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I bought a Cat D4C series III hystat ser# 5XK17822 knowing…

Customer Question
I bought a Cat...

I bought a Cat D4C series III hystat ser# ***** knowing it had an issue with holding the pressure on the blade angle. Whatever side the load is on the blade will give completely and swing to that side.

Mechanic's Assistant: Thanks. Can you give me any more details about your issue?

Removed both angle cylinders and had them pressure checked by the local hydraulic shop. Both checked out fine, packing not letting fluid to bypass. Talked to local cat dealer service manager and he said the problem had to be in the valve spool. Replaced the 3 bank blade control valve body, part#9T7249 G, with a re-build, after finding all the valves had been leaking for a long time. Start the machine up and the blade barely moves at all. After working it awhile, one valve will randomly work while the others are still sluggish. Seems like there is air in the system, pressure not set? Very discouraging! Can you give me a clue of what to try next?

Submitted: 2 years ago.Category: Medium and Heavy Trucks
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Answered in 1 hour by:
1/31/2016
Technician: cat_engines_only, Other replied 2 years ago
cat_engines_only
Category: Medium and Heavy Trucks
Satisfied Customers: 2,578
Experience: Repair CAT machines only
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Hello, Thanks for requesting me. I will be glad to help all I can. (FYI the serial number you posted is the engine serial number, it is linked to a 6BS00399 and this should be your machine serial number.)

Are you saying the lift and tilt do not operate correctly either? If so, it could be air. If the oil appears foamy and the pump is noisy, then air is likely the issue.

Did you change the filters? Cut open old filters and inspect them for metal or other debris that may relate to a system failure.

Is the oil level correct and are you using 10W hydraulic oil? This tractor operates all transmission and hydraulic functions on this one oil. It is critical it remains clean and the correct grade.

Has anyone tested the pump pressures for the implements?

Did this tractor have any add-on attachments?

Does the 9T7249 valve have an end cover plate with plugs in it is is it blank?

Have you tried swapping the main relief from the old valve to the new valve?

This hydraulic system is fairly simple and should not have this trouble. Do you trust the hydraulic shop that tested the cylinders or the one that resealed the valve?

Do you have any testing gauges?

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Customer reply replied 2 years ago
The lift and tilt operate randomly off and on. The pump doesn't seem to be noisy. Everything worked fine except for the angle until I changed the valve body. Oil level is fine. I trust the hydraulic shop. I'm not sure of the rebuild shop yet. I haven't had any pressure test done yet since it worked before. I don't have any gauges. I have not change the filters. It doesn't have any ad on implements. I'm not sure what a cover plate with plugs would look like on the valve body but it is the exact same that I took off. I haven't changed the relief valve either, is that the small adjustment screw, stem with a lock nut under the acorn nut on the side where the mainline in and out is? Forgive my lack of knowledge, I am a owner operator trying to save some money, not a hydraulic mechanic.
Technician: cat_engines_only, Other replied 2 years ago

Thanks for the reply. I will attach an image to help us get on the same page when discussing the valve.

Note: Items #1 and #2 is for a machine that has a ripper or other attachment. Since you have the 3 section valve, yours is missing this section.

If your valve was modified for "power beyond" to supply hydraulic power to another valve, item #8 would have a different end cover and it would include two ports. This is not common and is usually on machines newer than yours. Just want to verify the replacement valve does not have this either.

Item #6 is the main relief and has the adjustment screw in it as you describe. I would try the one from the old valve since you know it did make some pressure to lift the blade.

Since you are not seeing anything obviously wrong with the hydraulic oil, pressure testing is needed to know if the problem is in the pump. The hydraulic system must function well enough to purge out all the air or the blade will not hold correctly.

Will you be able to locate a test gauge and willing to preform a few pressure checks?

Some fittings and plugs or caps may be needed to block off the lines to pressure test the circuits as well.

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Customer reply replied 2 years ago
Mine has the same end cover as the one in the diagram minus the fourth ripper valve. I will try changing out the main relief valve tomorrow morning. Is it possible to build my own test gauge, If you give me the specifics. Possibly obtaining the parts from TSC or a hydraulic supply house. I guess I could ask one of the guys at the hydraulic repair shop, but I doubt if the cat service manager would loan me tools. I'll have to get back with you tomorrow. Thanks.
Customer reply replied 2 years ago
I did try screwing the stem up and down on the relief valve friday afternoon after I got the new valve on and it didn't have an effect on the blade movement.
Technician: cat_engines_only, Other replied 2 years ago

Ok, thanks for the information.

Since you have the standard end cover, that is one item to not worry about.

I will attach a guide to help with checking pressures. It contains the cycle times for a normal machine and the tool list needed to check pressure. Fittings to block hoses would be as needed to match you lines.

First item to check though is pump pressure as instructed in the guide.

Check pressures at low engine speed and then high engine speed both with cold oil and then with hot oil.

Also record the pressures you see when you operate each function in both directions.

Document your findings and post them here. We can go over them and see where it may lead us.

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Customer reply replied 2 years ago
OK, this morning I put the old relief valve in the new valve body and tested the blade. Everything moves like it did before I put the new valve bank on including the blade not holding its angle. Evidently the valve spool was not the original problem. I can't find your post for the procedure to check the pressures, have you posted it yet?
Customer reply replied 2 years ago
I'm beginning to think both of my hydraulic shops made a mistake. The valve bank had a bad or sticking relief valve, And I believe my Hydraulic man that checked the cylinders somehow missed the small check valve balls in the ports. The packing was good but I don't know if he checked the valves. I found a schematic of the cylinder and I'm going to take it to him tomorrow to see what he says.
Technician: cat_engines_only, Other replied 2 years ago

I am sorry, I guess I missed adding the attachment. Sorry for the late reply. I work during the day and answer questions at night. I hope this is not an inconvenience for you.

The think about this valve is it's a very simple valve. Your angle spool does not have any relief valves in it. This spool is how the load is blocked when the valve is in neutral. I would question the cylinders myself. A loose piston retaining bolt can easily allow oil to bypass just as a bad piston seal.

The only check valve in this design is for a load check. This does not affect the holding abling of the spool. A bad load check will only cause the load to drop slightly before the pump can pressurize the circuit. For this circuit, you would see a delay in the angle cylinders moving. You would not experience drifting cylinders from a load check valve problem.

The cylinders do not have any check balls or reliefs either. If you are referring to the items in the parts list, these are just used to prevent a hard slam for the cylinder when it is at full extension or retraction. They have no influence on the sealing of the piston when it is in the middle of the cylinder.

I have seen one cylinder that was worn in the middle and could not hold a load. It did not affect the operation of the cylinder though.

It worked normally but, just drifted slowly under use.

This is normally checked when measuring the bore and inspecting for bent tubes or rods.

Attachment should be below for pressure testing. I would check the pump pressures and then cap off all the angle cylinder lines and check the pressures again to see if it changes.

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Customer reply replied 2 years ago
Just to keep you up-to-date, I think I have isolated the bad cylinder. I angled the blade, removed and plug the hydraulic lines from the retracted cylinder and pushed with the extended. Did this for both sides, one cylinder pushed, and one cylinder collapsed. Took the cylinder back to my hydraulic man for a Nother inspection. He pressure tested both of them the first time but only took apart one. The one that collapsed was the one he did not take apart!
Customer reply replied 2 years ago
I haven't pressure tested the system yet, the pump seems to have enough pressure to operate the other two controls without a problem. I'm having trouble finding the correct parts to build a test gauge. Although I would still like to test it since I have played around with the relief valve.
Technician: cat_engines_only, Other replied 2 years ago

Thanks for the update. I am not sure about the test you did. The angle cylinders always extends one side and retracts the other side to angle the blade. The lever reverses the flow when you move it the opposite direction.

Yes, setting the pressure too high will damage the pump.

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