Hey fellas, got a 790 excavator with track speed issues, stuck in slow speed.... Can't seem to figure this out. Thanks for looking!
She's supposed to be a three speed right? Did it just quit working, and what have you checked? I'm not familiar but I see from the parts book there's two switches connected to the end solenoid group somehow, which sends pressure down through the swivel and tees off to go to the motor heads. Any codes from the CCU?
Thanks for the reply! Yeah it's a 3 speed, it quit working about a year ago, and I'm just looking into it now. It's a farm machine so it doesn't get too many hours. I have just started digging into it today, and so far I have been just trying to trace wires from switch, but have had no luck thus far. I'm not very knowlegdable with this kinda stuff so it's making it a little trickier! I found a couple solenoids, one is propel speed and one is speed change I believe , however not too sure how to test them, and the switch lights up when you choose desired speed, is there a relay I can check? Or what do you think my best plan of action is for diagnosing this problem? Thanks again any help would be gratefully appreciated!
Need a service manual, unless somebody else on here is familiar with these. First thing I think I would do is make sure it's not a hydraulic issue by taking the hose that comes from the solenoid manifold block to the swivel joint off and coupling a pressure guage on the end of it. 500 psi should be safe. There should be three pressure levels of course, low I would assume would be zero or near it psi, medium over 100 psi as a guess and high full pilot pressure or near it. The exact numbers aren't that important at this point, you'd just be checking to see if the electrics and the solenoid valves are delivering a different pressure in each speed. If the gauge never comes off the peg, the problem is above the swivel, if you get three widely different pressures then the problem is in the swivel or one of the motors bypassing too much oil. Should be the smallest hose connected to the center joint so it should be easy to recognize, might be a bugger to get at though. If the swivel seal has gone bad all the pressure is leaking off to the drain line back to tank maybe. The brochure says it's automatic shift, so I'm guessing the shifting is controlled by the pump Computer, maybe reading the pump pressure sensors to tell it when to shift down, could even be talking to the engine computer about engine loading. Modern technology.
So I put a gauge on hose from manifold to swivel, got 90psi at start up and no change through the speeds , at low or high idle, and while moving. Possibly solenoid?
I have a service manual for this machine but it doesn't really provide any information on this part of the machine..
You did disconnect the hose from the swivel, yes? Could be a bad solenoid, or something to do with the computer not sending the right signals. Anything in the manual about the solenoid valve block?
Good day sir! Yes I disconnected the hose at the swivel and put the gauge on at that point. I took off the solenoid at the block where the hose to the swivel goes, and the book says that's the propel speed solenoid. I tested the solenoid just on a battery and it functions properly, but I just found something underneath the cab, a sensor on a hydraulic block was broke in half so I'm just gonna look and see if that hydraulic block is related to the propel speed. Will keep you posted as to what I find! Thanks man !
According to electrical schematic in manual that broken sensor is called "propel speed pressure switch" so I will get a new one coming and see if that fixes the problem. Will keep you posted
Never mind upon closer inspection that component turns out to be a "boom Raise pressure switch" darn.