Bobcat 753 Arm Lift Problems

[decoursm]

Hey all!

I'm new to this forum and have been searching several sites for the answer to my problem I'm having with the lift function on my arms.

A bit about my machine... 1999 with 2,800 hours and normal flow aux hydraulics.

Pre-work. Hydraulic fluid full and looked normal with no major leaks noted.
Was using my grapple in the woods for a few hours clearing and piling brush.

Had the arms about half way up and pushed the pedal to raise it higher and the arms began to free fall until I went neutral on the pedal (then it stopped). Pushed up again on the pedal and it continued to fall. Now it is resting at its lowest position and will not go up. The pedal linkage feels normal and I can hear the actuator and hyds working when I press the pedal up, but the arms do NOT move. Tilt works normally and the aux hyds work fine as indicated by normal opening and closing of the grapple. Of note the aux hyd control switch is in the neutral position. Left and right skid steer movement is normal and is NOT laboring when moving (normal normal). The hyds are NOT under load (engine idling normally) with the lift pedal in the neutral position. I am not seeing any fluid leaking under the seat or around any of the hydraulic cylinders. No codes are set and the control/safety panel lights are all normal. When I shut the bobcat down and parked it I checked the hyd fluid immediately and noticed the level was normal, but the fluid looked "milky". I topped off the hyd fluid a few weeks back and it was changed 4 months ago.

Checked the fluid and started the Bobcat a few days ago and it still will not lift.

I've checked a few other forums and my situation is slightly different than all of them. The closest I could find was a 2005 S175 that was having lift control issues, both up and down, but it was setting a code. In that post he swapped out a few valves on the back side of the hydraulic control block (not sure if that is the exact nomenclature as I am struggling to refind the post) that sits directly underneath the right side of the seat (as you look forward in the normal seated operator position). Update: Found the link...
heavyequipmentforums.com/showthread.php?39062-Bobcat-s175-lift-problem

I am somewhat of a novice when it comes to mechanics, but I have changed hydraulics lines that were wornout after I first purchased the bobcat a year ago.

Thanks for the help!

Matt

 

[SSdoxie]

Your description sounds like lift function is going into position when spool is moved even tough lift up control is opposite direction of float. You mentioned actuators so I presume this is either ISO or ASC control (electric over hydraulic). At the control valve - locate the wire connections going to both lift an tilt actuators, unplug both actuators and plug them into the opposite harness connection plug. This will put the Lift control at the Bucket Tilt, try using the Tilt control to see if lift will operate. If it does the problem is in electrical side of lift. If no change in problem, then it is internal control valve.
the only thing I can think of in the control valve lift circuit that MIGHT allow going into Float when spool is shifted out of center is a small plug in back/rear end of the lift spool. To obtain Float, connecting lift up and down sides of the circuit, the spool is drilled through the center length to connect both ports then a plug is installed at the end to prevent oil from leaking into valve end cap and escaping to case drain.

 

[decoursm]

Dennis,

Thank you for the reply! FY info the pedal linkage is mechanically connected to the hydraulic control block for the bucket. The linkage pulls and pushes on pins that I'm guessing move pistons in and out on the control block. I lifted up the seat and noticed dripping from one of the compression fittings for a metal hyd line that goes to the right bucket lift cylinder. I fixed the leak by tightening the compression fitting. The big issue here wasn't the leak it's that I noticed air in the hydraulic fluid (milky appearance). I started the bobcat with the cab up and started moving the lift pedal and noticed the leak had stopped, however, the lift arms were still not raising. Again, it moves slightly (like it's taking some of the weight off, but will not go up. I used the tilt to push the bucket off the ground and again pushes up, but as soon as I hit the pedal (up or down) the bucket fell to it's lowest position. At this point in guessing maybe some of the seals on the lift actuator piston inside the hydraulic control block might be bad? May explain the air in the lift arm line?

I did swap two electrical connectors on the control block (not sure what they were), but it didn't change anything. Thanks again for your time.

 

[SSdoxie]

Foamy oil could be water or air. Given the explanation of recent fluid change and leak repairs I think air is more likely. Air entry would be would be somewhere on the non-pressurized side of system (suction side). Check hose clamps and fittings on suction side of pump. Air can enter through a slightly loose fitting without showing an oil leak. Fitting tight enough to hold oil when engine is not running but vibration from engine and pump allows air to enter under operation.

The 2 wires you switched on the valve are the Auxiliary controls for front couplers. Swapping them changes the direction of oil flow when using the thumb switch.

 

[truckdiagnostics]

I am impressed getting that valve section out of there is quite the project! best of luck
Steve

 

[SSdoxie]

I think you may have 2 issues going on, milky fluid and loss of lift. Contaminated/milky fluid should affect all functions not just 1 and I don't see anyway contaminated fluid would allow the lift to drop to the ground when pedal moved to lift/up position.

Suction side is ALL lines feeding fluid into the inlet of the gear pump.
A way to locate hard to find suction leaks it apply air pressure into the reservoir. This can be done using air compressor with regulator set to no more than 15 psi, or a shop vac that has hose fitting to exhaust port. Make a fitting (likely have to sacrifice reservoir cap) to apply air pressure into the reservoir and oil should be at normal operation temp (around 140 F). While under pressure look closely at ALL connections for slight oil drip.

A good way to identify if water or air in oil is to put a sample in a clear glass jar and set indoors and leave undisturbed and watch to see if it clears up or if water starts settle out. This could take 24 hours or more.

RE: lift arm dropping when trying to raise.
Are you moving the lift pedal to full stroke when trying this?
Is the pedal going to full stroke without hitting the floor or debris under the pedal?
A defective lift load check. Purpose of the load check is to hold oil in the rod side of the cylinder when trying raise the arms until the pump can build more pressure than the pressure applied by weight of the lift arms and load in bucket. If load check is by-passing, possibly blocking a portion of flow into the lift spool valve from the pump would produce this symptom.

 

[decoursm]

All,

Think I deleted my previous post. For info only. I took the hydraulic control unit out and removed the lift spool, main and port relief valves for inspection. All busings and orings looked normal. Hyd was still milky after the bobcat had been sitting for 2 days. Reassmebled and installed the control valve and tightened all fittings.

I drained all the fluid I could and will now replace the case drain and hydraulic filters. I'm going to replace the fluid with AW32 hydraulic fluid. Once I'm done replacing the fluid and filters I will start it up and check for obvious leaks. Once that is complete I will go thru the pressure and load checks as recommended by Dennis in the previous post. Thanks!

 

[decoursm]

All,

Was in the process of changing the hyd filter and noticed the fluid had what looked like whipping cream in it. Very seperated looking. Just got hyd filter and new AW32 bud oil from NAPA. Replacing tonight will let you know more tomorrow. Having a hard time uploading pictures.

 

[decoursm]

Hyd fluid and filter replaced. Started it up and lift still doesn't work. Tilt and aux work fine. Checked fluid in reservoir and the fluid in the dipstick has small bubbles. Before sacrifice the hyd cap for a pressure check, Dennis is it possible one of my lift are hyd lines has collapsed internally. Restricting flow to the spool as mentioned in the load check portion of your last post?

Thanks! Matt

 

[SSdoxie]

"Tilt and Aux work fine"- indicates supply from pump to control valve is OK. All 3 spools receive flow pump through same line. The load checks I mentioned are internal in the valve, look for 3 hex plugs - 1 for each spool. These plugs are located between the 2 outlet work ports for each spool. The Aux has a load check but is rarely needed for majority of attachments, if the Aux load check is there put it in the lift spool spot.
A collapsed line would restrict however I have never heard of 1 blocking flow completely.