Frustrating 809-series sprag!

OK guys. I have had a sprag issue with my XM818 since I picked it up. I had to pull the drive-shaft to bring it home from NC because the sprag was stuck in Reverse.

I have the truck running and driving great now and the only issue holding me up is the dang sprag. I started trouble-shooting the thing about a month ago.

The first thing I tried was disconnecting the air lines from the actuator on the transfer case. I then used an air nozzle and applied air to the lines. It shifted back and forth with a nice "thunk" like it is supposed to. Therefor I figured the problem must be in the valve.

I pulled the tranny access cover off and disconnected the air lines from the valve. I used the air gun and still got a nice "thunk" so I knew the lines between the valve and the t-case were good. I then pulled the valve off of the tranny and tore it down for cleaning and inspection. It was a little crusty inside so I cleaned everything up and re-assembled it. After putting it back on the tranny and re-hooking all of the air lines, it still wasn't shifting the sprag.

Then, I got an extra set of hands from my brother so I could supply air to the valve with the air gun while he shifted between R and 1. It would "thunk" once, but then would stop. I could get the output shaft on the t-case to turn in R right after he shifted and then briefly in 1st after he shifted, but then it would lock up.

If I applied air to the valve with the tranny in 1st, we could hear air coming from within the t-case. My guess is the seals on the piston in the actuator are shot and are moving the piston toward reverse and then the air leaks around and fills the 1st side and locks it into R.

Does that make sense? Have any of you guys had any luck replacing seals on the actuator? Could it be anything else?

Here are some pics from my experiences. Hopefully it might help some of you guys.

Thanks!

Forgot to mention this. Even though the valve has 8 bolts on it, only 4 hold it to the tranny. The other 4 hold the pieces of the valve together. I circled the mounting holes in yellow and the assembly holes in red. The pic is from the tranny side. You can see the threads in the bottom of the assembly holes. The mounting bolts are a lot longer so if you take a little stubby one out then you are on the wrong hole

looks very familiar!!!

Chris,
We just replaced the air shift valve assy with a new unit on our M812 a week ago. Good tip on replacing only the bolts that hold it to the tranny! Luckily we had the new unit to go by but either way it's not a treat to change. We could only hear the clunk noise going in reverse with the old unit. After replacing the unit we still only hear the clunk in reverse and not first and the other 5 ton's we have are the same way. After owning a duece with a shift on the fly front axle the 5 ton setup is not as user friendly. I wish somebody would come up with a manual valve setup to throw on when the driver wants 6wd. This way you don't have the clunky air valve assy on the tranny and you aren't limited to 6wd in only 2 low gears! It's hard to gain mometum in the mud when you are limited in first gear AWD.

I'm interested in reading what the transfer case experts say is your problem is as the new air valve assy. didn't fix the first gear clunck noise I'm supposed to hear on mine.
Andy

Re: looks very familiar!!!

Andydieselpower said:

I wish somebody would come up with a manual valve setup to throw on when the driver wants 6wd. This way you don't have the clunky air valve assy on the tranny and you aren't limited to 6wd in only 2 low gears! It's hard to gain mometum in the mud when you are limited in first gear AWD.


Andy

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Hi Andy,

Yes they have come up with a better idea with the 6wd.

On the M939 series five tons. There is an air switch on the dash to go in and out of 6wd on demand and on the fly while in High Range.

There is also another air valve tied into the low range shift linkage. When you shift the transfer case into low range the air valve opens and your in 6wd when ever your in low range.

Cheers,

C'dub

RE: looks very familiar!!!

Unless I am horribly wrong you do have 6x6 in all gears. You just have to make sure you move through 1st gear first after being in reverse. In other words you bank the truck up and get stuck. You then want to go forward. First put the truck in 1st gear to get the front sprag engaged and the rear sprag disengaged. Now you can put it in an forward gear as long as you do not put it in reverse again.

RE: looks very familiar!!!

hey 34! i'm not sure, but on this past sunday i took the 5-ton ou on the icy roads by our place (not my best decision) but i will say that during our questionable mission having gotten slid totally sideways on country road hills with only about 1-2 feet between truck and big trees on side of road, lets just say that i did a lot of movement between R and 1st. During these maneuvers my wife said that she could have sworn that the truck's fronts were grabbing and throwing in higher foreward gears. But I was busy shifting and hoping that noone else came over the hill(s) before I got straight again to pay attention.

you dont need to go into first to select forward motion sprag-as soon as you come out of reverse and into neutral the forward diection poppet air valve opens suppling air to transfer to move sprag-if you get chance physically look at sliding shaft in gearbox cover that operates poppet valves and you will see how this works!.too design it to be first gear related be wouldnt make sense=not everytime would a driver start in first especially when in low transfer, also it couldnt work because if it the poppet valve was opened by the first gear selector shaft when you came out and in to second then this shaft would move back causing poppet to then close again!.the sliding shaft that does operate poppets only moves when going in reverse and then back to neutral and is then static all other gear movments.

that makes a lot of sense. I guess the wife was right! cool! she'll get that smile when i have to tell her that she was right (not that i said she was wrong mind you!)

My main point was that you would have 6x6 in all forward gears not just 1st. I might have been confusing it with the deuce. You must go through R and 1st to activate the proper sprag. It seemed to make sense for the deuce. There are two sprags not just one. The deuce is activated by linkage and going into first activates the forward sprag and going in reverse activates the reverse sprag. The forward sprag stays engaged until the tranny is shifted back into reverse. In all other positions the linkage stays neutral. I would guess that the five ton is the same way. Movement of the trans into reverse causes the tcase to engage the reverse sprag then when he truck is shifted back into 1st the t- case would engage the forward sprag. It would then stay forward until air was applied to engage the rear sprag. I see what you are talking about but I doubt the 5 ton t-case is much different. I could be wrong. Also take note that I did not say you had to actually take off in 1st gear. You just have to shift into first to engage the forward sprag. After you do that you can start out in any forward gear you wish. Maybe I will break out the -20 and educate myself. Moving to neutral does nothing for engaging the forward or reverse sprag.

i took my 816 apart and physically watched how it works.the gearbox operating shaft has 2 detents machined in it which in reverse the reverse poppet rides up out of detent onto full shaft diameter causing poppet to open and air to transfer sprag .As soon as you come back to neutral the reverse poppet sinks into detent hence poppet shuts and no air to transfer and at same time the forward motion poppet rides out of detent onto full shaft diameter and is then opened and supplies air to transfer sprag =.none of this involves going into first gear.once in neutral the gearbox operating shaft does not move and keeps the forward sprag air supply open.. its the neutral position that selects the forward sprag not first gear selection.from reverse you could go straight to 3rd low and still operate the forward direction sprag for 6 wd

Whew! I think my experiment was easier and definitely more fun - but its good to know for sure!

you must have a problem cause when i plow i start in second and go up into 3rd and sometimes 4th and if the rears spin a little bit i can feel the front axel kick in and start pulling. you can go from second to reverse and vice-versa with no problem ,as long as you listen for the sneeze . been doing it for 4 years now and don't have any problems,.knock on wood.

It may be different then. On the deuce neutral does nothing. Putting it in 1st engages the front sprag. Reverse engages the rear sprag. So what you are saying is that neutral trips the forward sprag and when the shifter is in any forward gear no air is being sent to the t-case? That would make sense. The deuce is by linkage instead of air and that may be the difference.

I hope to have first hand knowledge with a five ton soon.

ida34 said:

So what you are saying is that neutral trips the forward sprag and when the shifter is in any forward gear no air is being sent to the t-case? That would make sense. The deuce is by linkage instead of air and that may be the difference.

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If I understand the operation correctly, when it's in neutral, there is air going to the forward side and in reverse it is going to the reverse side. If you cut the air supply, the sprag is in neutral...

Later,
Joe

yeah it seems to work as you say joe,when in reverse air is supplied via poppet valve to reverse side of sprag pushing it one way and then as soon as you are back in neutral and then consequently any forward gear air is then supplied to other side of sprag pushing it back other way.not sure if sprag will remain in a central position with no air supply.would be easy to test tho?

Crash_AF said:

ida34 said:

So what you are saying is that neutral trips the forward sprag and when the shifter is in any forward gear no air is being sent to the t-case? That would make sense. The deuce is by linkage instead of air and that may be the difference.

Click to expand...

If I understand the operation correctly, when it's in neutral, there is air going to the forward side and in reverse it is going to the reverse side. If you cut the air supply, the sprag is in neutral...

Later,
Joe

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I wish that we could get a definitive answer on this one! I would put a "sprag air release/isolation valve" on my truck tomorrow! I have already gathered the materials but I am reluctant to trash a t-case just to experiment. Has anyone ACTUALLY done the neutral sprag modification to the five ton yet?

the answer is yes, and i can say without a doubt you can get stuck if you have the air cut off which means forget to check the flip valve

just as a side note , the flip valve is a old mwo from m54 series days, though not many have it, it seems that it was done only here and there, the idea was that when lift towing, the front wheels would not turn

So, has anyone actually rebuilt or replaced the actuator in the t-case?

73m819 said:

the answer is yes, and i can say without a doubt you can get stuck if you have the air cut off which means forget to check the flip valve

just as a side note , the flip valve is a old mwo from m54 series days, though not many have it, it seems that it was done only here and there, the idea was that when lift towing, the front wheels would not turn

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So, you did this mod? How did it work for you? My trucks are on road 90% of the time and this would be a great addition to them. Overall, would you recommend it?