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F700 dump truck lift is weak

apm

Well-Known Member
Joined
Apr 1, 2011
Messages
171
Location
Northeast Tennessee
F700 dump truck, has two cables for dump control mounted in the floor. One engages PTO, the other is Up/neutral/down dump valve. Big red knobs that pull straight up.

With a full box, the dump bed struggles to raise. It makes a loud buzzing sound and jerks in fits and starts. If I load to heavily to the front of the bed, it will just buzz loudly and not raise at all. I have to get pointed down hill to get it to raise and then it's difficult.

With a light load it goes straight up and does it very quietly. All of the hinge points are well lubed and operated smoothly. It seems like some kind of hydraulic problem. When it is skipping and buzzing the PTO shaft does not seem to stop or operate erratically.

Any suggestions on where to start would be welcome.

Thanks,

Greg
 

mitch504

Senior Member
Joined
Feb 27, 2010
Messages
5,776
Location
Andrews SC
dead end your pump into a 3500 psi gauge and see what it reads. It probably just needs the relief valve adjusted. Don't do it without the gauge, you can break expensive things.
 

Delmer

Senior Member
Joined
Jan 3, 2013
Messages
8,865
Location
WI
When you say "pointing down hill" is that the front of the truck pointing downhill or the rear to dump downhill?

The jerking in fits and starts sounds like air, suction leak. Does it make any difference if it's cold, or hot outside and warmed up?
 

apm

Well-Known Member
Joined
Apr 1, 2011
Messages
171
Location
Northeast Tennessee
Thanks, Mitch. I'll try to put a gauge on it and see where it is.
Delmer, pointing the rear of the truck downhill seems to help, although it's not a cure. Outside temperature doesn't seem to make a difference, but it does operate better after cycling the bed empty a few times and warming up the hydraulic oil. It seems to be weight dependent. light loads dump fine.

The "buzzing" noise is loud and sounds mechanical, but the PTO doesn't seem to skip or miss.

Greg
 

Delmer

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Jan 3, 2013
Messages
8,865
Location
WI
The buzzing is either the relief valve operating, or cavitation. If pointing the FRONT of the truck downhill helped, I'd think it was cavitation. BTW, your oil level isn't low is it? Operating better warmed up a little, or maybe just working the air through a couple times, seems like cavitation or just plain pocket of air.

The pressure relief valve is pretty easy to get to, remove and adjust. Usually.
 

DMiller

Senior Member
Joined
Feb 21, 2010
Messages
16,432
Location
Hermann, Missouri
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Cheap "old" Geezer
I agree on the relief, the noise you hear is cyclic chatter at that valve, may have a damaged tension spring(heat or breakage), may want to do a Hyd oil change pull that valve and have a look. No damage reinstall and reset to spec pressure for the system.
 

apm

Well-Known Member
Joined
Apr 1, 2011
Messages
171
Location
Northeast Tennessee
So, it's a Muncie PTO driven pump. In the pictures I assume the relief valve is the one between the two hoses? It looks like a really simple hydraulic system. The reservoir stretches between frame rails and the single acting cylinder attaches to it.

Greg
 

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DMiller

Senior Member
Joined
Feb 21, 2010
Messages
16,432
Location
Hermann, Missouri
Occupation
Cheap "old" Geezer
make certain that bed is down or on multiple props before you work beneath it, know of three killed when a bed settled ever so slowly on them. Cannot get loose while knowing you are being crushed has to be the worst.
 

apm

Well-Known Member
Joined
Apr 1, 2011
Messages
171
Location
Northeast Tennessee
OK, feeling a little stupid, but thought I'd explain what I found for anyone else's benefit.
If you look at the third picture, you'll see the hydraulic reservoir between the frame rails. It's not possible to check the oil level with the bed down because the bed blocks access to the fill plug. The previous owner explained to me that it should be checked with the bed up, but the reservoir should only be about half full at that point because when the bed came down and the cylinder retracted, it would overflow (or actually stay up) due to the oil in the cylinder returning to the reservoir.

Apparently, that's not the case. When Delmer asked about the oil level, I replied confidently that it was good. After checking this thing out more carefully, I found that there's an oil line from the top of the cylinder that goes back into the bottom of the reservoir. Best I can figure is that the reservoir is too small, so they are using the capacity in the top of the cylinder to keep the reservoir full as the bed goes up.

So, by filling the reservoir full, even with the bed raised, the problem disappeared entirely. Works smooth and quiet. I haven't tried it with a heavy load yet, but have no doubt it was just way too low on oil and cavitating.

I should have studied it more carefully in the first place.
Thanks for everyone's suggestions.

Greg
 
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