Lmtv 1078 spare tire lift cylinder stuck. Again

[Ned81]

I’m on my 3rd lift cylinder and it got stuck just like the first 2. I can’t figure out what’s going on. I figured the first cylinder locked up so I bought another. Then it locked up after the 2nd down up cycle. So I got another one. It just locked up like the other 2 after one down- up. I have cleaned the screen and the restrictor screw under the raise /lower selector valve. That didn’t do anything. So then I replaced the o-rings on the valve. Still locked up. Then I cleaned and inspected the the bottom selector valve that switches between tire, cab, suspension. It looked fine. I topped off the oil. Still stuck. I even tried pulling the lift arm down with a ratchet strap while I pumped it by hand. Still won’t bugle.
Is my luck that bad that I went through 3 cylinders? Any suggestions?
Thanks

 

[Ronmar]

The cab and tire cylinders have safety checks in them. air trapped in the cylinder can cause them to lockup. Basically the air stuck in the cylinder compresses and stores energy, and when you switch the valve, it releases the pressure. The compressed air forces the fluid fast enough to lockup the check valve, holding it there and freezing any further movement.

You can remove the hose on the cylinder and the elbow fitting, then remove the clip and screen and push a small allen wrench or rod in thru the hollow setscrew and spring to push the check ball back off the seat and release any stored pressure. Depending on how much pressure is stored, you may need to tap on the rod you insert in to touch the ball. When the pressure/fluid is released you should be able to feel the spring on the other side of the ball when you push in on the rod. The spring on either side hold the ball centered between 2 seats so fluid can flow either way past the ball untill it trys to flow too fast and forces the ball against a seat, locking up the cylinder. They are supposed to leak a little to release by themselves, but the air pocket trapped in the cylinder means it has to leak a lot of fluid to release enough pressure to unlock…

Once you release the pressure, put the parts and hose back in place and lower the spare, then remove the rod end pin from the tire crane so you can stand the cylinder with the rod pointing up. This will let the air rise to the outlet pipe so it can escape ahead of the fluid when you cycle it, then cycle the cylinder fully in and out a few times to make sure you get all the air out of the rod end of the cylinder. Then without removing any hoses, lay the cylinder back down and reconnect the rod end to the crane and lift the tire back in place…

Aren't hydraulics fun

here is a video I did showing how it is put together, luckily the one with the safety is the easy hose to get to…

 

[Keith Knight]

Thanks for the explanation that happened to my cab lift once and freaked me out. It finally started working but I never understood why and what fixed it.

 

[Ned81]

The cab and tire cylinders have safety checks in them. air trapped in the cylinder can cause them to lockup. Basically the air stuck in the cylinder compresses and stores energy, and when you switch the valve, it releases the pressure. The compressed air forces the fluid fast enough to lockup the check valve, holding it there and freezing any further movement.

You can remove the hose on the cylinder and the elbow fitting, then remove the clip and screen and push a small allen wrench or rod in thru the hollow setscrew and spring to push the check ball back off the seat and release any stored pressure. Depending on how much pressure is stored, you may need to tap on the rod you insert in to touch the ball. When the pressure/fluid is released you should be able to feel the spring on the other side of the ball when you push in on the rod. The spring on either side hold the ball centered between 2 seats so fluid can flow either way past the ball untill it trys to flow too fast and forces the ball against a seat, locking up the cylinder. They are supposed to leak a little to release by themselves, but the air pocket trapped in the cylinder means it has to leak a lot of fluid to release enough pressure to unlock…

Once you release the pressure, put the parts and hose back in place and lower the spare, then remove the rod end pin from the tire crane so you can stand the cylinder with the rod pointing up. This will let the air rise to the outlet pipe so it can escape ahead of the fluid when you cycle it, then cycle the cylinder fully in and out a few times to make sure you get all the air out of the rod end of the cylinder. Then without removing any hoses, lay the cylinder back down and reconnect the rod end to the crane and lift the tire back in place…

Aren't hydraulics fun

here is a video I did showing how it is put together, luckily the one with the safety is the easy hose to get to…

That makes sense. Thank you! Good video too. I suspect I have 2 extra working cylinders now. It’s 13 out at the moment so I’m going to wait a bit but I’ll report back my findings.


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[Guruman]

Mine was all inoperative when I first got it, thought it was a bad cylinder. It turns out it was the restrictor behind the control valve. I took it out, cleaned the orifice with a small wire, and removed some gunk from inside the hole it came out of, it's been working like a champ since.

The hardest part was getting a wrench in to remove the valve. I had to remove the one above it, just so I could get a wrench on it to remove it. Then it was just a few tries to get the right size Allen key in there all the way to the back to get the little restrictor plug out. Took maybe 10 minutes total.

 

[Third From Texas]

That makes sense. Thank you! Good video too. I suspect I have 2 extra working cylinders now. It’s 13 out at the moment so I’m going to wait a bit but I’ll report back my findings.


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"Automated lowering bed access ramp" in your future?

 

[Ned81]

[mention]Ronmar [/mention] I took the elbow and all the parts out of one of the other cylinders I have. I see the ball and spring valve set up. It was free on that one ( been sitting for a couple weeks disconnected) I installed it on the truck. Extended the cylinder. Then stood it on end with the chrome rod pointing up. I let it sit like that for 10 min or so then cycled the cylinder in and out full stroke 3 times while still standing up. I attached it to the crane lowered it 85%. Raised it up all the way and it stuck again. That is exactly what it has done each time I replaced the cylinder. Goes down, raise the tire into place then it gets stuck and will not go back down. It’s getting dark so it’s going to stay stuck for now. What did I miss?


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[Ronmar]

Disconnect the line I showed in the video, place it in a clean container with some new fluid in it so the end is Under the fluid. Command a tire raise and fluid should flow out of that hose into the container. are there any air bubbles in the fluid coming out of the hose? A bad seal in the AOP might allow it to add air to the hydraulic oil, which would negate you carefully purging the air as soon as you raise the unit for the first time…

 

[Wingnut13]

Hmmmm, soon after I got my truck my tire lift cylinder locked up. The PO claimed to have just rebuilt the pump. I replaced the cylinder with a commonly available cylinder. I was sure it was corrosion in the cylinder that got it stuck.

I’m now gonna check it again. It’s helping to hold my steel rack down currently. I couldn’t toss it.

 

[Ned81]

Disconnect the line I showed in the video, place it in a clean container with some new fluid in it so the end is Under the fluid. Command a tire raise and fluid should flow out of that hose into the container. are there any air bubbles in the fluid coming out of the hose? A bad seal in the AOP might allow it to add air to the hydraulic oil, which would negate you carefully purging the air as soon as you raise the unit for the first time…

I’ll give that a try and post an update.


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[Suprman]

So when you remove the valve to clean the restrictor sometimes the old o rings on the valve dont reseat and you end up with pressure on both sides of the cylinder. You can use a generic o ring kit. Use the largest o rings that fit tight on the valve and still allow you to screw the valve back in smoothly.

 

[Ned81]

So when you remove the valve to clean the restrictor sometimes the old o rings on the valve dont reseat and you end up with pressure on both sides of the cylinder. You can use a generic o ring kit. Use the largest o rings that fit tight on the valve and still allow you to screw the valve back in smoothly.

I changed all 3 o rings on the valve when I had it out. When I put it back together the cylinder was still stuck. It wouldn’t move at all even after having the valve out.




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[Ned81]

Update.
So I ended up taking the whole truck to a local shop that repairs heavy trucks AND hydro cylinders. Oddly enough they had another used cylinder and a NOS cylinder in stock. ( what are the odds?) Turns our they are very familiar with these trucks. They thought the used one was good so we tried it. Locked up.
The mechanic cut one of the old cylinders apart to see what’s going on. He found that the cylinders are in fact serviceable. The end where the rod comes out can be pressed in about a 1/2” or so to reveal a lock ring. Remove the lock ring and the seal come out. He also bench tested and fiddled with the other bad cylinders and has come to the conclusion that over time the balls under fittings seem to get stuck. He’s not sure why yet but it seems like it could be age related. Maybe weak springs... He is experimenting on cutting a very small grove under the ball to prevent it from fully sealing. He doesn’t think it was designed to fully seal.
He put the NOS on the truck and it works great so it’s definitely the cylinder not the truck.
If he’s able to repair the old cylinders,I’ll post a follow up with the details.


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[coachgeo]

Update.
So I ended up taking the whole truck to a local shop that repairs heavy trucks AND hydro cylinders. Oddly enough they had another used cylinder and a NOS cylinder in stock. ( what are the odds?) Turns our they are very familiar with these trucks. They thought the used one was good so we tried it. Locked up.
The mechanic cut one of the old cylinders apart to see what’s going on. He found that the cylinders are in fact serviceable. The end where the rod comes out can be pressed in about a 1/2” or so to reveal a lock ring. Remove the lock ring and the seal come out. He also bench tested and fiddled with the other bad cylinders and has come to the conclusion that over time the balls under fittings seem to get stuck. He’s not sure why yet but it seems like it could be age related. Maybe weak springs... He is experimenting on cutting a very small grove under the ball to prevent it from fully sealing. He doesn’t think it was designed to fully seal.
He put the NOS on the truck and it works great so it’s definitely the cylinder not the truck.
If he’s able to repair the old cylinders,I’ll post a follow up with the details.


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petty sure the ball is check valve to protect soldier B. Keeps the tire lowering mechanism from falling too fast or failing then falling taking soldier B to cranium re-manufacturing plant.

some one posted a video that showed how to snake out an inner ring thru a hole to allow for rebuilding these.

 

[coachgeo]

some one posted a video that showed how to snake out an inner ring thru a hole to allow for rebuilding these.

think this is the secret to opening up these ...

 

[Ned81]

Yes. The ball is to prevent it falling by “floating “ in between to springs. It supposed to let it down slow but if the ball goes too far it seals.

Yes that is how our cylinders come apart.


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[Ronmar]

petty sure the ball is check valve to protect soldier B. Keeps the tire lowering mechanism from falling to fast or failing then falling taking soldier B to cranium re-manufacturing plant.

some one posted a video that showed how to snake out an inner ring thru a hole to allow for rebuilding these.

Yep, thats its purpose. It is supposed to be able to leak past to self release once the load is removed. Perhaps that bypass clogs over time…