Critical FMTV PM not covered in TMs

Awesomeness

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I wish I could get a better look at what the latch is doing when the cab lowers, but there is just too much in the way with no good way to view it. It seems like the cab is seating fully in the latch - far enough for the lock pin to engage - but I want to prove it. I wonder if I can raise the cab, push a screwdriver in the latch, and somehow mark at what point the lock pin is able to move. Then I can see if the cab is getting low enough in the latch, to eliminate that.
 

Awesomeness

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I rarely pump the cab all the way down. Once it's over center, I just let gravity bring it all the way down. It should self latch
I can say for a fact the cab will pivot forward if you slam on the brakes while moving.:rolleyes:
Once my cab is over center it does not gravity-feed down. It stays there unless I pump it. The same is true of raising. Is that abnormal?
 

Lmtv772

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As per Tm it should fall on it's own.
On the other hand , in PM they say you should keep pressing the button , or pump unit the cab is all the way down
 

LFGeorge

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So, when the cab hits the latch, and the pump is done, the hydraulic pressure on the pin is relieved, the pin spring is supposed to insert the pin over the cab latch. This might be impeded by latent hydraulic pressure, excessive friction from dirty parts, cab not fully seated or contaminates in the hydraulic line. My plan is to raise cab, put a block to support the cab where the catch is, while in the upright position, remove, the catch assembly (not the hydraulic hose) then lower cab. After lowereing, then the catch assembly can be taken apart for cleaning and inspection. Sound OK?
 

Keith Knight

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Thats how i did it when I discovered my latch not latching after it was in the down position. I notice some one in the military had it apart. so i took mine apart and discovered the spring that latches it after the hydraulic pressure is relived was missing. To get it working again I found a spring at the local hardware store that was the correct diameter and I cut it to a length that worked replaced the o-ring and now it works great. Oh and I polished/cleaned the shaft and bore.
 

tennmogger

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Couple of thoughts: The latch assembly as attached to the cab might be alignable by loosening the bolts. I don't know, but it seems logical to have some way to mate up the latch with the catch. Anyone know?

Also, have you tried the latch process with the cab in both modes, air bag support, or not? Maybe one or the other positions will allow the latch to catch.

As with others, my cab will descent just by it's own weight. Since yours will not, that may be a strong clue of retained hydraulic pressure.
 

Awesomeness

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As with others, my cab will descent just by it's own weight. Since yours will not, that may be a strong clue of retained hydraulic pressure.
This is my current leading theory. Any idea what would cause that, or where I should start looking to solve it?

Also just to clarify, what constitutes as the lock pin being fully latched? I'm guessing that fully locked means the head of the pin is submerged under the face of the nut, not sticking out at all? Is that correct, so if it's sticking out at all it's not locked "enough"?
 

LFGeorge

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I haven't messed with the cab air bag support. I will try that. My cab does descend on it's own. Seems to sit in place OK resting on the outside hooks and the center latch. I even got up in it and jumped around, didn't seem to settle any more. I pumped it down quite a bit with the hand pump, to no avail. I have been hitting it pretty hard and it doesn't budge.
Len
 

DSD277

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The pin through the nut is an indicator. On the other side is a similar pin that when the latch wraps around the strike, the pin blocks the latch from rotating. In simple terms, like a door latch on a modern car.

Other things to check. Remove the hydraulic line and pump fluid into a pail, in case there is 'gunk' in the line. Also make sure the cylinder passages don't have anything in them either. Any restriction in the fluid leaving the cylinder may hold the piston back. Lastly, with the cab raised ( and with a safety block) check to see if the latch rotates completely and freely, then check to see if the pin slides within the cylinder and extends to block the latch from rotating. DO NOT force anything. If you bend any of the pins, you are screwed and will need to replace parts.

I went through my LVAD and learn most of it on my own and with some help from BAE in 2012/13. I attempted to document most of what I did and saw on the truck for others.
Both in my thread in the sig line, and on my photobucket.
http://s10.photobucket.com/user/dsd277/library/FMTV?sort=2&page=1

I had taken thousands of pics while I worked on it, many for reference to prep for the next time I would go work on it. What I posted was mainly that the next person to work on their truck wasn't going into it blindly. I know I had pics of the latch assembly, but couldn't tell you where they're at now.
 

Awesomeness

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Just an update. No amount of raising/lowering, prying, jumping on, or clamping would latch the cab. I drove it a mile or two, and it was latched. It must just need a bit of jostling that can't be done by hand.

I'm wondering if maybe it just needs to have the pin taken out and a little bit heavier chamfer put on in with a lathe.
 

LFGeorge

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I took mine apart yesterday. The pin was binding and wouldn't easily go in. I ran my dremmel awhile cleaning the pin and the shaft it slides in, and it seemed to go OK, but not great. Obviously, too loose and it will leak hydraulic fluid. I then started pumping out oil, to see what it looked like. Came out as a milky orange colored oil. Pumped most of that out, replacing it with hydraulic oil. Mine does not have an o-ring on the small part of the pin, just the larger push ring has an o-ring. O-ring there looks good. There is a black metal ring in the shaft for the small part of the pin, but this seems OK, though tight. Reassembled everything and still no pin move. The cab seems to rest on the two side clips, not the center catch when fully retracted, so I lowered the center catch so it hits first. I did get some pin movement but not all the way. Took a close look at the catch slide and noticed a burr on the end. Dremmel came out again. Seems to slide a bit more, but not all the way. Got tired and quit. Plan to dremmel some more today. Anyone have any idea what I'm seeing for orange oil? Am I missing something?
 

Keith Knight

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Milky oil is an indication of water in the oil. My latch and cab catch were not parallel with each other preventing the pin from easily engaging.
So I installed some shims between the latch and the mounting surface in order to get the latch parallel to the cab catch. I also used longer bolts. Remember to readjust the cab catch height.
 

Awesomeness

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I took mine apart yesterday. The pin was binding and wouldn't easily go in. I ran my dremmel awhile cleaning the pin and the shaft it slides in, and it seemed to go OK, but not great. Obviously, too loose and it will leak hydraulic fluid. I then started pumping out oil, to see what it looked like. Came out as a milky orange colored oil. Pumped most of that out, replacing it with hydraulic oil. Mine does not have an o-ring on the small part of the pin, just the larger push ring has an o-ring. O-ring there looks good. There is a black metal ring in the shaft for the small part of the pin, but this seems OK, though tight. Reassembled everything and still no pin move. The cab seems to rest on the two side clips, not the center catch when fully retracted, so I lowered the center catch so it hits first. I did get some pin movement but not all the way. Took a close look at the catch slide and noticed a burr on the end. Dremmel came out again. Seems to slide a bit more, but not all the way. Got tired and quit. Plan to dremmel some more today. Anyone have any idea what I'm seeing for orange oil? Am I missing something?
My problems were not the same as yours. The inside of my latch cylinder was shiny and clean, and it all moved smoothly by hand.

There is (or should be) an o-ring that seals the large latch-side pin, but it's inside the cylinder housing, not on the pin itself.

The "orange" fluid is likely because the correct military fluid for the hydraulics are actually red, before the motor pool or civilians put whatever generic hydraulic fluid they could find in there. So "orange" is likely a a mix of the red transmission-fluid-like color of the military fluid, plus clear or amber. I was asking around about the red military fluid in a post last week... http://www.steelsoldiers.com/showthread.php?154182-Civilian-hydraulic-oil-for-cab-lift-reservoir

The #216 o-ring really isn't a perfect replacement for the original, on the large part of the piston/pin. It seems the overall diameter is about right, but it's not thick enough. You may run into issues using a #216, if you've Dremeled away even more material, since it probably already fits more lightly than designed. While you're putting it back together, I'd at least add a backup ring, such as this: http://www.mcmaster.com/#5288t259/=12js8ow Alternatively, you might figure out if there is an o-ring size that is thicker, or use a square or different profile ring.
 

LFGeorge

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Thanks for the help to all. This is an incredible website for someone like me. A little short on knowledge regarding these things but willing to try. I got the latch working. Mine has no o-ring on the small end, that I could find. Just a large o-ring on the fat part of the pin. After getting the pin to slide a little easier, I started messing with the latch alignment. I loosened the bolts where it attaches to the cab just a little so that with persuasion I could tap it around a bit. I got it into a better seated position and voila, the pin magically slid in. Tightened it up and it continues to work. Now for the headlights. I couldn't find a TM for the FMTV so I am using the LMTV TM. Seems to be the same, sort of. Symptom is no headlights when the switch is in the low beam setting. After a bit of time the breaker kicks off. I pulled the bulbs, they test out OK. I checked continuity between the bulb wire sockets and the ground wire socket. I assume 91 is ground. Got continuity on the socket that is supposed to be high beams, number 17, (left side looking into the bulb assembly), both sides. I assume now, I am looking for a bad wire issue, but why on the high beam wire, when the low beams aren't working? Suppose I should check the wire sockets and look for current to verify which one is hot with hi or low beams. Is there an electrical schematic for this somewhere? I've searched, but can not find one for the FMTV.
On a different subject, I found some breaker fuses on line that look like the original, but they are 12 volt. Is there any problem using a 12 volt fuse where a 24 volt fuse belongs? My truck came with a 24 volt fuse stuck in a 12 volt circuit, which I corrected. Thanks again to all who have helped, I am new to this adventure and this forum has a ton of good info.
 

coachgeo

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... I couldn't find a TM for the FMTV so I am using the LMTV TM. .....
possible your not finding the TM's you want because of a term confusion? FMTV's is the name for the whole Family of these trucks, be they any of the various LMTV 4x4, or any of the various MTV 6x6. What truck do you have?

If your looking for MTV (6x6) TM's there is a thread that tells you were to locate those but its hard to search for it since you can't search in a forum by 6x6 or MTV (not enough letters). Try this link
http://www.liberatedmanuals.com/search.mpl?q=M1088&Search=Search
 
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LFGeorge

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Thanks for the tip on the location of the TM files. I started following the troubleshoot guide on the low headlights. Got to the point where I check continuity between one of the relay socket slots and the headlight wire. I noticed the relay was "missing" the tab that inserts where I was supposed to be checking. Examined it more closely, and it was the wrong part for that socket. Some relays are four tab and some are five tab. Headlights use the five tab. So I switched it out and discovered the headlights are now fine. Sometimes it's right there in front of you, but you can't see. Oh well, got a bunch of connections checked and ground wires polished up.
 

DSD277

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Back in the day when I was working on the FDT, I had made sectional pdf's from the TMs of items I needed to address, and the PDP was the first.
This is for the "A0" cab. Most of the relays are SPST (4 pin) , but a few are SPDT(5 pin). Just pay attention to the trigger voltage
 

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LFGeorge

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Thanks for the PDP, that will get printed and sleeved for my cab notebook. Can you tell me if the voltage on the breakers matters? I have plenty of 12 volt breakers, but not finding any 24 volt ones. After today, I've about got it memorized. I went through every relay and breaker, looking for variations from specified. Found two others that were incorrect.
 
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