pc78mr swing overrun

[Mobilewrench]

Hello, HEFers (we should have t-shirts made.)
Got a lot of really good help here in my last episode (called "pc78 loss of almost all functions). Especially from JohnC and Lachau. Still cannot thank you two enough for the shop manuals.
The last episode seemed to have a happy ending with the machine miraculously repairing itself. I don't like it when machines do that. It makes it hard to write the bill and I always seriously doubt that the problem is resolved.
So.... Of course, the problem is back.
Machine ran well for about a week. Then ran well cold but got worse as the oil came up to temperature.
I finally got back to it today. With dead cold hydraulic fluid I have swing over run.

 

[Mobilewrench]

The logic tree in the shop manual just seems to indicate that I need to replace parts. But I am more than a little confused about what parts it means.

 

[Mobilewrench]

 

[Vetech63]

What have you positively eliminated on the troubleshooting chart??

 

[Mobilewrench]

So many questions.
The first line in the troubleshooting tree asks if the swing relief pressure is normal. And I apologize that the picture loaded sideways, I do not know how to rotate it. So, my first question is how do you stall out the swing on an excavator to figure out where the relief is set?
Next question is where do I plumb in a gauge to read it.

On to the lower part of the tree.
My control valve moves just fine. And I tried swapping the spool in the swing section of the control valve with the blade spool. Both functions remained the same.

So, now we get into the part where I cannot seem to decipher the nomenclature that Komatsu likes to use in their service manuals.
"When swing motor suction valve is replaced, is trouble repaired?"
Um... What is the swing motor suction valve? I see suction valves for several of the circuits in the main control valve. But not in the swing circuit.

Found nothing wrong with the reverse prevention valve.

"Swing motor check valve"
The check valves in the swing motor have some pitting to them where they contact the seats. It doesn't look like enough to cause the problem. I have new check valves and plan to replace them next time I get next to the machine. I would replace the springs as well, but the part number in my book and in the dealer's catalog does not seem to exist. The spring has the correct dimensions, so if anything I might try to shim the spring up just to see what happens.

"When swing PPC slow-return valve, swing PPC line, and swing PPC circuit choke are cleaned, is trouble repaired?"
If you have read my previous posts on this machine, you know that I have exhaustedly cleaned the hydraulic system.
I do not know what the PPC slow-return valve is. Ditto on the PPC circuit choke.

I have to say, I have missed this forum for the last few days. Glad it is back.
If anyone can help translate this Komatsu-ese, I sincerely appreciate it.

 

[Mobilewrench]

What have you positively eliminated on the troubleshooting chart??

Almost nothing, other than the cleanliness of the system, and maybe the spool in the control valve.

 

[Vetech63]

Well, first off, by what the chart is telling you it is making you a parts changer. You really need one of these Komatsu gurus that has access to a schematic for the machine....but since the site has been down some of those guys may not check in anytime soon. I can ask a few basic questions that may help us on the forum narrow this down.

1. Is the problem in the swing circuit alone? Does anything else act out of the ordinary?
2. Is there any type of error codes display, active or inactive?
3. Does the brake set once the movement stops?
4. How much over run are we talking about here?
5. Is this the exact same problem as before?
6. Has there been any work done on the machine recently?
7. Has the pilot pressure been checked and compared to the other circuits versus the swing circuit?
8. Filters been checked or changed?
9. What service technical info do you currently have?
10. Do you have the proper gauges to do the troubleshooting?

You can extend the stick all the way out, boom down with the bucket teeth down, and stab the teeth into the ground. This will usually stall the swing in most cases.
My uneducated general thoughts are that this is something that is sticking open, could be from contamination or severe wear. The pilot system seems to be working since you have swing in both directions. The brake is just a hold brake I believe and doesn't come into play until the swing is almost at a complete stop. Most systems I see have a counterbalance system in a block on top of the swing motor that uses hydraulic forces to act against
each other to slow the machine swing once you let off the joystick (its a bit more complicated than the description I just gave). If I were working on this without any tech assistance, I would be looking in the vicinity of the motor itself and how that system works. Since this is happening in both directions it makes me wonder if the hydrodynamic braking of the system is leaking oil internally.

Hopefully a Komatsu God will stop in and tell us more, or tell me I'm full of S*&T. LOL

 

[John C.]

I've never had the occasion to put hands on an MR so can't steer you to anything but general type stuff that has been stated above. The book that I have in my computer is a PC78US which I believe is different. You might check and see if the machine has a free swing function. A photo of the top of the swing motor might be a help. By your description of the problem going away and then coming back suggests a floating gremlin in the system somewhere. Most of the time for me if the problem is only in the one circuit, I'll take that component off and dismantle for inspection.

 

[Mobilewrench]

 

[Mobilewrench]

There is no free swing function. It also has no trouble codes.
The swing parking brake functions as it should. It engages about 5 seconds after you let go of the joystick. The upper structure just swings with gravity or inertia until it engages.
All other functions are top shape.
I have removed and inspected the reverse rotation prevention valve, the relief valve and all of the check valves on the motor.
The service manuals for the 78mr-6 and the 78us-6 are pretty much identical. All of the information for the swing function are word for word the same.

 

[John C.]

I looked over the explanation and studied the structural drawing of the US and UU machines and it looks exactly the same as your photo. The only things I can see that might be a cause of the run on are as follows;
1. The motor relief has something worn out, stuck or broken inside. I would run the stick all the way out and open the bucket all the way up and try to find something for it to push against while it sits on the ground. Check the relief valve pressure in both directions. It should be around 3,000 PSI.

2. The rotating group is worn out. Check case drain pressure on motor at full relief. Your book will tell how and what the spec is. It think it says to run the function over relief for at least thirty seconds and then run another minute and measure how much leakage you got? I might be interpreting that wrong. Usually if there is a problem, the drain flow will be considerably more than a garden hose puts out.

3. The last issue would be a problem in the supply valve from the main valve block. I believe the swing and blade pump is a gear pump so the control valves for those function are open center. The schematic is pretty straight forward in that the only way there could be a problem there is if the valve body were broken. That would certainly show up as low swing relief pressure.

Let us know if this helps or not.

 

[Mobilewrench]

I don't have a bucket on this machine at the moment. It has a flail mower mounted. But I can probably just butt the boom or stick against a tree to check the relief. But I haven't identified a test port on the swing motor. I will probably just tee into the pressure lines.
Thanks for the advice John C.

 

[John C.]

You should just be able to plug a gauge into a port plug by the pump. Usually they are on the outlet casting bolted to the pump. Plug will be an 11/16 wrench that has an O ring.

 

[Mobilewrench]

Thanks again JohnC. But wouldn't that be reading the main relief for the gear pump? I kind of assumed that since the swing relief cartridge was down stream of the main relief that the pressure would be set lower. But I haven't checked the numbers in the book in a while.

 

[John C.]

It's a load sensing system. If everything is working right, the main relief will only open when something hits the machine like a falling tree on the boom while it is being raised or lowered. The pumps will normally destroke at a lower pressure and hold it there. The swing pressure will be lower than the rest of the functions.

 

[Mobilewrench]

Yeah, I am a bit embarrassed about that. I woke up this morning and logged straight on hoping that no one had read that last bit.
Maybe that will teach me a lesson about drinking and posting.

 

[Mobilewrench]

2. The rotating group is worn out. Check case drain pressure on motor at full relief. Your book will tell how and what the spec is. It think it says to run the function over relief for at least thirty seconds and then run another minute and measure how much leakage you got? I might be interpreting that wrong. Usually if there is a problem, the drain flow will be considerably more than a garden hose puts out.

 

[Mobilewrench]

You called it. Went back this morning to double check everything that I had already.
Checked the relief on the swing motor and it seemed right by the guage, but it didn't sound right, couldn't actually hear it relieving.
Did the case drain test and it failed spectacularly. Book says to run for one minute and I should see 3.5 liters (service limit is 7 liters). Filled up a five gallon bucket in about 20 seconds.
I don't remember ever doing a case drain test on a swing motor before. I have done several case drain tests on travel motors and have never seen this kind of result. Kind of wondering how this thing even swings. But I suppose that track motors work a lot harder than swing motors, so the actual "stop moving" symptom happens a lot earlier.
Thanks again JohnC.
Pretty sure this is figured out. But I will make sure to post back when I change the swing motor.

 

[Krd96]

Hi Mobilewrench,

I have been down this same road with a PC78MR recently. Had same failing results on case drain test and stumbled on this thread while trying to source a new motor. Dealer cannot find me a motor and I am striking out at independent suppliers. Would you mind sharing where you purchased your motor from? Thanks.

 

[Mobilewrench]

I ordered mine from the dealer the day I posted last. Just got it and installed it last week. So they should be available again.