Tractor Repair: Case MX120 - 4wd not disengaging, damage occured, prop shaft

Question
Hi Arnie,

We currently have a problem with our Case Mx120 tractor, 2000 year.

Recently when driving on the road the tractor broke it's 4wd prop shaft, resulting in the tractor having being rescued.
When we got it back to our yard, we found that only the prop shaft and the first part of the shaft in the drop box housing - that joins prop shaft to clutch pack -  had been damaged.
We replaced the prop shaft, 2 bearings in drop box & oil seal - also the shaft coupling that inserts into the clutch pack was replaced.
The driver of the tractor insisted that the tractor had not been in 4wd at the time the damage occured, but that it had been at maximum speed (40km/h) going down a slight incline (150m), the shaft broke imediately when he began to brake the tractor.

As a result of the above we were concerned about putting the tractor out on the road again without investigating the problem further.

We recently had snow, and had the opertunity to easily test the 4wd system easily - what we found is that when the tractor is first started from cold, the 4wd imediately disengages, and stays off for about 5-10mins.
After 5-10mins the 4wd comes on by itself, and will not disengage - the switch, lights etc show it has disengaged - but by playing round on the snow we found that it isnt actually disengaging even when full pull is put on the 4wd.

We have checked the solonoid and there is a current coming to it, which switches correctly - if the back nut of the solonoid is removed, and the switch in the cab is switched, a second person can feel the solonoid moving in tune with the switch.

We have also switched the 4wd solonoid with the solonoid behind it (which is diff lock) - we swapped the entire solonoid (mechanical body, electrical coil & cabling) - and operated that solonoid by switching the diff lock switch in the cab - again 4wd did not disengage.

We are not sure what the problem now could be and would be grateful to have your suggestions on how we might further diagnose the problem?

Kind Regards,
Darren  

Answer
Hello,

  The 4WD clutch is spring applied and released by hydraulic pressure.  When 4WD is turned on, the switch is actually turning off power to the solenoid, and the springs in the 4WD clutch squeeze the clutch plates together to apply the clutch.  When 4WD is turned off, the switch turns on power to the solenoid so that hydraulic oil pushes a piston that compresses the springs to release the clutch.  If both brake pedals are pushed together or when the parking brake is applied, the 4WD automatically turns on to give you 4 wheel braking.  You have already verified that the solenoid works when the switch is turned on and off, so it sounds like you don't have an electrical problem.  It is most likely a hydraulic problem.  If it turns off when you first start the tractor, but then comes on by itself after awhile, then there is probably a hydraulic leak in the 4WD clutch that gets worse as the oil gets hot.  Put a gauge in the test port near the solenoid and turn the 4WD off to check the pressure.  Check it when the oil is cold, and then when it is hot.  Turn all the regulated pressure functions on and off and watch the gauge.  Diff lock, pto, brakes, shift powershift speeds, shift forward and reverse.  A leak in any of these functions will show a pressure drop.  The pressure should be between 300 and 350 psi.  If the 4WD clutch needs repair, the tractor will have to be split between the transmission and rear frame.