LMTV Cab will not go down

[Trey Bourette]

Greetings, new to LMTV world and Steel Soldiers. Hoping to get some smart input. There were issues with the hydraulic when I purchased the 1994 LMtV but the cab did go up and down though imperfectly. Now we have lifted the cab and we cannot get her to pump, pull, bleed, or in any way come back down. Thanks for any ideas.

 

[wheelspinner]

Trey I asked the mods to move this to
The correct spot to get you your answers

 

[gringeltaube]

Moved to the FMTV forum.

 

[Suprman]

Is the system out of fluid? How does the hand pump feel? Pump up firm or not? If it gets firm you may have a clogged restrictor. If not firm it may need more fluid.

 

[Ronmar]

What Suprman said. if the hand pump isn’t firm/solid, there is a fill plug for it’s reservoir to the left of the handle mechanism.

If the pump handle gets solid with the control in the “lower“ position but no cab movement, there is a restricted orifice inside the raise/lower control valve that may be clogged. The orifice controls the speed at which the cab moves up or down. There are safeties in the cylinder that cause them to lock if the hydraulic fluid flows too fast In or out of the cylinder, like if you blew a hydraulic hose. Occasionally if there are contaminants in the fluid, the safeties get clogged and they lock up and prevent the cylinder from moving. You can sometimes free/reset the safeties if you alternately pump in the raise then lower position several times.

you can determine which it is by loosening(don’t remove it, just crack it loose) the hydraulic fitting that feeds into the bottom of the cylinder and trying to pump the cab back down. If the cab cylinder starts to move and fluid comes out of that loosened fitting, the problem lies in the control valve, probably that restricted orifice.

a gentelman on youtube Helicools Helipad just made a video on how to disassemble and clean it...

 

[Trey Bourette]

Trey I asked the mods to move this to
The correct spot to get you your answers

Thanks

 

[Trey Bourette]

Is the system out of fluid? How does the hand pump feel? Pump up firm or not? If it gets firm you may have a clogged restrictor. If not firm it may need more fluid.

It all seemed good, including the hand pump, when the cab was raising With the hydro /air pump but now that it is raised we have tried “everything “ to lower it. The pump would not lower it in all kinds of control configurations, added fluid of course.

 

[Trey Bourette]

What Suprman said. if the hand pump isn’t firm/solid, there is a fill plug for it’s reservoir to the left of the handle mechanism.

If the pump handle gets solid with the control in the “lower“ position but no cab movement, there is a restricted orifice inside the raise/lower control valve that may be clogged. The orifice controls the speed at which the cab moves up or down. There are safeties in the cylinder that cause them to lock if the hydraulic fluid flows too fast In or out of the cylinder, like if you blew a hydraulic hose. Occasionally if there are contaminants in the fluid, the safeties get clogged and they lock up and prevent the cylinder from moving. You can sometimes free/reset the safeties if you alternately pump in the raise then lower position several times.

you can determine which it is by loosening(don’t remove it, just crack it loose) the hydraulic fitting that feeds into the bottom of the cylinder and trying to pump the cab back down. If the cab cylinder starts to move and fluid comes out of that loosened fitting, the problem lies in the control valve, probably that restricted orifice.

a gentelman on youtube Helicools Helipad just made a video on how to disassemble and clean it...

Thank you. I will look for that video. Much to consider in what you wrote, I will have a look. Not sure if this is important but the first time we raised the cab the spare tire Lift was moving As well. The spare tire controller was not set to raise or lower only the cab was. Also it does seem there is a leak at the cab lift / lower control valve if that make an impact on this.

 

[Trey Bourette]

It all seemed good, including the hand pump, when the cab was raising With the hydro /air pump but now that it is raised we have tried “everything “ to lower it. The pump would not lower it in all kinds of control configurations, added fluid of course.

I do appreciate it and will have a better look at the hand pump feel.

 

[Suprman]

If you have bad O rings on your selector valves, you may have hyd pressure ending up on both sides of the circuit at once. You can remove a valve they unscrew. The function selector is different but you can swap the cab suspension up down into the cab tilt up down. Just be careful it is possible for the cab to fall in some situations with the valve removed.

 

[Trey Bourette]

If you have bad O rings on your selector valves, you may have hyd pressure ending up on both sides of the circuit at once. You can remove a valve they unscrew. The function selector is different but you can swap the cab suspension up down into the cab tilt up down. Just be careful it is possible for the cab to fall in some situations with the valve removed.

Thank you. Picked the Covid so once out of this wilderness I will attempt to apply your ideas.

 

[Floridianson]

All good answers to question. My Silly question. On the passenger side Hyd. ram goes to the linkage that can go into the full cab lock position. Is there a way it is not pulling out of the full lock position? I do not have a pic of the linkage in full lock but it is talked about in some threads here and think it was M35 Tom that brought the question up about how it works. The fix and why it does not work sometimes when rusted up.

 

[MrMikey4026]

Has the cab safety lock been unlocked?
Check Helicools Helipad on youtube, He has a video of the cab cylinder cam and lock.
When mine does cam and lock, it is a pain to get the safety lock unlocked..
I am happier with a stuck cam, the cab tilts far enough that it cannot fall back buy itself.
If on level ground anyway.

 

[Ronmar]

Has the cab safety lock been unlocked?
Check Helicools Helipad on youtube, He has a video of the cab cylinder cam and lock.
When mine does cam and lock, it is a pain to get the safety lock unlocked..
I am happier with a stuck cam, the cab tilts far enough that it cannot fall back buy itself.
If on level ground anyway.

That is not a safety lock, as it requires no action to lower the cab if it is fully extended(it just folds right up on it’s own). Its purpose is to keep the cab from going too far forward in the event of a cylinder or linkage failure. With normal rod end function, the cab does not lurch past the balance point and that linkage should not fully extend. It is also beyond the balance point when the rod end is in the short axis. If it wasn’t you could shift the control back to lower and it would just start down by itself, but it doesn’t and you must pump it a little to get it back over the balance point.

if your dual function rod end isn’t working, you really should fix that as it is important that the rod end be alighned with the long axis when the cab is down to keep from damaging the cylinder or mounting structure as the cab floats on it’s air suspension...

I did a video on how it is supposed to function...

 

[MrMikey4026]

I will look at the video again. I have had the cam out. Cleaned the rust, greased it and still most of the time it it will still not turn into the slot, if I turn it with a wrench it will. When it does the the stay will not fold when I jack the cab down, I have to pry or reach down and pull it up to get it to fold. Maybe it is rusty? I don't know, but it sounds like I have more work to do.
It is possible that mine is working correctly, I can't get a good view, listen on my phone. I am going to have to look at it on the computer.
Thanks Ron

 

[Trey Bourette]

All good answers to question. My Silly question. On the passenger side Hyd. ram goes to the linkage that can go into the full cab lock position. Is there a way it is not pulling out of the full lock position? I do not have a pic of the linkage in full lock but it is talked about in some threads here and think it was M35 Tom that brought the question up about how it works. The fix and why it does not work sometimes when rusted up.

Yes, considered all this but definitely not. I did get her working after draining all the fluid and replacing it. Looked as if the wrong very thick almost motor oil was in the system. The cab lift now goes up and down but the cab lock is not working and the spare tire and suspension are not working. Going to bleed all and see if I can get them to function once the improper fluid has been removed.

 

[Ronmar]

You want 10wt hydraulic oil in the system, otherwise known as AW32 hydraulic oil.

 

[Trey Bourette]

You want 10wt hydraulic oil in the system, otherwise known as AW32 hydraulic oil.

Thanks!

 

[Floridianson]

Opening Cab | SteelSoldiers

 

[WassawBound]

If you have bad O rings on your selector valves, you may have hyd pressure ending up on both sides of the circuit at once. You can remove a valve they unscrew. The function selector is different but you can swap the cab suspension up down into the cab tilt up down. Just be careful it is possible for the cab to fall in some situations with the valve removed.

Can you share more information on how to do this, a photo, video or link? My spare tire is stuck on the rack and will not lower. This is the 2nd time it's happened. The 1st time the truck was in the shop for a while and it just worked after sitting. I need to mount new tires and now it's sticking again. The pressure just builds up and it will not pump. I think it's a bad valve as you described.