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 | jerrybeaner
Join Date: Posts: 1 
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2002-07-08 40175
I have a jd 750 which has a jd 7 back hoe. The back hoe has never been disconnected but now I want to disconnect and use the three point for a back blade. The problem i'mhaving is that when I disconnect the back hoe and reconnect the hydraulic lineslike all other jd's I've seen the rockshaft won'twork. A jd man told me that the top cylinder line should have a pipe plug installed inside the rockshaft behind this connection. Does this sound right?
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley 
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2002-07-10 40228
The JD factory hoe kits for some of the 4000 series compacts have an adapter line that must be installed between the hoe feed and hoe return lines when the hoe is removed. Without the adapter line the 3ph won't lift and the relief valve opens. If that's what's happening, the tractor shouldn't be run for very long.
With this kit, the hoe feed and return lines use different sized fittings so the hoe can't be connected backwards. The adapter also has different sized fittings on the ends. What ever the hookup, when you get around to remounting the hoe, it's important to not hookup the hoe backwards.
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| Halsey
Join Date: Posts: 1
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2002-07-10 40251
Not sure if your 750 is like my 790. but my auxilary rear hydraulics has a female coupler that is fixed and a male coupler on the end of a hose. These two couplers should be joined. When I installed mine, my 3 point wouldn't work, scared the bee gees out of me, thought I did something wrong. Problem was solved when I upped the rpm to around 2000 and tried to operate the 3 point. Heard a loud pop, and everything started to work. Checked with the dealer the next morning and he said that was the correct proceedure, didn't know why the directions don't tell people you have to blow the air pocket out. Since the air compresses, it takes a lot to get it pass the check valves in the 3 point. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-07-11 40267
That's the adapter hose I mentioned. I believe it's installed behind the loader valve in the PB line. In such a hookup, the 3ph wouldn't work if a loader valve wasn't centred. That might be another explanation.
I seldom differ with what works, and most people don't need explanation of things to get the tractor to do the job. I do wonder if the dealer's explanation might promote misconceptions about 3ph's and hydraulics though. As far as I know there'd be little reason for pressure to build and release the 3ph with a pop if the situation is just purging air in the adapter hose through the system.
If the 3ph was in neutral when the engine started, there aren’t many parts in the PB line and 3ph oil routes except a few valve orifices, which are more like holes than valves. Air usually is simply pushed through a system where it bubbles out of the oil while it is in the sump. It may take some time to push air through the system and more rpm would reduce the time. I can't think of what I would attribute a pop to. My interest here is promoting my own and maybe other people's understanding of tractor hydraulics, which as I noted isn't actually necessary to getting the work done. Doing whatever works is a pretty good argument.
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