Hydraulic Pumps and Oil Cooler

   / Hydraulic Pumps and Oil Cooler #1  


Platinum Member
Jan 18, 2004
east TN
Power-trac 422 2003 model and 428 January 2015 model
As to the best of my knowledge and investigation the only time that hydraulic oil passes through the oil cooler is when the tram motors are running. If someone knows different please let us know how it does or even could. It may be that some of the PTs are set up different but my is a 2003 PT422.

Ok the large motor or tram motor is in the front (of my 422). It is a piston pump that has a swash plate. The plate tilts one way to go forward the other to reverse. When the plate is not tilted the pistons are not displacing any fluid so you do not need any relief valves or dump valves in this circuit. You are not moving any fluid so none goes through the cooler.

Now with this said when you start up a hill and you try to go forward pushing down on the peddle and the PT is not going forward what is happing is that you are having fluid bypassing (leaking back through) the tram pump pistons. This is normal and can get worse as the pump wares. If you push the peddle further down and ask the pump to displace more fluid then the pump over comes the leakage and the PT goes on up the hill or the motor dies. I guess another thing that could happen is the tires could spin but we all know that doesn't happen

Another circuit on the PT is the (I will call it the cylinder circuit). This is ran by the smallest gear pump and it drives every one of the cylinders on the PT. I am not sure that it is by passing through the by pass valve all the time when cylinders are not being used or not. I think it is?

The third circuit is the PTO circuit. It flows at all times. If you turn the PTO switch on, a valve is closed and the only escape for the flow is out the PTO circuit to a mower (or what ever) or through it's by pass valve.

I hope this information is correct and will be useful to someone. I know I would like to have know this when I first started PTing. Thanks guys and keep up the good posts. I don't post as often as I once did but rest assured I will be reading each and every post posted.
   / Hydraulic Pumps and Oil Cooler #2  
The tram pump has a charge pump at about 4 gpm on the gas units. This pump is a fixed displacement at about 90 psi on the older units and 300 psi on the units with hydraulic displacement controls. Oil not used to charge the piston pump flushes the heat out of the case of the piston pump thru the oil cooler. This is the hotest oil in the circuit. The newer units have a lower pressure shaft seal thus requireing another part of the circuit to be used. PT then returns oil from the lift valve which is again 3 to 4 gpm constant flow. On that size machine those are the best choices.
   / Hydraulic Pumps and Oil Cooler #3  
Another important point to note is the location of the hydraulic filter. It is in the suction (input) line to the charge pump. That means that it is possible for the other circuits to suck in dirt should the hydraulic reservoir get contaminated. In other words, the hydraulic fluid is cleaned only when it passes through the drive circuit.

Eventually, all the fluid will go through the filter, but it can take some time before that happens.

I hope this makes sense. It is late, and I am tired.
   / Hydraulic Pumps and Oil Cooler
  • Thread Starter
Thanks Blackwell. That info on the charge pump explains a lot. Boy I would sure like to see a diagram or a cutout of the tram pump. Maybe someone knows a link to one like it.
   / Hydraulic Pumps and Oil Cooler
  • Thread Starter
Yes, I see where it will eventually circulate the entire contents of the reservoir. Thanks snowridge
   / Hydraulic Pumps and Oil Cooler #6  
The tram motor pump is a variable volum pump. I beleive it has a swash plate in it as you mentioned . The swash plate determines how much volum the pump puts out by the amount of tilt in the swash plate, the more the tilt the more the volum. When the pump is not on stroke it probably puts out some oil. I beleive that the tram valve determines the direction of the oil flow to the tram motor. The pump that supplies the oil to the cylinders is a constant volum pump, when the machine is running and the joy stick or the quick attach lever is in the neutral position these valves are open center valves which means that the oil pumps up to the valve goes through the valve and back to the tank without any resistance therefore the bypass valve is not bypassing. When you pull the joy stick it causes the implement to move. If the implement is weighted to where it would take more pressure to lift it than the releif valve is set for then the releif valve would bypass or it would stall the motor one or the other. That is kinda how I beleive it works, I guess I am living a charmed life because I haven't had to do any thing to my machine since I bought it other than a bent pivot pin that pt replaced and the normal matenance.