Lmtv cab lift safety question

[aaaiv]

Hello all,
I recently acquired a 1996 lmtv and have a question relating to safety during a cab lift for maintenance. The previous owner instructed me that the lift has built in safety catches as it goes up and therefore does not need to be secured, strutted, etc.

Now I left the cab up and noticed overnight that it had dropped significantly. Furthermore I left it up for a couple of days and it dropped all the way down.

My question is: How is this safe to be working under? If it catches, how can it drop over time?

 

[Ronmar]

The only safety in this system are the flow sensitive check valves at the outlet of the cylinder. If a hose or valve should fail, the increased flow out of the cylinder would cause it to block flow and lock the cylinder.

in operation when parked on level ground, the cab should raise past the balance point when fully raised, which pulls the cylinder into tension fully extending it. You should have to pump the cab back over the balance point when lowering to then get to a position where gravity can continue to lower it…

if you can simply turn the control from raise to lower and the cab starts to lower without pumping, it is not being lifted all the way. If it only raises until it stops moving, but does not go over the balance point, do you have enough fluid in the reservoir. It will stop when it runs out of fluid to pump…

Over the balance point, it can set this way indefinitely. If not over the balance point I am not surprised it leaks down, as I have yet to meet a hydraulic system that doesn’t leak in some regard.

i have some videos of the hydraulic system and cab lift on my truck on Youtube. Here is a link to one, you can find the rest from there(pressure tests, cab lift rod end proper operation, these ones on the manual system I switched to, ect)…

 

[aaaiv]

The only safety in this system are the flow sensitive check valves at the outlet of the cylinder. If a hose or valve should fail, the increased flow out of the cylinder would cause it to block flow and lock the cylinder.

in operation when parked on level ground, the cab should raise past the balance point when fully raised, which pulls the cylinder into tension fully extending it. You should have to pump the cab back over the balance point when lowering to then get to a position where gravity can continue to lower it…

if you can simply turn the control from raise to lower and the cab starts to lower without pumping, it is not being lifted all the way. If it only raises until it stops moving, but does not go over the balance point, do you have enough fluid in the reservoir. It will stop when it runs out of fluid to pump…

Over the balance point, it can set this way indefinitely. If not over the balance point I am not surprised it leaks down, as I have yet to meet a hydraulic system that doesn’t leak in some regard.

i have some videos of the hydraulic system and cab lift on my truck on Youtube. Here is a link to one, you can find the rest from there(pressure tests, cab lift rod end proper operation, these ones on the manual system I switched to, ect)…

Thank you for the critical details. It is very different than the instruction from my seller. It also appears that my system has been modified as I have an electric pump raise/lower with a control that looks much like a wired winch controller.


I'm going to quote my seller's incorrect instruction here in case it triggers other details that should be known/addressed for myself and others:

He said to "raise the cab simply press the up button and to stop before the cab tips forward, and that if I accidentally go to far up that I would have to push the cab back by hand until it gets to the point where the pump can work again. Additionally, if I go too far up beyond the tipping point I could possibly damage the cab or equipment."
Also note that my cab has a rack on top full of containers so perhaps he may be concerned that the weight of the load on top may too much to tilt forward. That is just a guess on my part.

Any additional responses are greatly appreciated.

 

[Ronmar]

Ok that has hack job written all over it…

I have questions:

1. Does it still have the military hydraulic cylinder and rod end in it? The rod end is important(i explain it in one of my videos).

2. Does it have the folding bars attached between the frame and cab where the top of the hydraulic cylinder is attached? That is a forward safety that keeps the cab from toppeling off of the front of the truck if the hydraulic cylinder should physically/mechanically fail.

3. Does it still use the original control valve manifold?

The previous owners instructions are ignorant for a few reasons. First off the greatest stress on the system is when you initially lift the cab off of the latch(watch my pressure test video). When fully raised and just over the balance point the stress is not very high. IE with the valve set to lower you can push the cab back over the balance point with your hands. More weight on top of the cab will make this a more difficult. Thats why y0u need to be on level ground. If the truck is parked uphill, it may not get over the balance point. The hydraulic cylinder being strong enough to even start the lift is certainly far more than strong enough to hold it fully extended at a significantly reduced load just past the balance point…

it is normal to pump the cab back over the balance point. The original control valve was designed to send hydraulic pressure to either port on the cylinder to push or pull exactly for this reason…

if the previous owner re-engineered it with an electric pump and did not make this provision, well back to my comment about hack job…

 

[GeneralDisorder]

Well you bought it from an idiot. Excellent. Be wary of anything that tool touched. Lucky he didn't get unalived by some other stupid notion.

Soldiers have been killed by the cab. I recall an incident where a soldier got their head stuck between the cab step and the tire. Fatality.

The Cab Support Tool is required when working on the cab with it in the partially raised position. The tool is part of the FMTV special tools kit that is fielded to every unit that has FMTV's. You can buy your very own here:

2590-01-559-7459 FMTV Tilt Cab Support US Army 12489328

2590-01-559-7459 FMTV Tilt Cab Support US Army 12489328

secoparts.net

When the cab is fully forward past the balance point and on a level surface you are safe unless big winds kick up - put it back down or on the support brace in that case

With the roof rack bull*hit up there you're going to bend the piss out of the floor of the cab unless you install the upgraded ram and reinforcement kit.

The hack job about not going fully forward or past the balance point is a huge red flag. Should have walked when you heard that. Get that system the hell out of there and put it back right.