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:

Has anyone done the hydraulic heavy duty cab lift upgrade?

Just rec'd, my cab hydraulic ram upgrade kit. There is a diagram, but no real instructions. Anyone else complete this upgrade?

www.steelsoldiers.com

The "hack job's" comments 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.

Lets see pics so we can determine what it is you purchased here

 

[Ronmar]

Well how much weight do you have on the roof? There is also a concern of that weight deforming the cab and popping out the windshield. For the gun mount installation they added A 1X2” steel support brace from the front of the dash up to the ceiling right in the middle of the windshield. If you look on dash and ceiling in the middle you will see the nutserts that hold the bracket in place… you might want to add one also…

 

[aaaiv]

Picture time in the rain.

 

[aaaiv]

Well how much weight do you have on the roof? There is also a concern of that weight deforming the cab and popping out the windshield. For the gun mount installation they added A 1X2” steel support brace from the front of the dash up to the ceiling right in the middle of the windshield. If you look on dash and ceiling in the middle you will see the nutserts that hold the bracket in place… you might want to add one also…

The cases are currently all empty.

 

[Ronmar]

OK so are you still using the cab control valve manifold seen in your pics to control raise and lower function of the cab lift, and the electric pump simply provides the hydraulic pressure like the AOP used to?

I might put a cargo box on the roof, I am certain I would not double stack them... I think I would also add a physical barrier, especially in the front, to limit their movement in addition to the cargo straps... I would also look very closely at the engineering of how that platform is attached to the roof...

 

[aaaiv]

I did not know about those controls until watching your videos. Here's a pic of the only controls my seller showed me. Now I need to get comfortable with them and see what's what. I've never used the bottle pump either. You and the rest of this forum are literal life savers. No joke, and I do not think this topic is a joking matter at all. Thank you.

Also, my truck does not use the ctis or ride height systems, and has the mme air ride kit for the cab.

 

[aaaiv]

I did not know about those controls until watching your videos. Here's a pic of the only controls my seller showed me. Now I need to get comfortable with them and see what's what. I've never used the bottle pump either. You and the rest of this forum are literal life savers. No joke, and I do not think this topic is a joking matter at all. Thank you.

Also, my truck does not use the ctis or ride height systems, and has the mme air ride kit for the cab.

My spare tire is on an UTV style electric winch with obviously again, it's own wired controller. It's interesting to see it listed in the panels control knobs. Many alterations have been performed over here. Wow.

When I push pump knob "in" a whirring noise occurs and a hydraulic fluid puddle forms on the top of the diesel tank.

None of the turn switches appear to do anything, but understand that I have no understanding on procedure.

The hand pump feels like it builds up pressure (harder to pump) and then the hydro fluid puddle grows in size.

 

[aaaiv]

My spare tire is on an UTV style electric winch with obviously again, it's own wired controller. It's interesting to see it listed in the panels control knobs. Many alterations have been performed over here. Wow.

When I push pump knob "in" a whirring noise occurs and a hydraulic fluid puddle forms on the top of the diesel tank.

None of the turn switches appear to do anything, but understand that I have no understanding on procedure.

The hand pump feels like it builds up pressure (harder to pump) and then the hydro fluid puddle grows in size.

With help I slowly took the cab up as high as possible.
There are installed off road lights on the top of the bumper that the grill comes into contact with, keeping you from getting into the safety zone. That's what the seller's instructions were protecting. Just completely ridiculous.

Concerning the weight on the cab rack, I'm going to follow your advice in addition to keeping the weight up there to a minimum.

I'm still alive and well. I haven't broke anything on the truck beyond what I bought pre-broken. The cab support bar is purchased. All is well. Thank you again.

 

[Ronmar]

well since operating the original cab hydraulic controls and hand pump deposits fluid on top of the fuel tank i would say they simply disconnected it and plumbed the electric into the cab lift latch and cylinder directly(hydraulic pressure releases the cab latch)? With the right type of electric, this can work as long as you can limit the flow enough that the downward flow does not trip the safety check valves in the cylinder base port. I have seen several electric setups but have yet to see one really properly engineered as most all commonly available electric Hydraulic Power Units(HPU) are grossly oversized for the tiny amount of flow(under 1/2 gallon per min) you need to raise the cab. They raise the cab but they need big wires and turn an awful lot of electric/battery current into hot motor and hot fluid... Pumps sized for this application are not commonly available which is why I went with the manual pump that I use(very simple)...

So if you push the up button on the yellow controller, the pump runs and the cab lifts?. if you push the down button does the pump run and the cab lower?

You should probably relocate those lights so you can get the cab over the balance point... As mentioned it is not the safest to have the cab suspended by/resting on hydraulic pressure. like I said, have yet to meet a hydraulic system that did not leak in some regard

 

[aaaiv]

well since operating the original cab hydraulic controls and hand pump deposits fluid on top of the fuel tank i would say they simply disconnected it and plumbed the electric into the cab lift latch and cylinder directly(hydraulic pressure releases the cab latch)? With the right type of electric, this can work as long as you can limit the flow enough that the downward flow does not trip the safety check valves in the cylinder base port. I have seen several electric setups but have yet to see one really properly engineered as most all commonly available electric Hydraulic Power Units(HPU) are grossly oversized for the tiny amount of flow(under 1/2 gallon per min) you need to raise the cab. They raise the cab but they need big wires and turn an awful lot of electric/battery current into hot motor and hot fluid... Pumps sized for this application are not commonly available which is why I went with the manual pump that I use(very simple)...

So if you push the up button on the yellow controller, the pump runs and the cab lifts?. if you push the down button does the pump run and the cab lower?

You should probably relocate those lights so you can get the cab over the balance point... As mentioned it is not the safest to have the cab suspended by/resting on hydraulic pressure. like I said, have yet to meet a hydraulic system that did not leak in some regard

Yes. The yellow controller operates the cab up and down. And yes, I need to move the lights. I am waiting on the safety bar to arrive before partially lifting the cab again, Generaldisorder's link worked cleanly.


I should probably start another thread for this but am curious how you feel on 6%+ grade downhill highway runs? These grades are all over Northern AZ. I do not have a tach and am concerned about running too high rpm. I saw you installed an engine brake, do you feel it is mandatory or insurance?Thank you Ronmar for your help.

 

[GeneralDisorder]

Put the hydraulic system back to stock. Remove the stupid electric bull*hit, rebuild the air-operated pump for a few bucks for the seals.

Too bad they deleted the tire crane. Fabulous design and mounted mid-chassis for weight distribution.

I should probably start another thread for this but am curious how you feel on 6%+ grade downhill highway runs? These grades are all over Northern AZ. I do not have a tach and am concerned about running too high rpm. I saw you installed an engine brake, do you feel it is mandatory or insurance?Thank you Ronmar for your help.

The transmission controller handles the engine RPM. No tach is necessary - think to yourself what an 18 year old kid would do when handed the keys...... The Allison was chosen precisely because the computer won't let you hurt the transmission or the engine no matter what the operator does with the accelerator, brakes, or trans controls.

Yes you should get a PAC brake. Call them - they make the military exhaust brakes for the A1+ trucks. There are zero drawbacks.

Also get ECO hubs if this truck doesn't have them. Hopefully it doesn't have highway gears already.

 

[chucky]

Ronmar has given you all the nuggets of wisdom there is to be had about the system and he is the GO TOO ... .... .... ..... ! My 2 cents is after the cab is fallen all the way past center of gravity ( it will scare you the first time it does ) measure from the back lip under the back of the cab with a long enough tape measure to either the ground or to a really stout place in the frame then either use a 4x4 post or a piece of 1 1/2 sq wall heavy tubing and this will be your brace to keep the cab from coming down on you if something bad happens . I was lucky enough to have had lived thru the cab coming down on my fmtv and resting on my skull on the airlift beam until i was rescued so dont be me and dont get too comfortable while your doing anything in or under these trucks cause they will kill you and never blink an eye !

 

[Ronmar]

You don't really need a tach. As general stated, the trans will protect and automatically upshift if RPM gets too high. You select a lower max gear with the down arrow but the trans will not shift unless RPM has dropped enough to allow the downshift. You use brakes to get the RPM back down to a point where the trans will downshift.. A pacbrake will make it easier for the engine to provide braking force.

From your pics, it appears as if you have a A0 truck with 3116 engine. What year is it and what transmission controller does it have WTEC2(green multi digit display) or WTEC3 (red single digit display)? They started phasing in the WTEC3's in mid 98... The WTEC 3 is already programmed for exhaust brake with a programmed downshift gear pre-select(it will only downshift if RPM is within acceptable limits). It however is not specifically necessary, I fit one on my WTEC2 and will simply downshift manually as I see fit... I did a video on how i have that configured in my truck/dash.

Call Pacbrake you will want to have your engine S/N from the sticker on top of the valve cover so they can confirm appropriate backpressure(earlier 3116's could not handle as much back pressure as later models) range for a PRBX type exhaust valve...

Eco hubs will make things a lot kinder on your engine and driveshafts(halvs running RPM at highway speed)... How much does your truck weigh?

 

[GeneralDisorder]

In his first post he said it's a 1996

 

[Reworked LMTV]

I did not know about those controls until watching your videos. Here's a pic of the only controls my seller showed me. Now I need to get comfortable with them and see what's what. I've never used the bottle pump either. You and the rest of this forum are literal life savers. No joke, and I do not think this topic is a joking matter at all. Thank you.

Also, my truck does not use the ctis or ride height systems, and has the mme air ride kit for the cab.

There is a lot of physics to get this cab open and shut. For example, the load on your hydraulics is non linear. Initially the load is high, then once you hit the center of gravity the force needed drops suddenly and dramatically. Add to the mass weight to a rack and you have more momentum. Tall items create leverage as hit the center of gravity. Thus, it is easy to over shoot. Your system does not allow dither the hydraulic valve as you reach maximum extension. It is a simple on and off electro mechanical, making a safe landing nearly impossible. It is true that you likely have too much flow, but you may be able to compensate for this by using something like a Prince Flow control valve. Regarding the rack, if your rack is steel, replace with an aluminum one that shunts some of the weight vertically to the gutters with gutter clamps. If the crane is attached to the cab top, I would remove it. Too heavy. The stuff packed on top should be light and bulky things. Find a local hydraulic shop. Fivver has hydraulic engineers on there too. Pick their brain. All is not lost, but hydraulic design is best left to engineers.

 

[Reworked LMTV]

Picture time in the rain.

Wait! Are those heavy duty electrical wires with no conduit or cover? !