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.
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
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...,
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.
The 790 was a Hitachi UH-103 with a Deere engine.
A straight 790 is going to be a totally mechanical machine, not pilot operated. So it's going to be a bit different to operate, make sure you check to see how worn out the linkage is.
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.
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?
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.
That is an old machine. Free is to expensive if it doesn't work properly.
based on my experience with a JD 595 hitachi-built machine, the weakest link was the jd engine. it had a 4276 eng IIRC
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.
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.
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
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.
I have the UH143 which is same as 792, Hitachi version uses a Hino engine which has been trouble free all these years.
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?
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.
Are the rods that go to the pumps stroking when you move a lever?
OK, That may explain why it not working if it needs air pressure. I'm unfamiliar with this type of system. I searched for a hydraulic schematics that show the air system, but couldn't find one. I just went to look at the machine the other day for the first time, and after 3 hrs of messing with it, with the mechanic, even he didn't know what to look for. The Hydraulic filters would not come out, even with a pry bar. They are loose and bobble around loose, but will not come out, as if they were hung on something, and that's on both filters. I can't remember if the rods are stroking at the pumps or not but it doesn't sound like there delivering full pressure and there was fluid dripping from the pilot pump. There are at least 3 hoses that need changing, and the rotory manifold if leaking, not sure how much it will cost to fix that. I'm still interested in this machine if I can figure out how it works.. Yeah, if you have a schematic of the hydraulic system with the air system that would be great, that might change my mind if it's worth getting or not.
Those filters should come out easy, maybe deformed on bottom. You have to check the hydraulic oil level with the stick straight out boom down and bucket in full curled position, if the sight gauge isn't to foggy. If the level is to low and your not reading air pressure on gauge that machine will feel dead. All excavators that I'm familiar with have some way of adding pressure to hydraulic tank. I suppose the real early machines didn't do that, not sure when they started to add pressure to tank. Maybe the early machines were gravity fed so not so important. I'll dig out the book for diagram tomorrow.
To bad you don't live closer, my Hitachi 143/JD792 is for sale.
Another thing to check would be the inlet screen if it has one. We had to clean the one on our Hyundai 290 recently, and there was a noticeable change in the hydraulic response.
As Puffie40 suggested, there is a screen inside the bottom of hydro tank.
The JD790 was same as UH09 japan market/UH103 european market. Those Hitachi models weren't allowed in the US because it violated the agreement JD had with Hitachi, not to allow the same size machines as they made for JD into the US. You were able to get them grey market though, as I did just that.
Thank You All for your responses, I have decided to pass on this machine. There's to many issues going on with machine, and by the time I put all the money into it, I could have already bought another machine that runs and works as it should without all problems.
Diminishing returns. I get it. Let us know your next find.