We just got a used John Deere 770D grader a short time ago and up till now it has been a good machine. It was well maintained by the previous owner. Anyway yesterday the operator brought it in and the steering is not working. My problem is I don't have a service manual yet for trouble shooting or a hydraulic schematic. I was wondering if anyone knows were to check the pressure to the HMU (steering valve as John Deere calls it) or what to check, if there is a priority valve, check valve or something that might cause this thing not to steer. Thanks in advance.
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John Deere 770D Steering Problem
- Thread starter owmamech
- Start date
Well i have an update on my steering issue. I sometimes get so mad at myself for not doing what I always tell everyone else KISS ( keep it simple stupid). I decided to start off the hard way by pulling out the hmu and tearing it apart, nothing wrong there. Decided to check the pressure coming into it so I put a tee in the line with a gauge. I had 1300 psi on stand by and 2850 when the pump stroked up by operating a control lever. Decided to to the same thing on the signal line. It had 30 psi in standby and around 1000 psi when i turned the wheel.I don't have a book with the spec's yet but figured the pressures should be enough to make it work. I got to thinking about things and decided to see if i had a cylinder bypassing. I unhooked the right hand steering cylinder and capped the lines. No change in anything so I did the same thing to the other side. As soon as i fired up the machine everything started working right. I took off the cylinder and was starting to take it apart and noticed the rod looks like it is bent and wont slide in the barrel. about that time another machine came in that was leaking oil everywhere so I had to stop what i was doing and take a look at it. I will update more on the condition of the cylinder when i get back to it.
I got back to the cylinder yesterday afternoon. I started looking at it closer and found what caused the bent rod, the barrel has a dent in it, its about in the center. I got to thinking about what could have caused this so i started questioning the operator. At first he claimed it just quit steering but when i told him there was a dent in it from it hitting something he said maybe a piece of concrete hit it. I showed him the cylinder. There was no way that the dent was caused by concrete. Then he finally told me that he had rotated the blade into the center of the machine and must have hit it with the blade. He couldn't tell me why he would have rotated the blade in between the wheels to start with. It really irritates me that they will keep this guy around, this isn't the first time he has done damage to a machine or something else by doing something stupid. This time its going to cost close to $1300 for a complete cylinder, maybe it will cost them enough to make the bosses see how great he really is.
My Brother did the same thing to a 670D. I love the flexibility of the blade and table. But you better pay close attention to where the "other" end of the blade is when trying to do some tricky maneuver. Friend of mine tore the step completely off the left side of the cab on a 672D. 
Tx33cowboy
Member
I have a 672d that has weak to no hyd pressure. New pump just installed. Worked right for a minute then went weak again. Does shying have a hyd schematic. And pressure checks