FMTV Air Hydraulic Unit Repair

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sjohn116

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Can you recall any specific issues when changing out your air over hydraulic pump with priming the system and/or getting all the air out? If I recall correctly, you had to take off and replace the pump a few times and I don't recall if it was clear what the difference was between the two pumps that magically fixed your issue? I got a brand new pump, switch out the old, filled the new one, and nothing works (didn't switch the hydro lines, nothing clogging the air line). New unit works, as tested on the bench with an air hose, spits hydro fluid out like a champ. I can't figure out what isn't working anymore. Old unit gradually leaked worse and worse, as everyone else's has... To the point that it would no longer pump the cab up. Hence the new pump.

I have heard superman reference the system getting air locked...can you explain how to test/ reverse this scenario?
 

Suprman

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I have swapped pumps without issue. I had to bleed out a tire lift cyl once took the line off. You could have some air in there. Make sure you didnt accidentally switch the 2 hyd lines. You can take off the pump return line and put it to a container see if you can use the pump to purge the system.
 

sjohn116

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Thanks to NDT. I disassembled the brand new pump, air chamber was so new and tight that it wouldn't slide back and forth. Forced the issue manually a few times and reassembled, bech tested, worked like a charm. Put on truck....success. So new and so quiet...you couldn't even hear the pitter patter of the pump lifting the cab.
 

sjohn116

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One last question on the subject...the inside of the air chamber has a chamfer/groove in it, and the TM specifies "orient it above half way," meaning anywhere you want between 9-10-----2-3 o'clock position. What is the significance of this groove and orientation? Lubrication was my first thought, but I dont see any way for oil to cycle into the cylinder, unless your seals are junk, in which case you'd be rebuilding the unit. I just wondered.
 

Andyrv6av8r

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I think I figured out why so many were having problems with the unit not working after a rebuild. It took me 3 times of removing and installing to find the problem. And it was a very simple one at that.
20170410_160522.jpg 20170410_160245.jpg

When I rebuilt the unit, I turned it upside down in my vice to hold it while I disassembled everything. After the air chamber is out of the way, you have to unscrew the cylinder, part #3, that has the packing in it. I didn't realize it but there is a ball check, part #41, that is in the pickup, part #39, with the screen in it. When you remove the part #3 with the packing, that ball will fall out of the pickup if you have the unit upside down like I did. It will actually end up inside of part #43 and it won't pickup any fluid after putting it back together and in the truck. So remove the pickup, part #39 and ball #41, and reinstall them AFTER you have the packing and housing put in. This assures that the ball is where it is supposed to be.
 

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Coffey1

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Ok read everything has anyone figured out why cab will go up and down like it should, but spare won't move with pump or hand pump like it's binding.
 
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you are awesome Andy, I have all the seals ready to do this for my truck, I will let you know if I get it right


Seth


I think I figured out why so many were having problems with the unit not working after a rebuild. It took me 3 times of removing and installing to find the problem. And it was a very simple one at that.
View attachment 674847 View attachment 674848

When I rebuilt the unit, I turned it upside down in my vice to hold it while I disassembled everything. After the air chamber is out of the way, you have to unscrew the cylinder, part #3, that has the packing in it. I didn't realize it but there is a ball check, part #41, that is in the pickup, part #39, with the screen in it. When you remove the part #3 with the packing, that ball will fall out of the pickup if you have the unit upside down like I did. It will actually end up inside of part #43 and it won't pickup any fluid after putting it back together and in the truck. So remove the pickup, part #39 and ball #41, and reinstall them AFTER you have the packing and housing put in. This assures that the ball is where it is supposed to be.
 

agazza2

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Thanks everyone for your information on this repair. Mine started to puff the hydraulic fluid out of the vent a few weeks ago. I have order all the seals and a extra hydraulic pump that I will rebuild and have a spare. I found information from SPX where as mentioned earlier in this thread they recommend a lubricator inline (airline). The diagrams they show use a compressor lubricator.

Once I receive all the parts, I look forward to this repair with all the information that was provided in this thread.
 

Andyrv6av8r

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Concerning the bumper washer, if you take a unit apart that doesn't have the bumper installed, make sure you put one in. Coffey's didn't have one and it wouldn't work until we tore it apart and put one in.
 

Reworked LMTV

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Thanks for all the information on this thread! I'll be posting photos when the replacement part are installed, it won't be for a while since mine really isn't that bad.

Forgot to mention that Terry at SPX said. It is recommended that a in-line lubricator be installed. I thought that I would replace my compression air fitting at the unit with a Push-Loc air connector so I can add a few drops of lubricant once in a while to help with the wear of the seals.
Any more specifics on the "in line lubricator" ?
 

simp5782

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Any more specifics on the "in line lubricator" ?
It is called a mister. We use them on the CAT 7155 transmissions to keep the Orings in the transmission moist so the air doesn't eat them. Around $150 on ebay. You would need to know the scfm rate and the port side you would need in order to find the correct unit.

It is similar to adding a few drops of marvel mystery oil to the supply line off of the air dryer for the system. This allows any check valves to get some lubrication so they function properly and they dont seize up or get rusty from the water in the system.

Shown below is a parker mister in black.
 

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