anyone know much about hydrostatic drives

[carlotpilot]

2 minutes ago, gearhead204 said:

I have a753 I think or 743 ( I can never remember unless im looking at it) Anyhow there is a red knob on the right side of the operators platform that is a dead machine boom lower by pass valve, could it be stuck in the bypass position? other than that you may have failed a cylinder piston packing or stuck port relief.

 I take it the bucket tilt function still works properly ?

 yes that works  

[gearhead204]

have you removed  the tube line at the  rod end of the  lift cyl. to see if you get a rush of  oil from either cyl. when trying to lift the boom ?

 

On another note I just got off the phone with my buddy that has the A-car, it has either a 262 or 250 in it and its stuck, but only oil in the crank case, he got the truck for the engine so he would have part for his old skoop mobile loader, he has since sold the loader so the truck and eng could be available.

 

[mrsmackpaul]

Maybe a key way  on the driven pulley 
Maybe the belt driving the pulley is slipping

But if you can hear cavatation that points to air getting in the system to me or maybe a blockage on the oil supply side thats floating across the inlet every now and then 
But Im way out of my depth here 

Paul

[fxfymn]

I think the real clue is that you can get it to operate if you hand prime it and that it operates at a reduced rate before failing.That says you are not getting enough oil to the motor to prevent cavitation. I'd check the supply valve to see if it has crap in it. Or maybe worn or blocked linkage that is keeping it from fully opening?

 

[Ezrider]

i am thinking whatever the problem is its going to be after the supply line enters the pump  when i blead out the line to the pump after the filter change i got a ton of oil flow with just 2 psi in the tank. no leaks prior to the pump. everything seems a-ok there. i suppose next i am going to have to pull the drive out and start taking it apart. i wish i knew were to find parts for it. although i have parts diagrams with part numbers gehl does not supply these parts any more. i am a little hesitant to take it apart without knowing i can get replacement parts for whatever i find wrong. 

 

my guess as to what is going on is i think the block that the pistons go into took some pretty heavy wear when it ran dry. im not really sure though.

 

i don't see any internal screens or filters in any parts diagrams i guess that doesn't mean there isnt one though. 

[Ezrider]

ok so i did make a little progress on finding parts availability on it, 

 

the drive units are a sunstrand 15 series u type. witch i am having some luck in finding some parts for although expensive 650 bucks just for one piston pump assembly from what i have seen. from a old thread i found on a skid steer fourm this same drive unit is used in some jhon deer tractors, with some minor differences but the internals parts are the same the swash plate is limited to reverse and the jhon dear ports hydro preasure off the drive to run implements where the skid steer has a separate pump for that. but i found a used unit on e-bay off a jhon deer 140 for 150 bucks. my thought is to maybe buy that and swap the internal parts to make a used part rebiult unit for my skid steer. 

 

if i get a chance today the check valves can be removed externally i will try pulling them and cleaning then and try it again. 

Edited October 19, 2017 by Ezrider

[Ezrider]

so i think were actually figuring some things out now, i just pulled the dang thing out of the skid steer today and started taking it apart. bearings on the input shaft  are completely fubar. separated the case half's the piston pumps actually look ok. the roller bearing on the back side of the case on the input shaft fell into peices. i got the sheer pin out of the input spline but the input spline is stuck on the shaft pretty good. but while trying to remove it peices of what looked like parts of the sheer pin to the small gear pump that pulls new fluid into the case fell out. so i would guess when the coupler broke it fubared my bearings extra shaft play broke the shear pin on the fluid gear pump and caused my current problems. 

i went ahead and ordered the used john deer drive. seems the only real differences are the swash plate throw to reverse is limited and there is a different pressure relief valve. and ill need to plug the implement ports.  swap my swash plate into the john deer and swap the pressure relief valve and it should work.

 

Edited October 19, 2017 by Ezrider

[carlotpilot]

21 hours ago, gearhead204 said:

I have a753 I think or 743 ( I can never remember unless im looking at it) Anyhow there is a red knob on the right side of the operators platform that is a dead machine boom lower by pass valve, could it be stuck in the bypass position? other than that you may have failed a cylinder piston packing or stuck port relief.

 I take it the bucket tilt function still works properly ?

i`m more than happy to report that my problem turned out to be a damn mud dauber filling up the under seat disconnect switch with his crap. a magnetic contraption that shuts part of the system off if you get unseated got that cleaned out and everything works fine

[carlotpilot]

4 hours ago, Ezrider said:

so i think were actually figuring some things out now, i just pulled the dang thing out of the skid steer today and started taking it apart. bearings on the input shaft  are completely fubar. separated the case half's the piston pumps actually look ok. the roller bearing on the back side of the case on the input shaft fell into peices. i got the sheer pin out of the input spline but the input spline is stuck on the shaft pretty good. but while trying to remove it peices of what looked like parts of the sheer pin to the small gear pump that pulls new fluid into the case fell out. so i would guess when the coupler broke it fubared my bearings extra shaft play broke the shear pin on the fluid gear pump and caused my current problems. 

i went ahead and ordered the used john deer drive. seems the only real differences are the swash plate throw to reverse is limited and there is a different pressure relief valve. and ill need to plug the implement ports.  swap my swash plate into the john deer and swap the pressure relief valve and it should work.

 

got my fingers crossed

[Ezrider]

thanks, im pretty confident as long as the replacement used pump is in good shape. my plan will work. i think most seals should be able to be locally sourced if i need to change a few but the plan is to just use the best parts of both pumps and try to get it working again. I'm ok with under 200 bucks to fix it. didn't want to pay over 2k to rebuild the pump on a skid steer that is only worth a few grand on a good day with everything working right. especially when the plan is to replace it not something i plan on keeping a whole lot longer. 

 

i was a little worried about how i was going to push snow this winter if it wasn't going to make sense to fix it. iv got a few spots where the snow has to be pushed quite a long ways. was thinking i might have to get a plow for one of my pickups witch inst the best solution either as i have a lot of area that needs to be back dragged. and doesn't have any of the other functional uses a skid steer has. one of them things your used to having a skid steer even if its a old small kinda junky one when it goes down you wonder how your get by without it....lol

[Ezrider]

i got the drive gear off today so i could get the charge pump apart took getting it red to get it off no idea why it was on there so tight when its not supposed to be a press fit.  got the drive torn the rest of the way apart. other than the relief and check valves ect probably safer to keep that stuff installed for now. i did take them apart and look at them all looked good then put them back in there housings. 

 

most defiantly a failure of the charge pump. looking at what i found i wonder if it wasn't necessarily the bearing failure that caused the charge pump failure. but a seperate issue maybe a small rock or something ended up inside the hose when i was snaking the new hose threw and got sucked into the charge pump. there is a little chunk of the housing that is broken off. quite a bit of debris in the charge pump.

also now that i know what pump it is rather than looking for information based on application. was able to find a service manual in pdf. seems i don't so much need to change a pressure relief valve to make the replacement work but delete one. looking at a flow diagram the implement relief valve is between the two implement ports. and the charge port is prior to the feed port, so in normal operation the flow is regulated to charge pressure then threw the open center valve of the implement then back to the pump. so the flow bypasses the implement valve until a load is put on it. so basically i will just need to remove the guts of the second relief valve and plug the ports. 

 

really its a pretty simple setup once you actually take one apart. 

 

[Ezrider]

got the john deer pump in the mail last week, finally got to open the box on sunday. pulled it apart everything is in pretty good shape. swapped out the swash plate, put it back together. changed the relief valves. shafts and everything were the same, only thing i didn't anticipate. was the cover on the charge pump was different. the one for the skid steer is machined on the outside. because the sheve bearing/pully assembly bolts ontop of it. the john deer one is not. so that is all i have left to do before the new drive is ready to bolt in. the old cover will work on the new pump but i cooked the seal getting the drive gear off. and it needs a new bearing. i didn't have time to try to find a new bearing and seal locally so need to do that and then get the machine back together and hopefully everything works good. 

[Ezrider]

well i just thought i would update this. i got the skid steer back together and its still acting like its airlocked. have been really busy latley and not had time to mess with it. 

 

however today i just purchased a bobcat 763 from a airport. for 11k 2900 hours on it. everything is super clean no slop in any pins every fluid looks like bran new every filter looks bran new clean fresh greeze ozing out of every joint. engine runs silky smooth starts right up with no hesitation. it was in the teens out when i first went to look at it and hold the glow plug switch in for a little bit and bump the key and started right up. no smoke every control is fluid and smooth tires are good it needs nothing. 

 

was primarily used to push snow. 

Edited November 18, 2017 by Ezrider

[Ezrider]