Komatsu pc120-6

[mattk]

Guys, I've only had this digger for 6 months, it hasn't done much work in the last 10 years. When I first got it, sometimes the hydraulics would operate at half speed even though the engine was still running at full throttle, but it would only do this for a second or two. It went away after some use and I assumed something was sticking and had freed up.

Today the machine would jam it's self in half speed for minutes (arm and tracks would both operate at half speed) at a time with the occasional split second of full power, the engine was running at full throttle in power mode and sounded normal. No amount of restarting, pushing buttons would fix it.

Any ideas of where to start looking would be appricated as I need this machine going.

Thanks

 

[heymccall]

The main system relief cartridge is designed far more pointed than I believe it should be. So, picture jamming a #2 pencil repeated into a 3/16" hole. Eventually, the pencil will form a wear ring on itself , and stick, and the valve works the same way. So, if you have full engine power and limited hydraulics, I'd suspect the main relief cartridge has beat itself to death. The valve is disassembleable for inspection, but be sure the to count every thread turn and even measure the adjuster protrusion so that you can set it back to the same value if it's still serviceable (not worn out).

As for trying to overcome it if it's sticking, you need to force it open. Open the bucket 90%, and with the arm 90% of the way extended, beat the bucket teeth off the ground. Because the cylinders aren't fully extended, you will spike the hydraulics, forcing the main relief to open. A couple of "shocks" like that will usually, temporarily, "cure" the symptoms.

 

[John C.]

The Dash 6 hydraulic system is a closed center load sensing type and the main relief will only actuate on the travel function or when you use the power up function button with the top button on the left joy stick. The pump will destroke to minimum flow at a lower pressure than the main relief. The more likely issue in with the load sense cartridge. There were several main control valves used on PC120-6 machines. As I recall early machines had the cartridge in the bottom of the main control valve. That cartridge was next to the main relief. Later on the main relief and load sense were incorporated in the same cartridge. Still later on the load sense valve was taken out of the main control valve and installed on the side of the pump. A photo of the main control valve and the pump would be helpful.

Beating the bucket into the ground will just pop open the circuit reliefs. I would be very surprised if it solved anything and as old as a Dash 6 machine is, you might very well break something else.

Unfortunately you will have to find someone local that has been into those systems. Your only other option is to start guessing at which part to replace and that is very expensive.

 

[mattk]

Guys, thanks for the advice. I'll have a look for the main relief cartridge tomorrow and inspect otherwise it looks like a job for a merchanic, will see if I can find somebody familar with this machine.

Thanks agian, will let you know what happens.

Matt

 

[mattk]

John C you were right. I spent a few hours on monday looking at things and found a merchanic ex Komatsu. He came out today. The digger I have is an early one and the load senseing valve is on the pump, the shaft between the solinoid and the working inside of the valve had very minor scoring on it, we cleaned this up, reasembled and it went for 4 hours without fault. Apparently this was a common fault with the early pumps.

Thanks again John C and heymccall.