tc40d low hydrstatic trans pressure

[jayhawk rv]

I have a TC40D that the Trans Pivot (87300017) broke the rod connecting at the transmission. After having to disassemble the fenders and floor deck to get to the connection, I decided to test the High pressure relief valve. Only have 450 psi in both forward and reverse. I also noticed that when I was disconnecting the gauge and fitting that the fluid was foamy. Before the test I changed hydro fluid and both filters. The repair manual says that the pressure should be 5000 psi, that is a big difference and the wheels still move both directions. Can anyone tell me how to test the suction side of hydro system for air leakage? Does anyone have any ideas? The high pressure relief valves are $300 a piece, and since both are about the same 450 psi I'm wondering if if could be something else, o-ring, pump, charge pump, etc. Cleaned suction screen when changed fluid and filters. I would really appreciate any guidance in this matter ! Thanks
Clayton Carr 620 496 8888 clayton.carr@yahoo.com

 

[dynasim]

Others may be able to help more, but I will give it a go.

The hydrostatic transmission has 2 pumps. The first, a charge pump, pressurizes the pump casing, and ensures lubrication of the pistons, valves, and sliding parts. This pump runs at a modest pressure(400 psig sounds right). This pressure is controlled by a low pressure relief valve.

The second pump/motor combination unit drives the wheels. The pressure on this pump is directly related to how much torque you are putting on the wheels.


If the tractor isn't doing any work, both the sides of the pump/motor combination will be at the charge pump relief valve pressure, and no higher.

The only way you will see the 5000 psi is if you do something to make the wheels not turn, and apply the pedal. This is not something done without care, particularly if you have to have the seat off to get to the port.

In essence, if you don't see 5000 psi, it is still likely nothing is wrong.

Chris

Be careful when messing with this sort of thing. There are lots of ways to get badly broken.

 

[jayhawk rv]

So can I set the park brake and turn direction control ? Have wheels off,tractor on stands.

 

[jinman]

First, let me say that Chris (Dynasim) has given you excellent advice. You cannot develop 5000 psi unless you set the parking brake for both wheels. That is how New Holland recommends doing the test. Put the transmission in Range II and Hi (Rabbit) position. This will allow the transmission pressure to build with much less torque to the rear wheels and less stress on the brakes.

I am very familiar with your problem regarding the Trans Pivot. I posted this thread way back in March of 2002 when my pivot broke. A picture and illustration are below.

 

[jinman]

I made a separate post to address your question about foaming, the charge pump pressure, and suction. Below is a piping diagram that shows the flow.

Suction from the main hydraulic pump and the HST charge pump are both applied to the same supply line. If you look just below callout #3 in this diagram, you will see the main hydraulic filter and a pipe going to a flange that attaches to the side of the transmission. The main filter is a suction filter. If it is not tight, you can draw air in around its gasket. As the fluid is drawn up the yellow-colored pipe, it comes to a "Y". That "Y" can easily be seen if you look under your tractor. it is a black plastic assembly with hose clamps holding it in place. This "Y" boot can be a source of suction leaking and air drawn into the line if it is not tight. Check it for leaks, damage, and tightness.

From the "Y" adapter, follow the yellow line up and into the snout of the HST transmission. (There's no need to follow the line going to the main hydraulic pump.) Where the yellow pipe goes into the transmission, there is a charge pump. I have added labels to the 2nd illustration below. You can clearly see where the charge pump is located.

The outlet of the charge pump can be as high as 200+ PSI. It sends fluid out the pink-colored pipe to the HST cooler. Notice that this pink pipe attaches to a fitting on top of the HST filter. That is a ball-check pressure relief valve that will bypass the HST cooler if the fluid is very cold and too much pressure is in the line. Normally all fluid goes to the cooler, but is some instances, this bypass valve may open. When it does, some people hear a clicking sound and think there is something wrong. This is completely normal.

When the fluid leaves the HST cooler (blue pipe) it goes to the HST filter under pressure and then is supplied at low pressure into the main HST pump. The charge pump's function is to draw fluid from the reservoir, send it to the cooler, and supply it to the HST transmission pump in sufficient quantities to keep the main pump from cavitating.

The main pump then varies pressure and flow depending on the swashplate angle. Because your HST pedal pivot plate is broken you cannot vary the swashplate angle.

Also, your Rabbit/Turtle switch peforms a unique function for our transmissions. It is a pressure actuated change in the HST motor swashplate angle. Very few HSTs allow changing of the motor's swasplate like ours does. When you press the Rabbit or Turtle button, you energize a solenoid that opens a valve. Pressure from the charge pump is used to also change the motor swashplate angle. The rabbit position of the solenoid sends 175 psi to the motor's swashplate. The turtle position sends zero psi.

So, by using the diagrams below, you should be able to troubleshoot your piping and find if you have any leaks. My guess is the frothing of the fluid you saw may be normal. You will have some idea when you can better test your internal pressure maximum and see if it is 5000 psi before the relief valves lift.

Good luck with your repair. Believe me, I KNOW just how big a pain this can be.

 

[dynasim]

Jayhawk RV,

I just want to reiterate, be careful. Depending on your model(I have the supersteer version with sensitrack), if the front wheels are on the ground, and your brakes slip while doing this test, it'll hurt.

Good luck.

Chris

P.S. I should have referenced Jinman's links in my response. He was the one who taught me about this(and many other things)!

 

[jayhawk rv]

Thanks for all the help,have 5000 plus on reveres side and 4600 psi on forward,will try to down load pic. Clayton

 

[jinman]

Glad you got your tests done. I too would caution you to be careful around 5000 psi pressures, but it seems you are very competent to do these tests. I would only suggest that since your reverse pressure relief is over 5000 and the forward is just 4600, you might swap them so the highest pressure is in the forward direction where you are most likely to need it. I'm guessing that the 4600 psi is because the forward relief has lifted so many times, it has weakened some. While you have your platform lifted will be the easiest time to make a swap if you want to tackle that job too.