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Dan, Technician
Category: Construction and Road Equipment
Satisfied Customers: 5268
Experience:  17+ yrs. shop and field experience with all major brands of heavy equipment and trucks - 1995 WyoTech graduate.
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I have a bobcat mini excavator with hydraulic drive/pump

Customer Question

I have a bobcat mini excavator with hydraulic drive/pump problems
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Heavy Equipment Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Heavy Equipment Mechanic can help you best.
Customer: the left track drives normally, the right track does not go at all. All of the other hydraulics are very low powered. One of the lines on the primary pump has air in it.
JA: Is there anything else important you think the Heavy Equipment Mechanic should know?
Customer: that's it for now
JA: I'm sending you to a secure page on JustAnswer so you can place the $5 fully refundable deposit now. While you're filling out that form, I'll tell the Heavy Equipment Mechanic about your situation and connect you two.
Submitted: 1 year ago.
Category: Construction and Road Equipment
Expert:  Dan replied 1 year ago.

Hi Paul my name is***** hydraulics being effected is not a good sign and points directly to either a main pump failure or a faulty or sticking relief in the main control valve. Check the filter for fine metal or debris that would indicate a pump failure. It will look like fine metallic glitter. If clean then inspevt the control valv and relief cartridges for damage or debris. Let me know

Customer: replied 1 year ago.
The filter is clean as can be. Where is the control valve and relief cartridges? Is that the block on the pump? If so, there is a plug on top and two bolts on the side. The bolts require a thin wall socket which I don't have..One correction - the left track drives forward and backward just fine. The right track is completely dead (sounds like a power steering pump no_fluid_whine when actuated) and all of the other functions are very low powered.If I press in the end most pump output "schrader" like valve, it is full of air bubbles. Thats the one furthest from the engine.
Customer: replied 1 year ago.
If the "cartridges" are the spring loaded parts on the control valve, they look very clean and move freely.
Customer: replied 1 year ago.
It seems to me that the outer most pump section drives the right side. That pump is full of air bubbles. I've bled a gallon or so through it but still no improvement. I managed to remove one of the check valves (the lower one) and it was clean and moving freely. The upper one won't accept a socket...The supply line for the outer pump comes from a valve on the left front compartment (all the way forward, all the way outboard to the left). It seems that the problem is an insufficient supply of hyd. fluid. The pump seems to get dribs and drabs but not a full flow supply. Is there something that would restrict the flow to the pump?
Expert:  Dan replied 1 year ago.

What's the complete model and serial number?

Customer: replied 1 year ago.
Bobcat model 331 C series. S/N###-##-####
Expert:  Dan replied 1 year ago.

Thanks for the info. The hydraulic tank has a suction screen that is restricted would restrict oil to the pump. Never remove a suction screen with the system full of oil as all contamination will go straight to the pump. Check the hydraulic filter for signs of fine metal/debris indicating a pump or component failure occurring. If contamination is found then the failing component needs to be identified and replaced. The online parts catalog is available online on the bobcat site to hel;p you identify the system components and locations. Let me know

Customer: replied 1 year ago.
As I said yesterday, we changed the hydraulic filter and it was clean as can be. It looked brand new.Where is the source for the right track pump? It is not fed directly from the tank like the left track is. As I said before, the supply hose comes from under the cab. I'm thinking something there is restricting the supply side of the third pump.Paul
Expert:  Dan replied 1 year ago.

Hi Paul. The pumps are gear type fixed displacement pumps. I there is no restriction or leak sucking air on the suction side and the pump is not picking up oil it means the pump is bad. The two inner pumps are for travel and split between functions and the last is your charge pump. disassemble the pumps ans inspect the gears, seals and housings for excessive wear and internal scoring. If so the pump assemble needs to be replaced. Let me know

Expert:  Dan replied 1 year ago.




Dual Outlet Piston with Gear

Pump Capacity


2 x 10.1 GPM (38,4 L/min)

System Relief Pressure


3480 PSI (270 Bar)

Auxiliary Relief


2600 PSI (180 Bar)

Auxiliary Flow


15.9 GPM (64,0 L/min)

Control V



(1) 10-S

pool, closed center

Drive Motors



Axial Piston Motors

Swing Motor


Orbit Motor

Slew S



9.2 RPM

Customer: replied 1 year ago.
What does the term charge pump mean? What circuits are affected? The charge pump section was repaired once before. It may well have failed again. It was not replaced...
Customer: replied 1 year ago.
I'm starting to think the pump repair was installed improperly. Should the two primary pump sections feed the drive motors? The hoses may be reversed. The charge pump is feeding the right and the right drive is feeding the charge circuit. That would explain low power everywhere...
Expert:  Dan replied 1 year ago.

Charge pressure is standby pressure for your controls etc which is a lot lower than the main pressure for you cylinders, drive motors etc. Here is a picture from the parts book showing the hose routing. Check to see if it looks like the way yours is connected.

Expert:  Dan replied 1 year ago.

Yes the two main pumps should feed the drive motors and major functions. Once that type of pump fails, they are not usually rebuildable as the machined housing and gears are usually worn out of specs.

Customer: replied 1 year ago.
The pump looks similar but the connections at the control valve look completely different. Is this for the right serial number? Also, can you give me the sheet with numbers and identification such as number 19 is such and such?
Expert:  Dan replied 1 year ago.

Hi Paul. You can access the same manual actually online at

Expert:  Dan replied 1 year ago.

Or if you go to you can find the parts link on the bottom right of the website

Customer: replied 1 year ago.
Hey Dan,I have fooled around for the past hour trying to get a web browser to view the bobcat CGM parts pages. No go. So, can you please send me a picture/attachment or two of my specific hose routing and I should be able to figure it out. There may be another problem as a result of the high pressure feeding the charge lines...Thanks,
Expert:  Dan replied 1 year ago.

My Laptop was giving me troubles as well. I saved that diagram but it wont post. You need that arbor view to be able to see the files and internet explorer is the recommended browser. I will see if I can get it to post that picture but it allowed me to save the first one but not the second for some reason

Expert:  Dan replied 1 year ago.

The last diagram is the only one showing tank to pump, etc. The service book will have more in depth schematics. I do not have the service book for your serial number machine but I found a PDF manual for download for under 10 dollars.


Customer: replied 1 year ago.
I bought the manual but there isn't a drawing that clearly shows the hose routing. I moved the hoses at the pump so that the to high pressure lines feed directly to the control valve, both feeding in right between the two drive control valves. That made the right tract move a little but all other functions (blade, arm, etc.) are completely dead. I don't have a way to measure the pressure but the two high pressure pump sections seem to have lots of pressure (relative to the charge pump). I'm not sure what to do now.
Customer: replied 1 year ago.
Apparently, the third section of the pump is for the right drive motor (which is different than a charge pump). The third section of the pump is clearly not working properly. I would think if there was a crack sucking air, it would be dripping fluid as well. I don't see anything like that.I'm thinking the pump may be bad and that the hoses were indeed hooked up correctly from the start.
Expert:  Dan replied 1 year ago.

Hi Paul. I agree that all signs point to a failed pump section. These are replaced as a complete assembly though. I still wanted to try to address the concern you had that the lines were hooked up incorrectly. It would be difficult since the diagram shows the two inner with the suction lines tied in together from the tank. If the machine has been working normal the way is is hooked up and recently developed this problem then I agree that you are looking at a pump problem.

Customer: replied 1 year ago.
Dan,The serial number I have is plumbed slightly differently than the one you have been referencing in that the 3rd pump section (furthest from the engine) is indeed the right hand drive pump (fed internally from pump 1's supply). Pump 2 is the left hand drive and pump 1 is the series slew/blade/boom supply. At this point, I'm convinced replacing the pump is the correct next step. The equipment was running OK but with low power everywhere but the left drive. It was parked for a while then never came back to life, so to speak. I am just trying to help a neighbor get out as cheap as possible. He can't afford the dealer. Thanks for your help.
Expert:  Dan replied 1 year ago.

I agree that you are looking at a pump issue. Id remove the pump and take it apart keeping the sections in order. Inspect to see if the gears have contacted the housings at all and wore into them creating too much clearance to produce enough GPM of flow. Also the bushings for the gears if worn will allow the gears to run loose and contact the housing. In both situations, once the gears have scored and worn into the housing, that pump section is no good. There are sealing kits and thrust plates available for any pump that has not worn to this extent. Just be sure not to waste your time trying to repair a section that shows scored gears or housings. Pleasure to help

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