Interesting M1078 Cab Hydraulic Problem

[wyocommo]

Have a '98 M1078 that has developed a problem with the cab hydraulics.( They used to work fine.) I have read through most of the hydraulic threads but this one is a pretty strange.
The spare tire system seems to operate OK and this issue is only with the cab.

The cab hydraulics seem to almost be locked up and working against itself. Cab moves very, very slowly both up and down. After disconnecting the cab lift cylinder from the anchor point on the frame it still has the same problem with no load. If I disconnect the cab cylinder hydraulic line that would be the bleed port(based on control valve being in raised or lower position) everything works as it should. If the bleed line is reconnected the system seems to pressure itself up and stop moving. It acts as if some check valve or such is not releasing the bleed side of the cylinder back in the hydraulic panel.

Tried doing this test with the selector valve in raise or lower with the respective bleed line disconnected from the cylinder and get the same result. The manual pump fights the same problem. The air driven pump slows way down when the system begins to go into the pressure lock condition. If I stop the pump for a few seconds and then re-engage it the cab cylinder will move slightly as the locked up bleed side appears to leak down a little.

Haven't been able to find a TM diagram that shows the internal works and passages of the particular hydraulic control box that is on my truck so I am stumped at this point.

 

[Suprman]

 

[wyocommo]

Is there a hydraulic schematic of the internals of this hydraulic control block that might designate the functions of the components, as in, check valves and reliefs?

 

[Suprman]

Maybe in one of the higher number tech manuals. There may be a repair flowchart for your issue in the tm. Maybe some dirt or something end up causing a partial clog. Maybe you can flush the system.

 

[agazza2]

What did you figure out on this problem. Mine is doing the same thing. I replaced the hydraulic cab cylinder with the newer armored cab model and it has the same problem.

 

[wyocommo]

What did you figure out on this problem. Mine is doing the same thing. I replaced the hydraulic cab cylinder with the newer armored cab model and it has the same problem.

Hello Agazza2
Just started working on this truck again. (its been an interesting journey)
Did you find the issue with your hydraulics?

 

[Ronmar]

Hello Agazza2
Just started working on this truck again. (its been an interesting journey)
Did you find the issue with your hydraulics?

If the tire lift raises and lowers fine but the cab does not, and when you remove the return line it moves OK, I would suspect the restricted orifice in the cab control valve is fouled. If you remove the handle and unscrew the valve the restrictor is screwed into the core of the valve. Its an allen head setscrew with a small hole drilled in it. I believe “Helicools Helipad“ (Dave) over on Youtube did a valve breakdown video that shows the location of that orifice…

 

[agazza2]

Hello Agazza2
Just started working on this truck again. (its been an interesting journey)
Did you find the issue with your hydraulics?

It turned out to be the control valve assembly being to worn. Even with new o rings it did the same thing. I replaced the whole assembly and it worked great. I tested a lever control hydraulic manifold and almost went that route. But then before putting it in permanently, I found a original one on Ebay.

 

[wyocommo]

It turned out to be the control valve assembly being to worn. Even with new o rings it did the same thing. I replaced the whole assembly and it worked great. I tested a lever control hydraulic manifold and almost went that route. But then before putting it in permanently, I found a original one on Ebay.

If the tire lift raises and lowers fine but the cab does not, and when you remove the return line it moves OK, I would suspect the restricted orifice in the cab control valve is fouled. If you remove the handle and unscrew the valve the restrictor is screwed into the core of the valve. Its an allen head setscrew with a small hole drilled in it. I believe “Helicools Helipad“ (Dave) over on Youtube did a valve breakdown video that shows the location of that orifice…

Great information.
I will hopefully get to work on this shortly.
Thanks for the replies.

 

[coachgeo]

IF CAB IS UP..... find some sound way to ensure it stays up ...... as you work on things....... and seek to have way to lower it in a controlled manner if a path way opens in hydraulics to let the cylinder retract.

 

[wyocommo]

IF CAB IS UP..... find some sound way to ensure it stays up ...... as you work on things....... and seek to have way to lower it in a controlled manner if a path way opens in hydraulics to let the cylinder retract.

Yep,
Been reading through all the post and that seems to be an absolute must.
I even find being under there, when all is working as it should, being a little spooky!!