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1995 pc50uu2 excavator boom wont lift up

707pc50

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Apr 19, 2010
Messages
123
Location
california
Excavator will not lift boom, my mechanic cant figure it out, engine runs fine tracks drive fine, all functions work except one direction on the off set boom, and the boom will not lift, but if you turn machine off, pull lever up and start machine it will move a couple inches, and then stop? We test wired around the safety lever made no difference. The control lever that does the boom raise function has been very sloppy feeling as long as we have owned the machine, but has never effected operation? Any ideas we have check everything we can think of?

Thanks
 

willie59

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Dec 21, 2008
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Knoxville TN
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The boom offset function that won't work, would it be the function that swings boom toward operater cab?
 

707pc50

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Apr 19, 2010
Messages
123
Location
california
two functions that dont not work are the offset boom in one direction (this machine has an "knuckle" in the mid boom that allows you do swing the boom over to one side of the machine so you can dig close to walls or objects that you wouldn't normally be able to get close to due to the tracks being in the way) the boom also will not lift back up off the ground? (except for a short instsance when the machine is first fired up as described in original post.

thanks
 

707pc50

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Apr 19, 2010
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Location
california
I cant recall which direction of offset does not work, it is controlled by a two direction pedal on the cab floor. I can check tomorrow?
 

willie59

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I cant recall which direction of offset does not work, it is controlled by a two direction pedal on the cab floor. I can check tomorrow?

If the problem is it won't offset the boom toward the op cab, I'm betting you have a problem in the cab protection system. It keeps an operator from accidently crashing cab with the bucket when the boom is in the offset position. The system is controlled by three potentiometers. One for boom offset position, one for stick/arm position, and one for boom position. You want to check each potentiometer to see if a tree limb or something has knocked the control linkage loose. Check all the wiring going to the potentiometers for any damage. Disconnect the harness connection to each potentiometer, and with engine running, check the voltage supply to each pot with voltmeter. Check the ohms value of the opposite wire of harness back to ground and see if all the harnesses have relatively the same ohms reading. You might also be able to check the ohms resistance of each of the pots and see if there's one that is way out of range from the other pots which would indicate a faulty pot. A faulty pot, or any fault in the system, would lock out the boom up, stick in, and offset left functions.
 

707pc50

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Apr 19, 2010
Messages
123
Location
california
Thanks for the tip, I will pass the word on to the guy who is helping me work on the machine, do you happen to have any advice on testing these potentiometers? Maybe my mechanic will have some knowledge on this, but I wouldn't know where to start. Also where would these be located on the machine?

Thanks
 

willie59

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Service Manager
There should be 3 potentiometers. The one for the arm/stick will be located near the pin that attaches arm to boom. The pot for the offset will be on top of the boom where the offset pivot pin in the boom is located. The pot for the boom, you have to remove that curved cover on the right side of the machine, unless your curved cover is designed as an access door and simply raises up. The boom pot is mounted unerneath that cover. They are all rather easy to find as they are fairly large components with linkage rods and control arms fitted to them. I haven't worked extensively on these components on Komatsu machines so I can't offer much in the way of testing procedures. I only know that if your machine won't boom up, arm in, or offset left, your problem is in that cab protection system.
 

707pc50

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Apr 19, 2010
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123
Location
california
Thanks very much, you nailed it on the head, great call, you just saved me some serious headache.
 

707pc50

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Apr 19, 2010
Messages
123
Location
california
yeah found a broken wire going to the pot on the boom. patched it up and it works like new, thanks very much
 

grapeape

New Member
Joined
Aug 22, 2010
Messages
1
Location
Montana
Pc 57 uu-2 has a similar problem.

I have a 1994 pc75uu-2 that has the depth control monitor on the guage panel. The guage doesn't work all the time and the excavator locks up unexpectedly. If I shut it off and turn it on it will work again. I was wondering if there was a way to bypass the system eithr electrically or hydraulically? The guage panel is $2000 to replace and I have never used the depth guage anyway.
 

toddjaeger

New Member
Joined
Dec 16, 2017
Messages
2
Location
72653
I now have the same problem
ive checked voltage and resistance
the pods seem to check out
happened twice before but had found broken wires
im lost at what to do now
 

Batkom

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Joined
Dec 24, 2017
Messages
202
Location
Idaho
New here, but I also have a 1996 PC75uu-2 that I recently bought. Have had a Hitatchi 50 and recently sold my 2014 John Deere 60g. Could not justify having that new of machine just sitting with my current needs reduced.
To Grape Ape
So bought this really ugly Komatsu!
So to the point of this thread. One of the previous owners has completely disabled the protective and depth controls on this machine. And he was obviously an idiot as he also crashed the bucket into the cab multiple times from the looks of it.
I don't mind it being disconnected- but it seems to also have disabled the safety lever / disconnect you lift to get out of the cab.
I don't like this- that cab is small and it would be easy to snag a joystick on the way in or out of the cab.
So I bought a service manual and have been tracing wiring. Reconnecting things and seeing what happens.
Where I am currently at is I have the wiring connector that goes to the boom and all the potentiometers and light on the boom, disconnected. This is located above and to the left of the batteries on my machine.
Then I have reinstalled the three fuses he had taken out, which made the buzzer sound - so I disconnected that-its under the seat.
Then under where your feet are when running the machine there is a plate. Under it there is the hydraulic solenoids that go to all the functions including the disconnect(I think they call it the PPC)
So I have plugged the PPC one in that was disconnected and the swing brake one.
So far the machine digging functions all work - but that still did not fix the Lockout.
So there are still all the dig functions solenoids unplugged down there.
I have not had a chance to get back to it and plug those back in to see what happens next- but my guess is that the PPC lockout will work then, but that it will also lock up the ability to actually dig with it.

My guess is that if you disconnect the solenoids I have not connected, you will be in business.
Again these are all located under the floor board.
Good luck!
Sorry this was awful long!
 
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