sudden hydraulic loss

michalakd · Sep 19, 2009

I have a NH1520 that I high centered on a felled tree. I used a tigercat (feller buncher) to pull it off the log with a tow strap tied to the upper mount of the box blade on the tractor. Afterwards, I had no hydraulics to the front, back, or steering. Would pulling the tractor in this fashion blow something out? PRV? It didn't seem like a big deal at the time to pull it off the log.

J_J · Sep 19, 2009

michalakd said:
I have a NH1520 that I high centered on a felled tree. I used a tigercat (feller buncher) to pull it off the log with a tow strap tied to the upper mount of the box blade on the tractor. Afterwards, I had no hydraulics to the front, back, or steering. Would pulling the tractor in this fashion blow something out? PRV? It didn't seem like a big deal at the time to pull it off the log.

There is a problem with pushing/towing any machine with hydraulic drive. There is usually a tow valve that should be turned to allow hyd fluid to go from one side of the pump to the other. Even turning the tow valve will only allow you to tow slow and not very far. If you only grab the tractor and pull it even for a short distance, things will break.

What happens here is, you are forcing the wheels to turn a shaft which is connected to a hydrostatic pump/motor combination, and is making things work in reverse. The hyd motor in the transmission now becomes a pump, and the fluid has to go somewhere. Perhaps if you held the clutch in if it had one, maybe.

An analogy would be the automatic transmission in a car, even in neutral, some of them will be turning a pump, and the tow guys are not supposed to tow over 45 mph. Very few do tow at that speed, more like 70 mph.

As to what damage, is anyones guess.

kennyd · Sep 19, 2009

I am thinking you ripped off or severely damaged the hydraulic filter under the belly of the machine, or possible the main return/sump line the goes to the pump.

Lot's of stuff under there to damage while your stuck on a log!

jinman · Sep 19, 2009

I agree with kennyD, some hydraulic line or filter is loose or loosened. If the hydraulic filter or pump's suction line has been compromised, it's probably drawing air instead of fluid from the reservoir. You need to inspect all filters and lines on the bottom of the tractor for damage.

I would not let the engine run very long in this situation because the hydraulic pump will self-destruct if it is running dry. Anytime the engine is running, the pump is spinning, so you need to get some fluid in there to provide lube and cooling. Don't try driving the tractor any long distance. If you can't easily find the problem, I'd trailer it out of the woods to the shop.

I suspect you had the transmission in neutral when pulling the tractor off the log, so any worry of damaging the transmission should probably not be considered. My guess is that this is not an HST tractor.

J_J · Sep 19, 2009

At least, some of the NH 1520s are HST.

michalakd · Sep 29, 2009

Thank you all for your input. I thought you would like a follow up (especially since I'm including a pic

. You were right on the money in thinking that being on top of that log fouled something up relating to the hydraulics on the undercarriage. There is a return hydraulic line that is metal tubing with a rubber splice underneath the tractor. The rubber splice (shown) was pushed nearly off the tubing. Not so much that any hydraulic fluid leaked out, but it must have been enough to allow air to suck in. All I did was to loosen the clamp, push the rubber hose onto the tubing, then retighten. Voila!! I did not even need to add a drop of hydraulic fluid. I don't think I've ever had this easy of a fix. Thanks again guys.