NH L325 transmission

[sions1]

I replaced the piston assembly in both left and right Cessna 70142 pumps and the drive motors looked good. I put everything back together and the machine ran great for about 2 hours. Then the right side quit working I could steer left but not right. I took the transmission back apart and everything looks the same in both pumps the drive motors look fine. When I put it back together I'm sure I will have the same problem. What could it be?

 

[SkidRoe]

Hi sions1 and welcome to the forum!! I think your best bet is to post this question in the NH section, if you have not already. There is a member there, mike10, who is the resident NH guru. He should be able to help. Cheers!!

 

[Mike10]

Did you check the check valves. It has been awhile since I have been into that model pump, but they are located on the back plate, the plate the rotating group flat end runs against. There are two of them for each pump. I think they protrude slightly from the plate and you can grab them with a pliers and pull them out. It looks like a round tube with a ball bearing inside and I think they are about 1/2 to 3/4" long. When they go bad you stop moving. If my description is not adequate to locate them let me know and I will look it up and give you a better answer.

 

[sions1]

Did you check the check valves. It has been awhile since I have been into that model pump, but they are located on the back plate, the plate the rotating group flat end runs against. There are two of them for each pump. I think they protrude slightly from the plate and you can grab them with a pliers and pull them out. It looks like a round tube with a ball bearing inside and I think they are about 1/2 to 3/4" long. When they go bad you stop moving. If my description is not adequate to locate them let me know and I will look it up and give you a better answer.

Thanks Mike I will check them tomorrow. I am wondering the right side camplate had some grooves wore in them and I had a machinist take a couple thousandths off to smooth it out. Do you think this would have any effect?

 

[sions1]

Thanks Mike I will check them tomorrow. I am wondering the right side camplate had some grooves wore in them and I had a machinist take a couple thousandths off to smooth it out. Do you think this would have any effect?

Mike couple more questions. What should I be looking for in the check valve? Also, the parts diagram I have shows a tow valve. Is that a dump valve or something? I'm not sure what it is or does.

 

[Mike10]

Mike couple more questions. What should I be looking for in the check valve? Also, the parts diagram I have shows a tow valve. Is that a dump valve or something? I'm not sure what it is or does.

The check valves have a steel ball in them which blocks the flow of oil in one direction. The tubes can break and the ball is no longer present allowing oil to free flow in both directions and not build pressure. The tow valves are turned a quarter turn when you need to pull the machine so the oil can escape from the hydro motors and that allows the wheels to turn.
When you put the hydro pump together did you have a slight gap between the back plate and the pump housing before you installed the bolts or did the plate fit right down on the housing? You need to have a gap so the rotating group is preloaded, when the bolts are tightened, to the back plate and swash plate for the pump to work. The gap is usually 1/8".
I thought about the tow valves not being in the right position but since you said it had been working I discounted that as a possible cause. The tow valves just lift the relief valves off their seats. Granted the result would be what you are experiencing, but the valves can not turn on their own. Another thought was the relief valves but again since it had been working it was unlikely that both failed at the same time. Another thought was a broken chain in the chain case, but I would think you would have noticed it when you removed the hydro motor. If you can't find a cause then you may need to reinstall the pump and motor but instead of attaching the large hoses to the pump cap the fittings on the pump. Start the engine and move the steering lever. If the pump is working it will pull the engine down. If it pulls the engine down then you have a problem in the motor. I mentioned the gap above in the pump and you should also have had a gap in the motor when you reassembled it.
You also have a coupler between the two hydro pumps but if it failed or the splines stripped your hydraulic pump would also quit working. Did the charge pressure light come on when the hydro quit? Did the hydraulics continue to work, boom and bucket?

 

[sions1]

The check valves have a steel ball in them which blocks the flow of oil in one direction. The tubes can break and the ball is no longer present allowing oil to free flow in both directions and not build pressure. The tow valves are turned a quarter turn when you need to pull the machine so the oil can escape from the hydro motors and that allows the wheels to turn.
When you put the hydro pump together did you have a slight gap between the back plate and the pump housing before you installed the bolts or did the plate fit right down on the housing? You need to have a gap so the rotating group is preloaded, when the bolts are tightened, to the back plate and swash plate for the pump to work. The gap is usually 1/8".
I thought about the tow valves not being in the right position but since you said it had been working I discounted that as a possible cause. The tow valves just lift the relief valves off their seats. Granted the result would be what you are experiencing, but the valves can not turn on their own. Another thought was the relief valves but again since it had been working it was unlikely that both failed at the same time. Another thought was a broken chain in the chain case, but I would think you would have noticed it when you removed the hydro motor. If you can't find a cause then you may need to reinstall the pump and motor but instead of attaching the large hoses to the pump cap the fittings on the pump. Start the engine and move the steering lever. If the pump is working it will pull the engine down. If it pulls the engine down then you have a problem in the motor. I mentioned the gap above in the pump and you should also have had a gap in the motor when you reassembled it.
You also have a coupler between the two hydro pumps but if it failed or the splines stripped your hydraulic pump would also quit working. Did the charge pressure light come on when the hydro quit? Did the hydraulics continue to work, boom and bucket?

Boom and bucket worked. The reason I asked about the tow valve is that when I removed the entire transmission agian I noticed the tow valve on the right side pump was loose. I can turn it with my fingers but the left side I couldn't. When I used my farm tractor to drag the skid steer back to my shop the right side wheels turned freely and left side didn't. I think you have found my problem. Now I just have to figure out if I can fix it without spending a lot more money. Just bought two rotating assemblies. Thanks for the education.