Diagnosing Low Pressure in a Hydraulic System

Low system pressure usually does NOT mean a bad hydraulic pump. In this video, instructor Hank Ayers demonstrates how to check the pump compensator valve. Check our website at www.GPMHydrauli... to learn about our two-part training program.

[@raulmarin1544]

Thank you so much,very helpful. Straight to the point.

[@award795]

Thank you very much for recording this video. I’ve found very few others that are actually helpful. I have a bobcat 863 that the auxiliary hydraulics lose pressure as the machine warms up. Any specific recommendations for this problem? Greatly appreciate your time.

[@shhs6127]

Did you find answer to your problem, a have similar .. lose power - pressure slowly... If engine is running or not... Boom will come to ground slowly...

[@jrac863]

Having a problem with our mini excavator.All hydraulics were slow on the machine it was down about half the usual power output.Blade barley picked machine off ground.Replaced all filters flushed all hydraulics and put in a new pump .Still same situation.Funny thing is when u first start machine for a few minutes machine has normal hydraulic power but once oil hydraulic oil warms up seems to lose power.My question is could a worn out main relief valve cause this .Or if a section spool valve was not fully closing on the control valve be the culprit.On our control valve there are a many relief valves that control different section of the excavator

[@JackWeeks]

Thank you for your question, Jay! Yes, indeed a worn relief valve could cause this - or a directional valve. Either could be bypassing such that it doesn't show itself while the oil is cold, but becomes apparent once the system warms and the oil becomes thinner. One easy way to detect what is leaking is to isolate different portions and see which will not hold pressure with a hand pump. You may also have some good results by using a thermal imaging camera to detect any bypassing. Look for hot spots around components in areas where they should not be. Hope this helps!

[@anythingunderthesun5586]

I need to save this video

[@JackWeeks]

So glad you found it helpful!

[@shhs6127]

Helou i have mini digger libra 216S.. it loses presure slowly... When i lift boom it wil came to ground slowly when engine is running or not.. What do you think problem is ??? Thanks..

[@akhilskumar4195]

Nice video simply presented

[@mustaphawahbeh]

kzbin.info/door/_GbYSHgKJL3g4J1aZ-RBSA

[@jrac863]

i have a bobcat excavator has very weak hydraulics is the torque limiter that is on the side of the pump the same as a compensator

[@Gpmhydraulic]

Thank you for your question, Jay! No, they aren't the same at all (though either of them could be out of adjustment or have trash inside and make for what you call "weak hydraulics"!). The compensator should be adjusted to determine maximum system pressure. The pump will deliver full flow until the compensator setting is reached, at which point it will de-stroke the pump (reduce the angle of the swashplate and therefore the flow delivery) to a point where the compensator setting is maintained in the line. The torque limiter, on the other hand, is adjusted to determine the maximum input torque to the pump. As input torque approaches the setting of the torque limiter, the swashplate angle is again reduced, reducing the flow output, allowing pressure to build to a greater amount without increasing the torque. In general, the torque limiter allows us to build the necessary force for the job to be done by a smaller prime mover than we would need at a higher flow. In other words, (since the Conservation of Energy law doesn't allow us to get anything for free...) it lets us sacrifice speed for an increase in force. Hope this helps!

[@mustaphawahbeh]

kzbin.info/door/_GbYSHgKJL3g4J1aZ-RBSA

[@dinneszozobradolaparan4601]

thanks you boss...

[@bobbieborror5339]

Hi, I just ran across your very informative video, and I am wondering if you could help me locate my Compensator on my pump for my plow truck?

[@JackWeeks]

Hi Bobbie, thank you for your comment! If your pump has a compensator, it will be either on the top or on the front (opposite side of the ports), but chances are that your plow truck uses a fixed displacement pump, probably a gear pump, which has no compensator. Instead, pressure is probably limited only with a relief valve. If there is no adjustment on your pump, the relief valve is probably your culprit. You can try disassembling it and checking for wear, varnish that could make it stick, or contaminants that have collected keeping the relief valve from closing. If, after cleaning the relief valve (don't forget to look for plugged orifices!) you are still unable to set pressure, you may need a new one. Also, often on mobile devices, there may be a relief valve built into the directional valve body. If so, check it the same way. Your pump could possibly be very worn and that be the problem, but unless this pressure loss has been gradual over a long period of time, that is unlikely. Hope this helps! Jack Weeks

[@bobbieborror5339]

@@JackWeeks Hi Jack. Thanks for the fast response! I didn’t know how to upload photos of the pump on this site, so I sent you them via an email. I sent it to: www.GPMHydrulic.com. Was that the correct one ? Thanks again! Merry Christmas!

[@JackWeeks]

@@bobbieborror5339 Unfortunately, no. GPM was forced to close its doors as a result of the effects of Covid. I've semiretired, but still do work on my own. You can reach me at [e-mail]

[@forory1]

With all due respect: A hydraulic pump has everything to do with pressure: not just flow. A hydraulic pump produces flow, but is must be structurally capable of overcoming the resistance caused by liquid and load resistance. If a pump is worn, it will not be able to overcome load and liquid resistance and thus will cause a decrease in pressure. When the pressure in a system decreases, ALL the components in the system are "guilty" until proven innocent. All components in the system, including the pump, must be tested for leakage to correct the problem. Also, it is impossible to find problems associated with actuator speed with a pressure gauge. Anytime actuator speed decreases, it can only caused by a flow deficit. A flow meter is needed to find the problem. Respectfully, Rory S. McLaren "Hydraulic safety does't just happen, it has to be vigorously pursued."

[@alcovyRIVERband]

With all due respect to Mr. McLaren's comments, a worn pump, depending on the amount of bypassing, can overcome the resistance in the system and move the load providing that the TOTAL pump output is not bypassing over the case drain line. In my years of personal experience I have found that the pump is usually the first thing that is changed when a low pressure condition exists. As Mr. Ayers states, many a good pump has been changed because of trash in the compensator. In the real world environment, it is not time feasible to install a flow meter as the downtime in plants can be as high as $10,000 an hour. Many times flow loss problems can be found by checking the pump case drain line or with an infrared camera to determine where the oil is bypassing in the system. Thank you, Mr. Ayers for imparting your knowledge to help mechanics and electricians improve their troubleshooting knowledge and skills.

[@jessemorton1947]

As long as volume of a solid or liquid or any other state of matter, the PSI is fictional when actually in reality is more like 1×1×1 inch cubed... maybe PCI??. Idk.. I can't one up anybody cause I am learning tons from videos and respectful comments.. Flow...pressure..To be a pump or not to be... Is the reservoir half full or empty?? Idk.. Is a square inch real or not?? I am being serious..I get confused...Only in cyberspace is co-planer a concept of sharing a face as one, and that baffles the computing out of computers.

[@MR-nl8xr]

Great comment. Made for a conversation starter.

[@MR-nl8xr]

@@alcovyRIVERband Great of you to share.

[@jasonhart1802]

This is the video I needed to see. I have a bad pressure relief valve. But I'm stuck between that or the spool valve on the differential lock on a grapple skidder. My pressure drops from 400 psi to below 100 psi. It is suppose to hold fast at 635 psi

[@Gpmhydraulic]

Jason, do you have a safe way to lock pressure in the line and shut down so you can listen to hear which may be bypassing?

[@jasonhart1802]

@@Gpmhydraulic I blocked off my lines going to my differential. That should have spiked my Pressure. Which it did what didn't hold. I'm not a 100% sure if it's the prv valve is bad, if the spool valve is bad that has the electric seloniod that controls the differential lock or if the block that all this screws into is cracked internally.

[@jasonhart1802]

@@Gpmhydraulic as far as locking pressure, I guess I'd have to remove the block. Fasten a porta power to it with a gauge and try that maybe

[@Gpmhydraulic]

@@jasonhart1802 Yes, that sounds like a very good method to me. Take extreme care - never expose yourself to pressure!

[@clayrosencutter285]

I enjoyed your video, I have an older Bobcat 853 that I just purchased, and it has some hydraulic issues with the lift/tilt function, The gentleman that I purchased it from said he installed new seals on the spool valves (they were leaking) but now the lift/tilt hydraulics appear to work well when the machine is at idle but as soon as I start turning up the throttle the hydraulics pretty much stop working, occasionally they will wove slightly but I have to move the throttle back to idle to get them to do anything. Any thoughts or suggestions would be greatly appreciated?

[@JackWeeks]

Thank you for your question, Clay! If I'm understanding the symptoms correctly, your flow is being diverted once it reaches a certain level. Remember that hydraulic oil will always take the path of least resistance. This path seems to change once the flow gets to a certain point. Not many components will do that, but the system relief valve is one that will snce it is held closed by a spring. I would check the relief valve first. And, if you have any other pressure controls in the system, check those next if the relief valve checks out OK.

[@michaelcostello6991]

Thanks

[@Happy_Harry]

Great video, but I don't believe my pump has a compensator. I have a Roper Log Splitter that won't split anymore. The cylinder goes out, but will not push into the wood anymore. No leaks. So either its the pump, the actuator valve or the cylinder? Normally the actuator valve handle would self return to the neutral position when the cylinder fully retracts. It will not do that anymore either. When trying to push into the wood, the engine bogs down. It normally would bog down if it encountered a very hard piece of wood or at the end of cylinder stroke. Any thoughts or help would be much appreciated. I still have a lot of wood to split before winter.

[@JackWeeks]

Thanks for your question! You are correct, your pump has no compensator. Your pump is likely a gear pump. I'd be interested to know if your pressure is low, but I'm guessing you have no pressure gauge on your splitter, so you would not know if it was above or below normal even if you measured it, correct? I'm thinking your pressure is fine, though, because you say it is bogging down the engine. It wouldn't do that if your pressure was low, it would simply stall. First thing I would check is your relief valve. Probably you have no separate relief valve, but there would be one integral to the directional valve. Since you say that the valve handle no longer springs back, the relief valve sounds like the most likely culprit. You probably only need to clean it out. It is probably the only adjustment on the valve. WITH NO PRESSURE IN THE SYSTEM AT ALL, remove the components of the relief valve and clean everything, Put it all back together and test it. If that doesn't work, you may have to replace the whole valve assembly. Hope this helps!

[@Happy_Harry]

@@JackWeeksThank you so much for your answer & insight. I'm not sure if it even has a relief valve. I wish I could post some pictures here. I'm gonna start by simply changing the filter to see if that helps. I'll look for a relief valve on the directional valve. The splitter is at my brother's right now. I'm gonna head over there shortly. What would be safe to use to clean the relief valve? Ether maybe? Or just some cloth?

[@Happy_Harry]

@@JackWeeks also no guage. But I pulled a guage off of a different hydraulic unit. I have see if I can adapt it on with some fittings on hand. But yeah, I have no clue what the pressure should be.

[@JackWeeks]

@@Happy_Harry Definitely start by removing the directional valve, taking it apart and cleaning everything. Look for a built in relief valve and see if it is stuck open. Carburetor cleaner is good for cleaning these valves, just be sure to remove the cleaner with new hydraulic oil afterward. Good luck to you!

[@boarderlinesk8media433]

I’ve got a 755 jd tractor I’m looking to buy a backhoe with a recommended psi of 300 more than my tractor says it will produce. Can I still use the attachment with 300psi less than it says it needs?

[@Gpmhydraulic]

Probably not. If you are unable to build the necessary pressure, the backhoe may not move. And if it DOES move, it will certainly stall under heavy load.

[@anilramdasbaviskar6347]

Thank for useful video related to hydraulic systems, our machine Turret Punch Press made by Amada model Aries 222 also facing low pressure problem, in this hydraulic unit used Gear type pump made by Uchida Oil Hydraulic Model GSP2-A0S12A06AR-A0-901-0 & unload relief valve by Uchida Rexroth DA10-2-A0/80-998-0 so, kindly give us a right reason to solve the problem.

[@Gpmhydraulic]

Thank you for your question, Anil. The best way to test your pump is to deadhead the flow and measure the flow through your relief valve (the safest way to do this is with a flow meter). Begin with the relief valve set at a very low pressure. All or very nearly all of the pump flow will be delivered, even if the pump is badly worn. Then, increase the pressure setting of the relief valve, continuing to monitor the flow. If, as the pressure is increased, the flow decreases, the pump is worn. If you can get the pressure up to normal system pressure with no significant reduction in flow, the pump is good. Naturally, if you are unable to adjust the relief valve, then the valve is likely to be the problem. Hope this helps!

[@keithcarpentersr.4353]

GPM Hydraulic Consulting - I have an old backhoe with no real power to the backhoe unit. I tested pressure according to manual and seems fine but when put under a load it’s very weak. Do you think my pump is bad? 60 year old machine. Do you sell pumps?

[@nikkoBcool]

Do small hydraulic pumps such as 12v KTI ones used in dump trailers have compensator spools as well? I'm trying to understand if my pump is weak or the cylinder has an internal leak because I'm supposed to be able to lift 6000lb with this trailer but max out at around 3000lb, the pump doesn't sound like its "struggling" and I dont see any extrernal leaks anywhere on the cylinder or the pump, what could be the issue?

[@JackWeeks]

Thanks for your question, Nikko! No, those pumps usually don't have compensators; they're usually fixed displacement so their flow rate is fully dependent upon the RPM of the drive motor. Usually those pumps lose flow when they wear, meaning that they will still move the heavy loads, just not as fast as usual. A pressure problem in a system such as that is more likely caused by a worn relief valve or internal bypassing in the cylinder or directional valve. Bypassing generates heat, so look for a component that is running hotter than the others, that will likely be the culprit. Hope this helps!

[@nikkoBcool]

@@JackWeeks thanks for the reply. So you are saying in these pumps as they age because of wear all that should happen is a reduction of rate of flow so it effects speed but not the pressure so it should be able to lift it's original weight? Can the cylinder have an internal leak and allow some of the incoming pressure to be routed back the pump or is that not how they work? The cylinder is dry and it holds the weight up without lowering down once the pump is not running. I will see if it's a current issue as well since it's a 2 battery pickup but it's crew cab and the cable is probaby 10-12 feet, looked like a #4 awg. Also I saw that the reservoir tank can get clogged with the pickup up mesh since this does not have an external filter. Thank you once again!

[@JackWeeks]

@@nikkoBcool Yes, I think bypassing in the system is where your trouble probably is. Though it could be a battery issue if your motor is electrical - I was thinking a PTO when you first asked.

[@nikkoBcool]

@@JackWeeks I've seen a video where the guy says the issue sometimes is ok the spools (he called it something else) basically two spools that sit on the top of the aluminum pump assembly and are responsible for the double action, I will try to take those out and see if there are any issues. I did notice that the pump ran stronger when I turned the truck on as opposed to just running off the two batteries up front (new) thanks for the input, will try to report back once I figure this out

[@TrailMaker2]

hello, I have the problem with my pump, every time I turn it on, it cant build up pressure but when I open the valve all the way open after 10 seconds it grabs hard and after that it works fine, like nothing ever happen. Can you help me solve the problem? I had this problem before I put new oil and new oil filter.

[@JackWeeks]

It's definitely caused by trash somewhere. Possibly the system relief valve not seating at first, or if you have a variable displacement pump, it could be the compensator valve. Try disassembling the relief and cleaning it. Same with the compensator valve if it exists in the circuit.

[@TrailMaker2]

​@@JackWeeks thanks for answering my question, here is a video from my channel if you want to take a better look on what the problem looks like kzbin.info/www/bejne/rF68fKaHjql0p5I

[@JackWeeks]

@@TrailMaker2 Yes, you definitely have some trash keeping either a relief valve or the compensator from seating properly. The trash is getting blown out and all is working properly in just a few seconds, then when it is shut down, lodging again and keeping pressure from building. The ideal thing would be to flush the system, but second best would be to remove and manually clean the compensator, relief valve, and even the directional valve (sometimes directional valves will have a built-in relief). Hope this helps!

[@ativorevans8584]

Is it always that the compensator spool is located on the hydraulic pump?

[@JackWeeks]

Thank you for your question, Ativor! Yes and no. When a pump is pressure compensating, there must be a compensator valve mounted physically on the pump to de-stroke it. But it is also quite common for the compensator to be remotely adjusted, so in that case there will usually be a "master" remote valve controlling a "slave" compensator. Hope this helps!

[@k.j.renterprises5816]

Super sir tnq

[@Gpmhydraulic]

Thank you!

[@junickescanilla4335]

Thanks much sir

[@mustaphawahbeh]

kzbin.info/door/_GbYSHgKJL3g4J1aZ-RBSA

[@hdfan5742]

So this could be why my digger boom is slow?

[@Gpmhydraulic]

This certainly would slow your boom down if you have a pressure compensating pump, yes.

[@maimagsucang6720]

Sir ask how about the pc78 slowly power swing motor also weak.

[@JackWeeks]

I presume you are talking about a Komatsu PC78 crawler? If so, it sounds as though either you have some bypassing or a badly worn pump. Is your pump case very hot? That's a strong symptom of wear. Particularly if it heats up rapidly under load. Listen to the pump - is it cavitating? Aerating? Check for bypassing across the relief valve. If the relief is bypassing, it will be hotter than the surrounding components. Hope this helps! Jack

[@mustaphawahbeh]

kzbin.info/door/_GbYSHgKJL3g4J1aZ-RBSA

[@rajmohanprabhudesai4009]

Dear hank my self Rajjmohan, and i am working in maintenance field as fresher. i am facing problem of low cutting pressure in NAMSUNG a korean make multi punch press bussbar machine. large dimension buss bar cannot be cut bacause of low cutting pressure. kindly give me trouble shooting thank you

[@Gpmhydraulic]

Thanks for your comment! In our experience, low cutting pressure in a multi-punch is usually caused by contamination or wear in either the system relief valve or, in some cases, crossport relief valves at the cylinders that are intended to limit the shock of impact. Check those first. It is also possible that the compensator valve on your pump has an issue similar to the one in the video. Remove and inspect/clean the relief valves and the compensator. These are your most likely culprits!

[@nahalmouloude2996]

Rajmohan Prabhudesai

[@stuarth43]

i changed the pump but still, no pressure, checked relief valves, removed electric valve spools, still nothing, the pump is in tank, been working on it 3 weeks, if I took a pic on the system could I send it to you please, or to ANYONE!!:))

[@PolymathU]

Did you get your system up and running?

[@kansasadventure1831]

I feel like this is what's wrong with my bobcat. I checked it once and it had debris but my issue wasenr fixed

[@mustaphawahbeh]

kzbin.info/door/_GbYSHgKJL3g4J1aZ-RBSA

[@joebojanic1905]

Flow is a byproduct of the pressure generated.

[@adamsorell4851]

what about a gear pump?

[@JackWeeks]

Well, since gear pumps don't have compensators, this wouldn't apply. Usually, in a gear pump circuit, low system pressure is the result of a problem with the system relief valve or system bypass. It is possible for a gear pump to be so badly worn that pressure cannot be developed, and if your low pressure problem occurred as a very gradual thing, it may be that badly worn. Test it by deadheading the system such that the only flow path from the pump is through the relief valve. Compare the pump output with the relief valve turned down very low to its output at normal system pressure (or as high as you can get it if you can't get it all the way to normal). If the flow decreases drastically as you increase the relief valve setting, the pump is worn. If, however, you can get all the way to normal system pressure with no significant flow loss, the pump is good and your problem is elsewhere.

[@timhanna870]

if only they were all that easy. what if u don't have a compinsater and no pressure being generated out of 2 spool valves. but the other 6 valves operate fine?

[@JackWeeks]

Hi Tim, yes, it would be great if all hydraulic issues were straightforward, but sometimes they do hide around corners! In your case, clearly your pump is not the issue, though (which was the primary message Hank was trying to convey in the video). And in fact, pressure problems rarely are the pump. You are losing flow somewhere - and it is either somewhere common to the two valves or possibly the two valves themselves. The best way to find bypassing is usually by looking for heat, because bypassing fluid will always generate heat. Often you can tell by feel where the heat is being generated, but the best tool for this is a thermal imaging camera. Find where the heat is being generated and you will find your bypass point. Hope this helps!

[@timhanna870]

@@JackWeeks that helps me more than you know. Thank you!! Been dealing with this issue for almost a year. Yesterday I was going through the spools and realized the valve where flow is being bypassed is a 2pc spool with holes drilled for passage. different than all the rest being solid spools. There are sure signs of heat on the valve. I was unable to take the spool apart but cleaned thoroughly. Unfortunately theyre generic valves with no part available. Fingers crossed. Thanks for your help

[@fernandosalas6402]

That is old piece!

[@yogeshvaidya8526]

मीटा कंट्रोल वाल पुरा व्हिडीओ बताईये

[@Gpmhydraulic]

This sounds like a subject better addressed in a different type of video.

[@yogeshvaidya8526]

@@Gpmhydraulic धन्यवाद सर

[@yogeshvaidya8526]

@@Gpmhydraulic मिठा हायड्रोलिक का व्हिडीओ बताईये कंट्रोल का

[@user-kv6zd2re3c]

If you have no pressure its meaning that all pistons inside destroyed 🤣🤣🤣🤣, not just compensator dirty))

[@ForensicCats]

1:37 and 2:12 are two DIFFERENT parts! Jack Weeks video has a little white lie in it... I do not like any lies! Do a real video and not white lie video... thank you anyway.

[@Gpmhydraulic]

Yes, you caught us! We did in fact take a little license with the making of the video since we didn't actually have a pump that was bad that we could use. I suppose we should have included a caption of something along the lines of "Simulated Pump Failure - Video for Instructional Purposes Only". We apologize.

[@ForensicCats]

@@Gpmhydraulic , based on my "amygdala theory and being triggered"...your reply is AWESOME... Outstanding, to the person that replied...