My 973, ser. no 90L loses forward drive when it pushes into a load, and had steering problems. After changing port plates in transmission and seals, hoses, etc the unit steers much better but still stops under load. A transmission mechanic Mr. T has been been trying his best where others gave up. Now with the transmission cover off, tests have gotten us here; 1) Right motor Case pressure is at 10 psi in forward. we are to pull this to examine. 2)Underspeed valve only works in forward when oil is cold, giving test pressure 0f 200psi in reverse & 398 in forward. 3) Charge pump pressure is 400 psi, to adjust down. 4) Servo pressure is 255psi. can adjust 5) Drive pressures - right & left in reverse is 5600 psi, in forward the right is 2200 psi, while the left is intermittent from 0 to 5200 regardless of selector lever position The venturri upsteam gives 410 psi while throat pressure is at 150 high idle & 250 low idle. We can see the left servo hoses flexing as pressure build and drops while in forward. The unit always drove well in reverse. Now Mr. T and I are stuck. Really need some help here. It is frustrating considering the really good condition of everything else.
Drawing on experience I would say the drive motors are leaking excessively inside. Forward leaks the most as it does the hard work. They have swivel joints inside the trunions and the seals leak. The best way to check for LOOP LEAK is to have a small (50GPM) inline flow meter in the cooler line. Cooler flow is around 30 gpm at high idle and drops as you load the drives. You can compare cooler flows as you load the drive in forward and reverse and as you load each side independently. Find the RPM as you load the entire system then throttle down to that speed as you only load one side to get meaning full readings. This will give you total system loss and side to side loss. To determing if the loss is in the pump or motor, disconnect the drive hoses and block at the HPCU and retest. Then subtract the first readings from the new readings and you have the motor loss. Now for the reason you have no drive, the cooler flow is charge pump flow that goes through the venture to the servo and charge valve. Charge pressure holds the system in drive and if it falls below 125 psi it will vent out and you need to reset(go to park then back to drive) to move again. Loop loss is charge oil loss. Later Bob
Thanks Bob. Mr T will try your test. Quite likely to look at taking out each travel motor, right first and replace with a rebuilt unit. Thanks
Yes I think so, case drain pressure under load should be only 2 psi with fill cap loose. Later Bob