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View Full Version : dump bed won't dump with load! HELP


vntgrcr
03-26-2005, 08:46 PM
HI Guys,
I just purchased a 95 Isuzu with a mason dump body. The dump mechanism is a 12volt electric unit, no PTO. I put a load of dirt/mud on it yesterday of about 5 skid steer buckets, and it wouldn't dump. I had to pull some out before it would go up. It has a Fenner pump on it and goes up and down fine empty. It also has some kind of regulator on the output with a twist knob that says "faster" if you turn it counterclockwise. Can somebody help me diagnose the problem. Do I need a pressure gauge on one of the lines and how much fluid and what kind goes in the resevoir? I am bummed because this thing supposedly could lift 5 ton of stone. At least that is what the previous owner lied to me about!
Thanks
David

hosejockey2002
03-26-2005, 09:11 PM
First of all, with 5 skid steer buckets of mud you may be overloading the hoist. Does the motor stall, or does it just not lift? If the motor stalls under load, you may have a bad electrical connection or the motor itself may be bad. If it runs, but does not lift, then the pump may be bad or low on fluid. BTW, if this is an NPR, it's WAY overloaded with 5 tons of stone on board. Capacity for this truck should be around half of that.

grass_cuttin_fool
03-26-2005, 09:25 PM
I could be wrong but most 12 volt electric pumps/ hoist are not rated for that much weight, but I havent seen them all so.............. AS for fluid you need to check the owners manual, most take hydraulic fluid but I have known some that took regular motor oil also

gene gls
03-26-2005, 10:45 PM
You over loaded your hoist. I have a Venco hoist rated for 8 ton under my 10' platforme dump. It will only pick up 2 ton because there is not much overhang.

Gene

vntgrcr
03-27-2005, 06:45 AM
Thanks for the tips guys. My only reference to electric lifts is a freinds old dump trailer that I have had 4 ton on and lifted no problem. But it sounds like I may have just overloaded it. Thanks for the info and I will be more careful.
David

Gravel Rat
03-27-2005, 05:43 PM
Thats the problem with scissor hoists is they don't have any power. My old 88 F-450 with a 11.5 box and a telescoping post hoist would dump 8500lbs of gravel with no problems. The hydraulics were PTO so that does make a difference. I think the hoist would bend the frame behind the cab before it ever stalled.

When is the last time you checked the hydraulic fluid level ?

I wouldn't class 5 skid steer bucket fulls to be much unless its a yard per bucket which I doubt.

The pump should put full PSI and GPM to the hoist so there shouldn't be a regulator to adjust the pressure. You may want to go with a PTO and pump set up if you want more reliability and save on batteries.

specialtylc
03-27-2005, 05:44 PM
The hoist should have a model number which will tell you what it is rated to lift. I have an Isuzu NPR that I had built with a 12 ton electric hoist on it. And I have had 9000lbs of gravel on it before and it lifted it just fine.

vntgrcr
03-27-2005, 07:28 PM
The hoist should have a model number which will tell you what it is rated to lift. I have an Isuzu NPR that I had built with a 12 ton electric hoist on it. And I have had 9000lbs of gravel on it before and it lifted it just fine.Can you tell me where that rating might be? I looked around and didn't see anything. The piston on this bed is pretty good size, that is why I was surprised it wouldn't lift it. I did put some fluid in it and will try it again tomorrow and see what happens. Thanks
David

cet
03-27-2005, 10:08 PM
If you are low on fluid then it would not lift to the max even if it was empty.

StealthDT
04-06-2005, 02:15 AM
I might be able to clear up a few things. The valve could be a flow control rather than regulator. We use a check valve flow control to slow down the bed on the downstroke for safety. This way if the bed is loaded, it won't hit the frame so hard. I've seen flow controls without the check valve, which would work in both directions.

The next thing I noticed was you didn't mention a high pitched squeel coming from the pump under load. This noise is usually associated with the relief valve opening at full pressure. If your pump is worn out, then it will never output the pressure to trigger the relief, it just bypasses at the gears. Most 12VDC dumps have outputs between 2,000 and 2,500 psi. A pressure gauge is handy for troubleshooting.

When we sell a dump kit, we advise using Dextron 2 transmission fluid in the pump for cold climates. Tractor hydraulic fluid can also be used. For commercial applications in hot climate, use 30 weight motor oil. This increases the life of the pump, but can cause high amp draws in cold weather. So it's a trade-off on what your purpose is....

Yes, scissor lifts are weakest when closed. Typically they need a larger diameter cylinder for the same capacity of a telescoping cylinder dump. Load the dump bed more to the rear to help it lift. The geometry of the scissor determines the leverage it's cylinder can use to open the mechanism & dump. There is also a great deal of difference between a forward opening and rear opening scissor. A scissor that pivots from pins & mounts as far forward on the truck frame & bed will lift more, but it's dump angle suffers. This type is the more conventional design. If a scissor is mounted to the rear, it dumps less, but the angle is higher. In this case the cylinder diameter is usually increased, and the geometry changed to add leverage.

Not trying to write a book, but figured I could help. Check the fluid level, pressure, and open flow valve.

vntgrcr
04-06-2005, 08:26 PM
I might be able to clear up a few things. The valve could be a flow control rather than regulator. We use a check valve flow control to slow down the bed on the downstroke for safety. This way if the bed is loaded, it won't hit the frame so hard. I've seen flow controls without the check valve, which would work in both directions.

The next thing I noticed was you didn't mention a high pitched squeel coming from the pump under load. This noise is usually associated with the relief valve opening at full pressure. If your pump is worn out, then it will never output the pressure to trigger the relief, it just bypasses at the gears. Most 12VDC dumps have outputs between 2,000 and 2,500 psi. A pressure gauge is handy for troubleshooting.

When we sell a dump kit, we advise using Dextron 2 transmission fluid in the pump for cold climates. Tractor hydraulic fluid can also be used. For commercial applications in hot climate, use 30 weight motor oil. This increases the life of the pump, but can cause high amp draws in cold weather. So it's a trade-off on what your purpose is....

Yes, scissor lifts are weakest when closed. Typically they need a larger diameter cylinder for the same capacity of a telescoping cylinder dump. Load the dump bed more to the rear to help it lift. The geometry of the scissor determines the leverage it's cylinder can use to open the mechanism & dump. There is also a great deal of difference between a forward opening and rear opening scissor. A scissor that pivots from pins & mounts as far forward on the truck frame & bed will lift more, but it's dump angle suffers. This type is the more conventional design. If a scissor is mounted to the rear, it dumps less, but the angle is higher. In this case the cylinder diameter is usually increased, and the geometry changed to add leverage.

Not trying to write a book, but figured I could help. Check the fluid level, pressure, and open flow valve.
Please right a book for us dummies! That was very helpful. It is like you were looking at my truck. Due to frustration last weekend, I totally disconnected the flow control valve. It was connected to the line that pushes the cylinder up, and since I have been dumping concrete waste and actually had to dump 3.5 tons today and it had no problem. But, isn't there always a "but", when the bed is coming down it hops and jumps all over the place like there is air in the lines. If I stop midway and let it settle down it is better but not smooth by any means. Sometimes have to stop it a couple of times. With the flow control inplace, when I adjusted it, I would get the hop when it was open, close the valve down some and it would go away. Should I put it in the down line or will that decrease my lift again due to a restriction in the fluid going the other way? Also I do have a really good dump angle, but as you stated at the sacrifice of having the scissor mounted more towards the rear. I guess just a compromise. Thanks for any help.
David

StealthDT
04-07-2005, 12:11 AM
Does your pump run during the down stroke? Are there two lines into the cylinder, one at the top, one bottom? The flow control should be in the lower line. It should be adjusted to eliminate a rapid downstroke. Do two other things, grease the scissor pivot pins and hinges, use white lithium spray grease if it doesn't have fittings, EP2 if it does.. Check the cylinder rod for straightness at the top of the stroke with a ruler. I've seen some bent cylinder rods from overloading. Also, is the rod scored or is it bright chrome finish? Scoring means the rod is galling the bushing. Does the relief valve squeel at the top of the stroke when the scissor is locked?

vntgrcr
04-07-2005, 07:03 AM
Does your pump run during the down stroke? Are there two lines into the cylinder, one at the top, one bottom? The flow control should be in the lower line. It should be adjusted to eliminate a rapid downstroke. Do two other things, grease the scissor pivot pins and hinges, use white lithium spray grease if it doesn't have fittings, EP2 if it does.. Check the cylinder rod for straightness at the top of the stroke with a ruler. I've seen some bent cylinder rods from overloading. Also, is the rod scored or is it bright chrome finish? Scoring means the rod is galling the bushing. Does the relief valve squeel at the top of the stroke when the scissor is locked?
Stealth, Thanks for the quick reply. To answer your questions, the pump does run on down stroke. The flow control was on the line at the bottom of the cylinder, top being where the piston comes out. So I assume that was the line for the up function. I will grease the scissor pivots as well. And I will check the cylinder for straightness. Never thought of that. And it is bright chrome and not leaking any fluid. The relief doesn't really squeal, you can just tell that the pump/cylinder has reached it's max. Nothing unusual. Also there is a small box with one of the wires for the solenoid onthe side of the pump. It says something like fluid dynamics or somthing like that. I guess it has something to do with a safety for the pump. Too much pressure build up and shut's the solenoid off? Thanks for all of your help.
David

StealthDT
04-08-2005, 12:47 AM
Post some pictures of the pump & scissor. Where does the line from the top of the cylinder go into the pump? I doubt the box is a pressure switch. Double acting pump and scissor is rare, could be a valve problem, or the pump is worn out. Double acting means the pump runs on the down stroke, rather than gravity down.