Dump bed ... Slow going down

steve terplak · Apr 5, 2014

Hey guys,
I have a "95 chevy 3500 dump with an electric hydraulic dump..
The dump is slower than molasses lowering the body after dumping. It is not the fastest on the way up, but it really isn"t too bad as compared to what I had been expecting for an elec. dump.
When it goes down, however, it takes 5x as long to go down as up, there is no hesitation, it doesn"t get stuck in any position it is just super slow and steady.

What should I look for to trouble shoot this. It is a used truck that is new to me.

From what I have read online it could be
a) incorrect hoses - should have a larger diameter hose for fluid exiting the cylinder? Is this true?
b) plugged relief vent - where on the unit should this be - hose? near the reservoir?
c) malfunctioning switch/actuator - not opening the valve all the way...

Hopefully you guys can get me looking in the right direction.

thanks,
Steve

jon f mn · Apr 5, 2014

Not likely the hose, a lot of oil will go through even a small hose. The vent is likely the fill plug in the tank. Could be that, but lowering it with a load would probly blow up the tank if that was it. Could be the valve going bad, but I doubt that too. A lot of those pumps have an adjustment for drop speed and that could be it also. Post a pic or post the model # of the unit would help a lot. Also check the pivots for the hoist and bed. I can't tell you how many hoists I've rebuilt because they were never greased and seized on the shafts. Slow dropping is the first sign of a problem there too. I think that is the most likely problem.

steve terplak · Apr 5, 2014

Thanks Jon! I will head out later and take a few pics

greygoat · Apr 5, 2014

Is this a new, or new to you dump truck ?
Over the years many people have been killed by
dump box"s coming down too fast, it can happen
when a hose breaks, or is tampered with.it can
happen when someone is checking, or maintaining
the system, and didn"t block the box up properly.
A CORRECTLY installed hydraulic system will have
a check valve installed. The check ball will be
pushed open by fluid lifting the dump box. Then it
seats. The check ball has a hole drilled in it.
when you lower the box, the check ball remains
closed, and fluid can only get out through the
small hole in the ball, thus the box lowers
slowly. It may be frustrating, but it"s there to save your life. I can"t remember the specific
name of that check valve, but different sizes are
available. A hydraulic schematic should show
one in your circuit. Replacing it with a larger
one, would be like putting a 50 amp fuse in a 10
amp circuit! Don"t do it! Live with it!

Bob Bancroft · Apr 5, 2014

I'll bet Jon has it. Easier said than done, but you have to separate the hyd. system from the mechanical system. Here that means I would lift the bed with a loader, and/or bypass the truck system and go right to the hoist cylinder from a tractor, etc.

A couple years ago my then 10 year old "no lubrication required" "Olympic" hoist in my dump truck started dropping slowly. It was the main pivot of the scissors/stabilizer. It was flexing the cylinders enough to cause them to leak! Turns out it was never assembled correctly, and the plastic "seal" washers weren't keeping the crud out. They can put all the plastic parts in there they want, but as long as they're next to steel, with no lubrication, the salt can get in and cause things to corrode and swell.

Paul Janke · Apr 5, 2014

I think a small hose could be a problem, but it would need to be quite small. Could somebody have put thick oil in it? Is the temperature low while you are checking?

jon f mn · Apr 5, 2014

Yep, should have thought of that too. If it's cold and they have thick oil that could be the problem as well.

Welding man · Apr 5, 2014

Is it a single action or a double action pump and hoist. The double action has 2 hoses going to the cylinder. The up pressure is set at 2500 PSI and the Down at 500. They are made to go down slow for safety reasons.

Steve@Advance · Apr 5, 2014

Graygoat, I think you're on the right track. There should be a block bolted or threaded directly on the bottom of the cylinder that contains the check ball. However, I think there may be a problem, like a broken spring or a stuck shuttle causing the problem.

I ran into a similar problem with a forklift once. It would go up but down very slowly. Took the block off and replaced it, worked fine.

Dealer said it was a safety device that monitored flow rate. Going down through the restriction of the valves and plumbing kept the safety valve open, but if a hose blew allowing too much volume too fast would close the valve and slow the fall.

Pops1532 · Apr 5, 2014

The safety valve others have mentioned sometimes have a knurled knob you can turn to adjust oil flow/speed.
I'd guess your problem is electric. I have a manlift that was electric/hydraulic. Those solenoid valves gave us a lot of trouble. You could have a bad ground, loose connection, or a faulty solenoid/valve.

jon f mn · Apr 5, 2014

I've never seen a truck hoist with that valve on the cylinder. They may have them on new ones in some cases, but I just worked with someone to install a new scissor hoist in a truck and there was no valve on the bottom of the cylinder.

ericlb · Apr 5, 2014

i have a electric over hydraulic dump on one of my 1 tons too, now mine is a old truck, a 83 model but it will do the same thing if the relief is plugged with dirt, mine is on the resivor and doubles as the fill spot for hydraulic fluid like on some ford loaders, might see if there is a small round deal on the resivor about a inch and a half around with a hex sided spot where you remove it, and see if its full of dirt and debris

gene bender · Apr 5, 2014

Have you cobtacted the manufacter of the product. Thats what i do when i have a problem as they made it and will be glad to help.

David G · Apr 5, 2014

Plugged return filter?

ss55 · Apr 5, 2014

Record the model numbers and serial numbers, then contact the manufacturers of the pump and of the hoist. If they are still in business, they will likely send you their owners manuals for free.

It sounds like it could need a flow control adjustment or cleaning.

If you lift the bed a little with a load on it, and then lower it back down with a load still on it, does the lower faster?

Oil flow volume through a restriction is dependent on the oil pressure. An empty bed at a steep angle may not have much down force to push out the oil with much pressure until it is nearly all the way down. You need to keep enough resistance so you can still lower a fully loaded bed slowly enough not to damage anything. That's my thoughts, check with the manufacturers to be certain.

jerrybigrig · Apr 6, 2014

Check your voltage on your lowering solenoid...should be battery voltage ...then make sure you have good ground if questionable add extra ground wire ..

Dump bed ... Slow going down

steve terplak

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jon f mn

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steve terplak

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greygoat

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Bob Bancroft

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Paul Janke

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jon f mn

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Welding man

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Steve@Advance

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Pops1532

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jon f mn

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ericlb

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gene bender

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David G

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ss55

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jerrybigrig

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