TN70a loss of high pressure hydraulics

[horse7]

Was out moving dirt-- hard digging into old dirt pile, which causes the overload valve to bypass at times.

After moving a few buckets of dirt with no issues, I was driving back to the pile when I got a phone call, turned off the tractor (bucket and implements off the ground), then restarted and proceeded to the dirt pile and found out that the high pressure hydraulics were out (no bucket (except to drop the bucket), no 3pt raise, no auxiliary... unfortunately, I dropped the bucket while fiddling around at first). No unusual sounds or anything... just high pressure stopped working. Mechanical connections to spool valves seem to operate normally.

Low pressure hydraulics ok (steering and EHSS fine), and fluid levels look ok (I did not check the engine oil level, did not think to do that); no error codes on console; no sound of the overload valve bypassing. Manual implies either failed high pressure pump or failed bypass.

Any other possibilities on a TN70, and is there a reasonably simple way to differentiate the overload valve vs. the high pressure pump. Otherwise its another dealer repair... time is pretty tight for the next few months.

Thanks.

 

[RickB]

We have seen a couple pump drive failures that would be sudden and complete, as your symptoms seem to indicate. There is a test port plug directly above the high pressure pump on the outlet tube connection flange. You might try removing this plug and cranking the engine (without starting it) to see if you have ANY flow whatsoever.

 

[horse7]

Looked at the tractor again today-- oil level was OK so there isn't bypass into the oil, the hydraulic filter is on tight.

Started tractor, and the high pressure was working! Everything worked normally (except that I don't have as much confidence working on some things, since getting stranded in some spots is problematic).

So, my guess is that the (expensive) high pressure pump is working, and the problem is somewhere else.

Is there a thermal bypass for the high pressure?
The only somewhat unusual thing I did was shut the tractor off w/o the usual cool down (only 15S vs. 3min) and idly pressed the joystick buttons (which should not be connected to anything, since I don't have forward hydraulic auxs).

 

[RickB]

The only system protection is the pressure relief valve located in the hydraulic filter housing,. The PRV works on pressure only, not temperature. Not sure what might have happened, let's hope it doesn't recurr.

 

[horse7]

Talked to the repair manager at the NH dealer, and the system is pretty simple-- no thermal bypass, only a high pressure pump gear driven off the engine (if the engine is running, so is the pump, unless something is mechanically broken) and the overload valve.

So one candidate is a stuck full open overload valve, which apparently is pretty much soundless. I don't see how the valve can get into this state just from turning off the tractor-- bucket off ground, box blade off ground, but no active use of high pressure hyd.

Another possibility is an air leak on the suction side of the HPPump. The original symptoms were: tractor running, fully warmed up, with fair amount of load, then shut off (less than usual cool down), then restarted. If sufficient air leaks in/oil leaks out through a hot joint/O-ring in the 5-10 min the tractor is off, the pump might not acquire prime especially if there is a leak already present (just not enough to cause a problem normally). Overnight, the hydraulic oil can leak down and the pump primes again, or the leak closes up when cold and after starting up, the leak is once again tolerable when hot.

No leaks were noted during normal operation, although there are small hydraulic leaks present after the tractor sits for some number of days (stains on the ground) although the leakage is pretty small.

With a working hypothesis, I guess I wait for the problem to re-occur, or get worse and expose the root cause, or never happen again and leave us all wondering what actually what the problem is/was.

 

[RickB]

Gotta say, I'm not surprised. Make sure you don't have a remote valve sticking in a stroked position if this happens again. That's not completely consistent with your symptoms, but is partially so. 'A' series TN's use a nearly completely common suction circuit, so the chances of a major suction leak NOT affecting the low pressure pump is fairly small. Fact of the matter is, my experience would point to a major suction problem affecting the lower capacity stering pump first.