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JD 790

TMJD

Active Member
Joined
Nov 20, 2017
Messages
31
Location
Florida
Looking at a JD 790 excavator, is there anything to look out for other than the obvious things like oil leaks bad pins and bushings or undercarriage. Went to the JD part's page and found out it was made by Hitachi. I've ran a John Deere 892Elc and 992 Elc, which were alot newer, and bigger, just haven't messed with the earlier models before and would like some insight from someone if this model is one to stay away from.
 
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uffex

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Jan 23, 2012
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Lincoln UK
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Admin
Good day
I suggest you make a thorough inspection its a good way to become familiar with a machine. Protocol attached meet any questions give us a shout.
Kind regards
Uffex
 

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lantraxco

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Jan 1, 2009
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Location
Elsewhen
Good machine, I don't recall the size match, but the 490 was I think an EX120 no dashes with a Deere engine as I recall, so...,
 

JDCrow

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Joined
Sep 25, 2020
Messages
125
Location
Redmond, Or
Without looking it up, 790 is around a 220

490-120
590-160
690-200

I believe your right, most are Hitachi based with deere engine, except 690, which was last US deere made machine in the era. Was a long time ago, so don’t quote me LOL. Much smarter people here than me.
 

lantraxco

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Elsewhen
Without looking it up, 790 is around a 220

490-120
590-160
690-200

I believe your right, most are Hitachi based with deere engine, except 690, which was last US deere made machine in the era. Was a long time ago, so don’t quote me LOL. Much smarter people here than me.

690ELC was indeed 100% USA made, and man did they sell like hotcakes... we had contractors ordering them in three at a time for a while there. I sold undercarriage at the Portland Deere dealership in one of many former lives, not that I retained much from any of them, lol.
 

Welder Dave

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Oct 11, 2014
Messages
12,260
Location
Canada
The UH series Hitachi's are some of the best machines ever made so no reason the JD labelled machines aren't also very good. Have heard the JD engines may have been mounted higher in some machines and don't get as good of fuel economy but that could be a minor detail.
 

TMJD

Active Member
Joined
Nov 20, 2017
Messages
31
Location
Florida
Thanks for everyone's response, I went and looked at the machine and it has major problems with the hydraulic system. It is just a regular 790, with the mechanical linkages going to the control valves, and when you pull either of the levers to move a function with the engine at full throttle, it just will barley move and some of the hydraulic hoses going to the right boom cylinder are jumping like they have air in them. Even when I wanted to travel forward or backwards, I could barely get it to move. The pumps don't sound like they have air in them and aren't making any noises, could the pumps be completely gone? Could there be air getting in somewhere causing the issues?
 

masterwelder

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Joined
Jul 28, 2010
Messages
144
Location
NY
How’s the hydraulic oil level?
Check condition of hose from tank to pump. Could draw air in but not necessarily loose a large amount of oil.
On the pressure side it would be nearly impossible to draw air in.
 

56wrench

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Dec 4, 2016
Messages
2,074
Location
alberta
based on my experience with a JD 595 hitachi-built machine, the weakest link was the jd engine. it had a 4276 eng IIRC
 

lantraxco

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Lots of possible causes, starting with the drive coupling at the flywheel. It's just prior to pilot joystick controls so it has some assist and is fairly complex hydraulic and air systems.
 

TMJD

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Nov 20, 2017
Messages
31
Location
Florida
Lots of possible causes, starting with the drive coupling at the flywheel. It's just prior to pilot joystick controls so it has some assist and is fairly complex hydraulic and air systems.
Yeah, notice it has an air compressor, but thought it was only used for the air horns. It does have a smaller pump like a pilot pump between the two big pumps, haven't seen a system like this before. The hydraulic tank is full.
 
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TMJD

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Nov 20, 2017
Messages
31
Location
Florida
Could it be possible that the filters are plugged up , or a relief valve is stuck open in the system? I did notice that there are two different sets of linkages going to the pumps from the valve body, could they be out of adjustment, and have something to do with the pumps stroking? The machine has been sitting for a long time, probably 6 or 8 years.
 

lantraxco

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The Mitsubishi hoes of that era used air pressure on the hydraulic tank to supercharge the pumps, not sure but guessing this might be the same. That mechanical linkage hooked up to cam rollers on each valve spool? If so that brings the pumps on stroke, we had some early EX120s like that. If it's not moving or the rods are bent out of adjustment that may be part of the problem
 

361brock

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Jun 24, 2020
Messages
207
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staten island ny
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retired
There is a hex nut on top of each hydraulic pump that allows for bleeding system in the event a hose was blown and oil was lost. Just crack the plugs while engine is running. Those machines used air compressor to charge hydraulic tank, so if a line blows 1st thing to do is shut engine and relieve air pressure from blow off valve on top of hydraulic tank or oil will continue to push out of line from tank pressure.
 

361brock

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Jun 24, 2020
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staten island ny
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retired
I have the UH143 which is same as 792, Hitachi version uses a Hino engine which has been trouble free all these years.
 

TMJD

Active Member
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Nov 20, 2017
Messages
31
Location
Florida
Thanks 361brock, so that could be the problem. When I was pulling any of the levers about half way whether it be booming up or down, moving the dipper stick, bucket, or travel, they would barley move. But when I moved them all the way, the function would stop completely. The owner thought that the hydraulic filters were clogged up, so he had a mechanic try to remove them, and they were loose but wouldn't come out. Is there something down in the bottom of the hydraulic tank that needs to be disconnected to get them out?
 

361brock

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Jun 24, 2020
Messages
207
Location
staten island ny
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retired
Is the machine building air pressure? The gauge should be in the green, the adjustment for the pressure is under the big lift door on top of machine. When you lift the door and step in there it's on the side also there is a small air tank that needs to be opened once and a while to bleed off any water or oil that my have gotten in the air system. There are 2 hydraulic full flow hydraulic filters that just drop in from the top, should be able to pry them up with little effort, they are on top with the cap bolts in circle holding covers on. The air pressure relief valve is also right there sticking up with a plastic screw on cover. If you unscrew the cover and lift up on the pin it will relieve the air pressure in hydraulic tank if there is anything in there, which there should be if it's building air. I have all the books if you need a specific diagram.
 
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