thanks for point it out.

All the best,

Peter

 

[marrt]

I'll bite.

How do you measure the pressure, and where, and under what conditions? e.g. cold, warm, hot oil?

Thanks for point it out.

All the best,

Peter

I can tell you generally but you'd have to call Terry for instructions on a specific pump (or, read your pump manual if you have one). On some tractors you can connect the gauge to the same location you connect the hose when bleeding air after a hydraulic filter change. This is the charge pump that's pushing oil when you're turning the engine over to remove air. Some pumps also have dedicated ports to take a reading...just remove a plug and connect a gauge. Usually, these readings are taken when the tractor is at operating temperature. What should the reading be? Again, varies by pump. All VDP's should have a spec for charge pressure. It's usually a few hundred PSI.