Bobcat 742 no charge pressure to hydrostatic pump

[Dmc10140]

This is a continuation of my no movement thread. After rebuilding the hydrostatic pump and putting the machine back together I had little to no movement from the wheels. The machine sat for several years. The charge pressure relief valve was inspected during re assembly of the pump. When I removed the charge pressure sensor to install the manual gauge no hydraulic fluid leaked out which I thought was weird. The gauge reads zero at all Rpms. I have replaced the fluid and filter (bobcat brand). What else should I look at? All other hydraulics functions seem to work ( tilt and lift) so I would assume the hydraulic pump is in Good shape. Thanks for the help.

 

[Tazza]

That is an odd one.... Is this an older style machine with a square block on the hydraulic pump with a bunch of hoses coming from it?
Wondering if there is an issue with a hose in the wrong spot.

 

[Dmc10140]

That is an odd one.... Is this an older style machine with a square block on the hydraulic pump with a bunch of hoses coming from it?
Wondering if there is an issue with a hose in the wrong spot.

Yeh I thought so as well. Unfortunately it is the newer setup without the bronze filter. I can re check the hoses with a drawing, sorta hard to mess up I believe due to the sizes. These types of issue come with taking over someone else's projects. Would a stuck pressure relief cause no charge pressure?

 

[Tazza]

Yeh I thought so as well. Unfortunately it is the newer setup without the bronze filter. I can re check the hoses with a drawing, sorta hard to mess up I believe due to the sizes. These types of issue come with taking over someone else's projects. Would a stuck pressure relief cause no charge pressure?

It can be interesting working on other peoples machines.
With a gauge installed into the sender port on the pump, so you get any pressure here? do you get any oil coming out of this hole?
The only other thing i can think of is a plugged filter or oil cooler, as this is the path the oil takes from the control valve, oil cooler, filter than port block and back to the pump inlet.
Another possibility is the port block, the alloy block with the fat hose, something could be stuck.

 

[mmsllc]

It can be interesting working on other peoples machines.
With a gauge installed into the sender port on the pump, so you get any pressure here? do you get any oil coming out of this hole?
The only other thing i can think of is a plugged filter or oil cooler, as this is the path the oil takes from the control valve, oil cooler, filter than port block and back to the pump inlet.
Another possibility is the port block, the alloy block with the fat hose, something could be stuck.

check for blockage at bottom of hydraulic tank. It might be full of oil, but blocked in a way that no oil gets to your pumps. If it is a steel tank, it could be rusting in a way that it has blocked fluid flow to your pumps!

 

[Dmc10140]

It can be interesting working on other peoples machines.
With a gauge installed into the sender port on the pump, so you get any pressure here? do you get any oil coming out of this hole?
The only other thing i can think of is a plugged filter or oil cooler, as this is the path the oil takes from the control valve, oil cooler, filter than port block and back to the pump inlet.
Another possibility is the port block, the alloy block with the fat hose, something could be stuck.

When i removed the pressure sender on the top of the hydrostatic pump to install the gauge my oil came out of it which i thought was weird, but yeh i have no charge pressure. I think my plan of attack should be to remove the lines in order backwards from the pump to see if I can find the blockage. If it gets to the port block i may have to remove it to inspect it, it seems like that could be the easier way to get a good look at it. I believe the port block also has a cooler bypass valve that may be an issue. The filter is brand new so hopefully that is not it.

 

[Dmc10140]

When i removed the pressure sender on the top of the hydrostatic pump to install the gauge my oil came out of it which i thought was weird, but yeh i have no charge pressure. I think my plan of attack should be to remove the lines in order backwards from the pump to see if I can find the blockage. If it gets to the port block i may have to remove it to inspect it, it seems like that could be the easier way to get a good look at it. I believe the port block also has a cooler bypass valve that may be an issue. The filter is brand new so hopefully that is not it.

Ok, i had some time to look at the machine. I decided to remove the port block for inspection after i found little fluid in the cooler. As it turns out the poppet for the cooler bypass valve was missing. This allowed the fuided to escape to the drain. I was able to order a poppet, the charge pressure is now at 100psi, and the machine is moving on its own. I still have some leaks and such to deal with, but thank you for the help.

 

[Tazza]

Ok, i had some time to look at the machine. I decided to remove the port block for inspection after i found little fluid in the cooler. As it turns out the poppet for the cooler bypass valve was missing. This allowed the fuided to escape to the drain. I was able to order a poppet, the charge pressure is now at 100psi, and the machine is moving on its own. I still have some leaks and such to deal with, but thank you for the help.

Glad you found it, this is one of the fun things of working on a machine that you buy not working....
I actually bought a machine at auction years ago that we believe was messed with by someone that worked there to try and get it cheap. Naturally it was not working, the starter was even removed. The inejctors pump had the springs at the top of the delivery nozzles removed, so you would not get any fuel pressure. I had a donor pump that i took springs from and good injectors. So it didn't cost me a cent to get going again

 

[mike_belben]

Glad you found it, this is one of the fun things of working on a machine that you buy not working....
I actually bought a machine at auction years ago that we believe was messed with by someone that worked there to try and get it cheap. Naturally it was not working, the starter was even removed. The inejctors pump had the springs at the top of the delivery nozzles removed, so you would not get any fuel pressure. I had a donor pump that i took springs from and good injectors. So it didn't cost me a cent to get going again

Ive been dealing with exactly this issue on a basket case 742 for a year now. prior owners and their hodgepodge. Cant get over 40psi charge pressure. Refurbed the charge pump, reseated all valves, bought new oem springs at quite the ob$olete premium. Im gonna try shimming the "non functional" charge relief up at the high pressure pump inlet. After that im at a loss.. The loader works fine and i guess next id have to dig into the tandem pumps just to feel good about them.

 

[Dmc10140]

Ive been dealing with exactly this issue on a basket case 742 for a year now. prior owners and their hodgepodge. Cant get over 40psi charge pressure. Refurbed the charge pump, reseated all valves, bought new oem springs at quite the ob$olete premium. Im gonna try shimming the "non functional" charge relief up at the high pressure pump inlet. After that im at a loss.. The loader works fine and i guess next id have to dig into the tandem pumps just to feel good about them.

Mike, My issue was at the filter bypass poppet on the port block. If there is a problem there you will bleed charge pressure. Just another thing to look at. My biggest problem now it addressing all the remaining leaky hydrostatic hoses $$