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Author  [Modern View]

02-13-2014 17:23:20

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Ok, if you can't tap into the main line try hooking a gage into the line going to the other end of the tilt cylinders and the lines to both ends of the boom lift cylinders. In all cases take the cylinder to the end of it's stroke and see what kind of pressure you've got. This will give you a good idea what the system pressure is without having to tap into the main line. If it were me I'd check the pressure on the boom lines first since that function seems to be OK. That said, just the fact that fluid comes out of the line really doesn't mean anything as even a worn out pump will usually create flow, but not pressure. It all depends on the wear, but I've flow tested systems before and seen pumps act perfectly normal until the system pressure reached a certain pressure at which point both the pressure and flow dropped to near nothing. At that point you could remove the load/pressure and the flow came right back up, but as soon as a load was applied and pressure started to rise it was right back to nothing. Unfortunately the only way to really check things that way is with a flow meter with valve to load the system. That is the best way to check a hydraulic system given that pressure and flow are tied together like they are, but in a situation where nothing but a cylinder is involved, the flow doesn't mean as much as the pressure since the only effect flow has on the cylinder is the speed it works, and has nothing to do with the amount of force it can exert.

As far as why one function works and not the other, like I said before, the tilt back and boom down functions are both working off the rod end of the cylinders where the tilt forward and boom up are working off the tail end of the cylinder without the rod. Using round numbers think about it like this. With 100 psi on the rod end, the rod takes away surface area from the piston so there is only 1 sq inch of surface area for it to act on. With the one sq in, the force put out by the cylinder is only 100 lbs. On the other end, without the rod, the 100 psi is acting on the full surface of the piston which is, say 4 square inches of area. This gives the cylinder 400 lbs of force. Ultimately between the differences in the area of force the pressure has to act on, and the mechanical advantage achieved by the way the cylinder is attached, it's not uncommon to see one function seemingly act normal, while the others show a definite problem. This happens until enough weight is put on the system for the problem to show with every function. It's hard to say if that's an issue here without factoring in the weight of the loads you have been carrying, the length of the 'levers' involved, etc, etc and doing the math, but I have seen that make a difference and cause a problem to show at point A but not B and C until a situation occurred that pushed the 'stronger' functions past their limit. Hope that makes sense as I don't know any other way to explain it.

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Kevin Wright

02-13-2014 20:08:12

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 Re: GEHL HL4400 BUCKET GOES DOWN BUT NOT UP in reply to NCWayne, 02-13-2014 17:23:20  
I am in the process of locating a pressure test kit, i had borrowed one when i tested the first time, but i only could borrow the gauge and one fitting. every set of lines on this thing has a different type of fitting, new hoses, old hoses, couplings, close nipples, etc...its a nightmare! i need a gauge and an assortment of fitting so i can do tests on each line at the point it connects to the cylinder, and where it comes out of the control valve. ive talked to everyone i know locally about what the problem could be but they just scratch their head like ive been doing for the past month. you have been a big help, i really appreciate your patients. when i find some gauges ill do some more test and let you know what i find.

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02-14-2014 19:02:50

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 Re: GEHL HL4400 BUCKET GOES DOWN BUT NOT UP in reply to Kevin Wright, 02-13-2014 20:08:12  
Testing at just two locations will tell you a lot. Since the lines going to each lift and tilt cylinders are Tee'd together, all you need to do is put a gauge on the lines going to both ends of one of them either at the same time or seperately. With a gauge tee'd into one of the lines move the cylinder to the end of it's stroke and read the max pressure. Then put the gauge on the other end and take the cylinder to the other end of it's stroke and read that pressure. That will tell you whether the pressure the pump is making is the same on both circuits.

If the pressure is higher on the lift circuit than the tilt circuit, or different between the ends of the cylinder on the same circuit, that will help you pinpoint the problem. Now if its the same (low) on both ends of both cylinders then you could have either a bad pump or the main/system relief could be stuck open/damaged and dumping the pressure . Unfortunately the only way to test for that problem is to tap into the main line going into the valve and put a load on the system like I talked about before.

All you really need to do is put a load on the system and the actual flow rate isn't an issue, all you need is a Tee with a needle valve on the outlet/downstream side of the tee to give you the load capability. Slowly closing the valve should cause an increase in pressure. All you need to do is slowly close it and watch to see if the pressure reaches the spec. If it stops rising before you get there you know the pump is bad, if it continues to increase as you close the valve a little more then all is OK with the pump and it has to be the main relief valve.

If you want to get yourself an assortment of fittings and gauges to check this problem, and to have for future use, give Surplus Center a look. They are about the cheapest place I know of to get pretty much everything you might need to tap into and check the system.

Once you get everything checked out, like I said, shoot me an email, or post back and I'll be glad to offer up any assistance I can. Wayne

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