518 CAT dozer control

[Caseybow123]

Good afternoon all. Has anyone had any experience with the older 518 skidder control valves? My dozer blade goes down alright but is very, very slow to come up. When pushing snow, you actually have to put in nuetral or reverse to lift the blade and it is painfully slow. Just wondering what to look for in the dozer control valve. Possible O-rings?

[mike_belben]

Is there any hydraulic function on the same pump loop that works perfectly? Like steer or a grapple? 

In general we are looking at pump is shot and is not creating flow, or pump is flowing but a huge leak is preventing the circuit from building pressure.  It can be a relief valve stuck open by rust or a chip or bur, can be a cracked valve section or failed O ring.  Or i guess maybe plumbed wrong if you bought it with this issue.  

If its flowing over a relief valve that area will be hotter than others and an IR temp gun can be used to narrow it down.  

[Caseybow123]

When I bought the skidder last year, I noticed that the blade did not raise fast but was useable. However, the other day it went from annoying to useless. The down function works great and fast, the steering is very quick which to me eliminates a bad pump. There is no pump shake or noise. No grapple on skidder. 

[Skeans1]

Do you know the pressures for that circuit? If so flsuh your compensator then run it past your relief pressure double check them, then set your compensator.

[Caseybow123]

I have no idea what pressures are and it doesn't say in the books I have. When I quickly looked yesterday, I never seen a relief valve either. Just 2 lines running to the dozer control valve.

[Skeans1]

You have to have a relief for both sides of your blade one side will be up and the other will be down then a compensator that's slightly under the reliefs. You might call your local cat dealer or better yet go in get a break down of the valve as well as ask them to print out a copy of the pressure setting then go through your book and write down the others in the front on that same sheet.

[Caseybow123]

Thanks for the info. I don't believe it is anything serious but with hydraulics it doesn't take much. Just glad logging is done and I have some time to work on it. I will post my findings. 

[mike_belben]

If it has a bunch of force and speed downward, will lift the machine off the ground and hold it up...  That will reasonably allow us to cross off the pump, the main relief and the rams.

First test id do is pull the hoses off at the tee and let them blast into a bucket clean enough to notice any trash blasting out.  Chunks of rubber o ring, conposite gasket, plastic wiper or rusty metal blob, gear teeth etc.  Its always smart to ID your FOD.   Both the A and B hose so you have a basis to compare flow and intensity, probably need a helper.  If they both squirt the same im looking downstream toward the blade.  If not, im headed upstream toward valve.  

Crushed hardpipe under the belly pan?  Delaminated internal hose with a flapper inside? Working back to the valve i would pull the hoses at the AB ports, screw on some other junk hoses to some cylinder and try it.  I know thats a tall order for people with insufficient quantities of junk but its what id do.

If the test cylinder is still acting screwy id pull valve apart and find the crack or bad seal.


Dont know too much about how an accumulator would be plumbed in or why (into ram or steer circuit?)  It doesnt have any load check on the rams right?  What about an individual relief valve just on the blade spool section? Look for hex plugs on that section.  If so see whats under. 

[Skeans1]

Mike I get this exact issue with our harvester it ends up being a particle in the relief or compensator 10 minutes with two guys fixed up and good. Same situation boom would push down good but would lift slow flushed the relief and reset everything good to go. One side of your valve is for up and one is for down, then you'll have the two lines coming off the valve to a splitter block to go to each cylinder.

[mike_belben]

Yeah same here.  Bobcat relief picks up metal chips once or twice a year. 

[Firewoodjoe]

I've had the nut on the back of the spool loosen up so u think your actually pulling it but inside it's not. The down motion is pushing directly on the spool. Pop the back off and see.

[mills]

There is a check valve on the side of the control valve that might have a piece of trash stuck in it. It's surprising how a small spec can cause so much trouble.

[Oliver05262]

  joe made me thing of another place you might have a problem--could the nut that holds the piston on one of the blade cylinders be loose? The piston retracts when lifting the blade, and if the nut backed off, oil could leak off between the piston and the cylinder rod. When you push down, the piston moves up against a shoulder and pretty much seals itself off.
  Mike's test to see if the flow from each hose might tell you this, if you can direct the flow coming out of the cylinder into a bucket. One hose sends return oil back to the tank. There should be no flow if the cylinder isn't moving.

[coxy]

try changing the hydro filter i know it sounds crazy but i had trouble with the blade coming up slow the steering was great not sluggish and i changed the filter problem solved ???    i know if you try steering and use the blade at the same time the blade don't respond   it was a pita to get use to that my other skidder has a double pump don't know what cat was thinking by putting a single pump on there   the drain for the hydro tank is on the left side sitting in the seat by the foot step