753 boom won't come down, tilt won't curl back.

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scottyk

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Sep 17, 2007
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Hi. I am new here and need some help. I have a 753 1993 model. The booms powers up quickly, but doesn't go down. The tilt is strong downward but won't curl back up. The machine has no problems driving, turning etc. I tried isolating the boom cylinder by raising it all the way up and removing the top hose for downward pressure, but when I started the machine fluid came pouring out immediately even without touching the pedal. The pedals and linkages appear to be okay and not out of adjustment. I don't bleive it's the cylinders because the boom stays up all night and next day without dropping. I'd like to add that when pedals are in nuetral the machine's hydraulic noise seems to be more labored than when I press the pedals. It seems the machine doesn't labor at all when pedals are depressed. I hope someone has experience with this and is willing to help me. thanks for reading. Scott.
 

sterlclan

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May 1, 2004
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I don't know much about the newer ones sounds like the aux hydraulics are stuck on or a sticking spool valve someone with more knowledge is sure to have some information good luck Jeff
 

scottyk

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Sep 17, 2007
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I don't know much about the newer ones sounds like the aux hydraulics are stuck on or a sticking spool valve someone with more knowledge is sure to have some information good luck Jeff
Thanks for your suggestion Jeff, Here's an update. I checked all the relief valves, solenoid valves, linkages, spools etc. Everything worked freely, engaged when powered and was hooked up correctly. I did try and put the bucket on the boom and found that now the boom doesn't go up all the way so it seems that I don't have enough power to lift the boom with the bucket on more than half way up. I am thinking it's the pump now. Does anyone know a good way to check pump output without buying a very expensive tester. Or do you think it's something else?
 

Tazza

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Thanks for your suggestion Jeff, Here's an update. I checked all the relief valves, solenoid valves, linkages, spools etc. Everything worked freely, engaged when powered and was hooked up correctly. I did try and put the bucket on the boom and found that now the boom doesn't go up all the way so it seems that I don't have enough power to lift the boom with the bucket on more than half way up. I am thinking it's the pump now. Does anyone know a good way to check pump output without buying a very expensive tester. Or do you think it's something else?
You can get a pressure gauge that reads up 3,500 PSI and hook it to your quick couplers. Activate your aux hydraulics and you should get between 2,500 and 3,000 PSI depending on what the pump is set to.
It almost sounds like your oil is going ove the relief vavle giving no pressure.
 

scottyk

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Sep 17, 2007
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You can get a pressure gauge that reads up 3,500 PSI and hook it to your quick couplers. Activate your aux hydraulics and you should get between 2,500 and 3,000 PSI depending on what the pump is set to.
It almost sounds like your oil is going ove the relief vavle giving no pressure.
Hi again everyone; thanks for all the suggestions on this problem. I just got a hold of an OTC flow/pressure tester. I was able to test the main relief valve and heres what i got. 1. Main relief valve opens at 2300 PSI 2. Flow is 11.5 GPM with tester valve fully opened and 10 GPM with restriction up to 2300. At 2300 PSI, I believe relief valve is opening and GPM drops all the way down to 0-2 GPM. (is it supposed to drop?) 3. I need to adjust relief valve tomorrow; book says it should open at 2550-2600. 4. Pump output is a little low. manual says it should have 13 GPM instead of 11.5. I will test the pump tomorrow after adjusting main relief. One thing I noticed is that when I start the machine the front auxilaries are already flowing at 5 GPM at idle and 11.5 GPM full throttle. (and this is with front auxilaries button disengaged.) I tested the front auxilary solenoid valves and they both were in the closed position and only opened when voltage was applied. I even disconnected the wiring harnesses to make sure they weren't being energizd by a faulty switch. I still had same result. Does anyone know if the front auxilaries are supposed to be fully pressurized on start up and auxilary switch off. If auxilaries are not supposed to be flowing at start up, I don't have any idea how they are being opened. Any help would be greatly appreciated. Thanks for your time. Scott.
 

skidsteer.ca

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Jan 20, 2006
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Hi again everyone; thanks for all the suggestions on this problem. I just got a hold of an OTC flow/pressure tester. I was able to test the main relief valve and heres what i got. 1. Main relief valve opens at 2300 PSI 2. Flow is 11.5 GPM with tester valve fully opened and 10 GPM with restriction up to 2300. At 2300 PSI, I believe relief valve is opening and GPM drops all the way down to 0-2 GPM. (is it supposed to drop?) 3. I need to adjust relief valve tomorrow; book says it should open at 2550-2600. 4. Pump output is a little low. manual says it should have 13 GPM instead of 11.5. I will test the pump tomorrow after adjusting main relief. One thing I noticed is that when I start the machine the front auxilaries are already flowing at 5 GPM at idle and 11.5 GPM full throttle. (and this is with front auxilaries button disengaged.) I tested the front auxilary solenoid valves and they both were in the closed position and only opened when voltage was applied. I even disconnected the wiring harnesses to make sure they weren't being energizd by a faulty switch. I still had same result. Does anyone know if the front auxilaries are supposed to be fully pressurized on start up and auxilary switch off. If auxilaries are not supposed to be flowing at start up, I don't have any idea how they are being opened. Any help would be greatly appreciated. Thanks for your time. Scott.
You have a spool (stem the coil slides over) that is sticking which is not allowing the aux hydraulics to shut off. There should be no flow until you activate it. Relief is slightly low but that only affects the max pressure. Try removing the spools and cleaning them with solvents and air. You can also switch them from side to side to verify thats the problem.
gpm will vary with engine speed. Will it make the same pressure at a idle or does pressure fall off as you reduce rpm? If the spool won't close it could be that it won't fully open either??
Ken
 

Fishfiles

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Feb 8, 2007
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Hi again everyone; thanks for all the suggestions on this problem. I just got a hold of an OTC flow/pressure tester. I was able to test the main relief valve and heres what i got. 1. Main relief valve opens at 2300 PSI 2. Flow is 11.5 GPM with tester valve fully opened and 10 GPM with restriction up to 2300. At 2300 PSI, I believe relief valve is opening and GPM drops all the way down to 0-2 GPM. (is it supposed to drop?) 3. I need to adjust relief valve tomorrow; book says it should open at 2550-2600. 4. Pump output is a little low. manual says it should have 13 GPM instead of 11.5. I will test the pump tomorrow after adjusting main relief. One thing I noticed is that when I start the machine the front auxilaries are already flowing at 5 GPM at idle and 11.5 GPM full throttle. (and this is with front auxilaries button disengaged.) I tested the front auxilary solenoid valves and they both were in the closed position and only opened when voltage was applied. I even disconnected the wiring harnesses to make sure they weren't being energizd by a faulty switch. I still had same result. Does anyone know if the front auxilaries are supposed to be fully pressurized on start up and auxilary switch off. If auxilaries are not supposed to be flowing at start up, I don't have any idea how they are being opened. Any help would be greatly appreciated. Thanks for your time. Scott.
I think it is a aux stem valve stuck causing your problem , the boom and tilt being effected in one direction is classic example of the problem ----no , you should not have 5 gpm idle and 11.5 at high rpm's on crank up at aux coulpers
 

scottyk

Active member
Joined
Sep 17, 2007
Messages
26
I don't know much about the newer ones sounds like the aux hydraulics are stuck on or a sticking spool valve someone with more knowledge is sure to have some information good luck Jeff
Thanks guys. The GPM does vary with RPM. At idle it's putting out 6GPM at the front aux. at full RPM it's putting out 11.5 GPM. The problem is the front aux is constantly pressurized and flows at start up. I checked the solenoid valve and stems and they were closed without power and opened properly when energized. when de-energized, they closed again. Are there any other valves that fluid can go through for the front aux. I'll try switching them today after work and let you know. Thanks for the help. Scott
 

skidsteer.ca

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Jan 20, 2006
Messages
3,853
Thanks guys. The GPM does vary with RPM. At idle it's putting out 6GPM at the front aux. at full RPM it's putting out 11.5 GPM. The problem is the front aux is constantly pressurized and flows at start up. I checked the solenoid valve and stems and they were closed without power and opened properly when energized. when de-energized, they closed again. Are there any other valves that fluid can go through for the front aux. I'll try switching them today after work and let you know. Thanks for the help. Scott
Scott
The needle valve inside the stem is not seating and therefore the flow continues, weather there is power going to it or not
1 of the 2 stems is likely still good and if you swap them my bet is the opposite coupler on the front will have pressure coming from it.
Ken
 

scottyk

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Sep 17, 2007
Messages
26
Scott
The needle valve inside the stem is not seating and therefore the flow continues, weather there is power going to it or not
1 of the 2 stems is likely still good and if you swap them my bet is the opposite coupler on the front will have pressure coming from it.
Ken
Guys I'd like to thank you for all your advice and help. The problem was the auxilary spool valve which is hidden behind steel caps on the hydraulic control valve. Unknown why it was stuck but when I opened up the caps which were secured with allen head screws, I immediately saw that the spool was sticking out on one side more than the other. I had to hit it with a hammer to unstuck it. I also had to push the spool back and forth many times to free it enough to pull out. I cleaned and replaced it; working it back and forth. Put everything back and now the bobcat works like a champ. Thanks for the help. Scott
 
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