Hi All, I have been browsing this forum for a little while now looking for solutions to my problem, but haven't found one yet. I am working on a Hitachi EX120-3 and am having issues with the pump flow control. When I start the machine, the pump seems to be stroked to minimum As I start working the machine, the pump seems to slowly increase flow until it reaches max, and then stays there. If I shut it down and start it up again, the pump will start at min flow and then creep up again. At that point the excavator functions are flying and if I deadhead a function it stalls the engine. What I have checked so far: The pump angle sensor has 5.3VDC going in and 1.5VDC coming out. As I run the machine, I observed the voltage begin to rise to 4.9 volts. I then checked both stroke control solenoids on top of the pumps. They have 28VDC on the positive side. The voltage drops off as I begin to move the boom, and that is when the pump starts creeping up the stoke. It is my understanding that the solenoids are controlled by the computer by pulsing the ground, so the positive should always have voltage. Correct? Also, how sensitive is the computer to the battery voltage? Is the 28VDC too much for a 24VDC system? What is causing the power to drop off the solenoids. It then comes back after shutting down and restarting. Any suggestions to what I can check next. I want to try and confirm what is wrong before buying parts. Also if anyone has an electrical schematic I would appreciate it. Thanks in advanced!
I just had this same issue. Ended up being the computer behind the seat had a burn spot underneath it. Had to take it apart to find it.
28 volts is not too high on a 24v system. With the pump at maximum displacement, adjust the angle sensor position to achieve output voltage of 4.35 to 4.45 volts. If 24V is actually dropping off at the two pump solenoids check that the pump control relay is closed. @LACHAU posted a link to a EX120-3 tech manual with electrical schematics in the thread below, see post #30. .