Have a problem with a hydrostatic system that is comprised of a variable displacement pump and two fixed displacement motors. Wont operate in either direction and have established low charge pressure. What could cause this? Adjusting charge relief valve made no difference. Thanks for any help in advance.
Restriction to the charge pump or too much internal leakage (worn out) in pump Are there bubbles or foam in the case drain line back to the tank coming off the pump?
What make and model of machine would be an important consideration before any suggestions could be made as to what is wrong.
Thanks guys. No sign of air in the return oil. Hadnt thought of a restriction. Machine in question is an Elgin Pelican street sweeper with a Sauer Sundstrand system. Have had one with damaged internals in the motors due to improper towing but it wouldnt develop any charge pressure. This one will fluctuate between 100 to 200 pounds depending on engine speed.
200 pounds does not seam that low. Is the fluctuation rapid or smooth and what are the conditions
Many times the charge pump oil is used for other circuits as well. Such as brakes, steer, cylinders, etc. Before condemning the charge pump, check what other circuits are connected to it. A hydraulic schematic is worth a trillion to determine where charge flow may be going. Capping off external circuits and then checking charge pressure may reveal leakage at a remote point.
No this has all seperate hydraulic circuits for other machine functions. Book calls for 300 psi at idle and I checked its brother and it goes from 300 up to 400 at high idle. Pressure does not rise smoothly but is somewhat jerky as it climbs.
To determine where you are losing charge pressure you must seperate the pump from the motors by plugging off the propel hoses that go to the motors. make sure the charge pressure relief is close to where it was before it was turned in. Now crank the machine if charge pressure come up malfuntion isw in the motors, if not it is in the charge pump or drive pump. Charge pump can be located inside of drive pump or it can be external. Can help with further diagnosis if you get to this point. dead heading the drive pump will not hurt it as it is internally regulated. This is a closed loop system so if any failure is found that released metal into the system it all has to be disassembled and checked for contamination , both motors, pump propel hoses case drain line(s). My experience has been that sauer pumps are good and the motors are not. Likely if you engage the travel the charge pressure will drop to nothing, and you will find a piston ring blown off in one of the motors. If the test shows it to be in the motors it can be isolated by seperated case drain test on the motors. the one that has fluid coming out like a fire hose will be the one that is bad. Should only be about 2 gal. a minuted out of case drain. Hope this helps
I believe the Elgin Pelican used the series 90 pump and series 51 motors. Here are the links to the service manuals at Sauer Danfoss for those components: There are troubleshooting and diagnostic guides included. http://www.sauer-danfoss.com/stellent/groups/publications/documents/product_literature/520l0603.pdf http://www.sauer-danfoss.com/stellent/groups/publications/documents/product_literature/520l0440.pdf
Wow! Thank you guys!! Was curious about isolating pump from motors. Definitely gives me something to go forward with. Kudoos to the great members of this forum
Did you find your problem? If its a 90 series pump it has multi function valves and charge reliefs built into the pump. You can use the test procedure given to you by partsandservice. Is there a loop flush valve in the system?
Yes one of the motors had failed. Determined by isolating one motor and then the other. Motor was replaced and the failed one to go in for service yet. Unsure of exact cause at this time. Thanks to all for help . Great forum here.