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catmastertech, Technician
Category: Heavy Equipment
Satisfied Customers: 2219
Experience:  19 years Cat, 3 years Case
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I have a Mitsubishi MM45 mini excavator that has lost all hydraulic

Resolved Question:

I have a Mitsubishi MM45 mini excavator that has lost all hydraulic functions on the right side of the machine. The left side functions work just fine. I had the hydraulic pump tested and it checked out fine. My question is what could be causing this power loss?
Submitted: 7 years ago.
Category: Heavy Equipment
Expert:  catmastertech replied 7 years ago.

I see your question is getting old and no one has answered. I will try to help, if your machine is the equivalent of a 301 CAT. The systems are shared on many small machines but, not exactly the same. Cat has a straight travel valve and a combiner valve. These valves change flow from the pumps and if they stick, they could block flow.



1) Bank valve. (2) Valve for the boom swing. (3) Valve for the bulldozer blade. (4) Swing valve. (5) Combined flow valve. (6) Valve for the auxiliary hydraulic circuit. (7) Stick valve. (8) Valve for the right piston motor (travel). (9) Straight travel valve. (10) Valve for the left piston motor (travel). (11) Boom valve. (12) Bucket valve.



The stack maybe standing up on some models but layout should be the same. Remove and inspect for damage spools 5 and 9. If they only have minor scratches, just polish and reassemble. If damage is severe, you may need to replace valve and find source of contamination.


If you need more info, include as much detail in your reply to help me understand if the systems work the same.


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Customer: replied 7 years ago.



I did see where a MM45B was the equivalent of a Cat 304.5 (but ours is a MM45). I have this unit down on the farm where I was doing some ditch work. My plan is to follow your instructions this weekend and check the combined flow valve and the straight travel valve. If the system does not match your diagram then I will try and take a pic. As far as I know there is only one hydraulic pump that feeds all the banks and as I mentioned in my first note it tested fine. Again all functions on one side of the machine do not seems to point to that comibned flow valve. Thanks for your suggestion



Expert:  catmastertech replied 7 years ago.

The main relief is a possible source for you failure as well. This is different for the larger machines were they have one relief this on has two. Switch the reliefs from the location 1 and 3 in the drawing. The single relief on the end is for swing and has a different setting.




Main control valve

(1) Main relief valve (P1)

(2) Main relief valve (P3)

(3) Main relief valve (P2)

The main relief valves are attached to the main control valve. The settings of the main relief valves for ports P1 and P2 on the main pump are 24500 ± 500 kPa (3553.48 ± 72.50 psi) at a flow rate of 45 L (11.9 US gal). The setting of the main relief valve for port P3 on the gear pump is 20600 ± 500 kPa (2987.82 ± 72.52 psi) at a flow rate of 42.5 L (11.2 US gal). The relief valves protect the hydraulic circuits from damage that could occur from pressure that is too high.

I hope this helps,

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