Front axle not engaging, not air related.

[Smokinyoda]

M939 Front axle not engaging, not air related.

I found out the hard way that the front axle on one of my a2 trucks does not engage, it involved getting stuck behind my house. After spending several hours troubleshooting the issue I believe it is something inside the transfer case. With the air engagement cylinder off, and pushing the exposed rod in all the way I can still rotate a front tire that's jacked up. And since the front driveshaft turns that seems to rule out an axle issue.

Anything else to check or do I need to start looking for a replacement transfer case? Other than this issue truck runs and drives just fine.

Has not been a good week for my trucks. One down for a bad compressor, and now this...

 

[73m819]

Not really, could be the other axle that is broke. with the truck jacked up on both sides of the front, engage the front drive, IF you can turn the front drive shaft, the issue is in the transfer, if you can not turn it, the issue is in the front end. Note, it may take a bar to turn the front drive shaft.

 

[Smokinyoda]

Not really, could be the other axle that is broke. with the truck jacked up on both sides of the front, engage the front drive, IF you can turn the front drive shaft, the issue is in the transfer, if you can not turn it, the issue is in the front end. Note, it may take a bar to turn the front drive shaft.

Front end jacked up, front drive engaged, driveshaft turns freely.

Couple other things I noticed. When I went to remove the airline from the engagement cylinder it was under pressure. Even with cab switch off. Air continued to blow until tank pressure bled down. Also the interlock cylinder and its air line at the top of the case were both capped.

So sounds like I got some other problems to address even after finding a replacement case.

 

[jcappeljr]

I have a good used transfer case for sale.$500

 

[Smokinyoda]

I have a good used transfer case for sale.$500

Thanks for the offer but hoping to find one a little closer to home to save on shipping.

Trucks to big to fit in my shop so will have to swap them out on gravel. TM says to use a transmission jack but that isn't going to roll very good unless maybe a sheet of plywood over the gravel. Anyone else done this on gravel, and if so what did you use?
May have to look into renting a bobcat with fork attachment.

 

[gimpyrobb]

I did a transmission in a 939 truck and it SUCKS. Since there is no tunnel to open the floor up, you HAVE to use a jack/lift of some sort. We had nice 3/4 plywood under the truck, but had to lift the truck with a crane to get the hoist with the transmission sitting on it, under the truck! No way could we scoot them under individually and then set the trans up on the jack. The T-case isn't much lighter, I'd think about making an access hole in the floor to use a come-a-long or something.

I don't envy you.

 

[Castle Bravo]

I helped a buddy put a new transfer case into a M923A2, and we did with a forklift and chains from above. It seems like I remember that we took the spare tire davit off, but realized that it would've been possible by just removing the spare tire only.

 

[Smokinyoda]

I helped a buddy put a new transfer case into a M923A2, and we did with a forklift and chains from above. It seems like I remember that we took the spare tire davit off, but realized that it would've been possible by just removing the spare tire only.

Will definitely have to look into that. I have a 1 ton crane with electric winch on the back of my deuce, if I can back up close enough I might be able to use it.

 

[99nouns]

I think you need to figure out why that line is capped and what happens if you connect it properly, just swapping stuff just because it doesn't work doesn't mean it will fix it, I had a truck and front axle was disabled by same way capping as a part of demilitarization, and all had to do is connect it back. You need to check every line on the t-case and sometimes they will use rubber plugs inside the line, and that is even nastier, because then you have to remove every fitting on t-case and check if it has stupid rubber in it.

It also no unusual they switch air lines around too, so say sorry to your better half (if you have one) and you will be spending rest of your week's free time with TM

 

[Smokinyoda]

I think you need to figure out why that line is capped and what happens if you connect it properly, just swapping stuff just because it doesn't work doesn't mean it will fix it, I had a truck and front axle was disabled by same way capping as a part of demilitarization, and all had to do is connect it back. You need to check every line on the t-case and sometimes they will use rubber plugs inside the line, and that is even nastier, because then you have to remove every fitting on t-case and check if it has stupid rubber in it.

It also no unusual they switch air lines around too, so say sorry to your better half (if you have one) and you will be spending rest of your week's free time with TM

Thanks but I have ruled the air lines out. Granted I know I have airline issues but they are currently not causing my front axle to not engage. As explained in first post.

 

[Monkeyboyarmy]

Was your transfer case lever in high or low when u took that line off? I'm pretty sure that the front end engages automatically when transfer is shifted to low range. Not that will help your transfer case problem.

 

[Smokinyoda]

Was your transfer case lever in high or low when u took that line off? I'm pretty sure that the front end engages automatically when transfer is shifted to low range. Not that will help your transfer case problem.

It was in high. Thats why I was shocked when line was pressurized. Probably lines swapped like so many others.

 

[M543A2]

There is always pressure to one line. It depends on the transmission shift lever position. On my M series trucks the front drive is engaged by a two way overrunning clutch in the transfer. The air shift cylinder on the front of the transfer shifts from one side of the clutch to the other. One side is for forward engagement when in a forward gear. The other side, shifted to by the air valve on the transmission cover directing air pressure to the other side of the air cylinder, engages the other side of the clutch for reverse drive. Which side of the cylinder is pressurized depends on shift lever position. Any forward gear including neutral directs air to one side, reverse shift lever position directs air to the other side. With air up and the engine off you should hear a clunk sound of the transfer air cylinder shifting when going from neutral or any forward gear to reverse. It is an overrunning clutch because there is a ratio difference built in between the rear and front drives. The rear needs to slip a percentage before this clutch will engage the front drive. As long as it senses the front drive turning faster than the rear the clutch free wheels or overruns. When the rear slips it engages.
Sometimes problems are caused by the air valve on the transmission tower that directs pressure to the TC cylinder. It has face type valve seals in it that can become deformed due to the constant air pressure, especially those in the forward gear/neutral side because they are under constant pressure. The seals on the reverse side of the valve are under pressure only when in reverse. When these start to leak, then mixed signals are sent to the TC case valve. I found that little valve to be expensive so I took it apart and removed the old face seals and glued on ones I made from some smooth side conveyor belting I have. 3 years later is is still working fine.
I really wish they made these trucks like the deuces where there is positive engagement. These trucks are frustrating because the front will drive until the rear gains good traction again then disengage. The right kind of conditions cause a cyclic engaged/disengaged/engaged situation for the front axle that can damage to parts, probably what fails the clutch. We are looking at modifying one to positively engage because we are frustrated with the undependable operation of the clutch in one truck we have. We will have to be careful where we use it in lock because of the ratio difference. We have a transfer in an M139 we need to take apart because we lost high range. It appears there is a mainshaft snap ring that has become dislocated. When we get it apart we are going to look at the front drive clutch to analyze possible ways of making it positive engagement. I am not going to replace the whole transfer or go to a new clutch because I just do not trust them in the long run. Why pay for something that might go out again?
We have looked at this and we think it might be possible to take the internals out of the TC case with the case still in the truck. Has anyone tried this?
Regards Martin

 

[sandcobra164]

Martin,
Your description is perfectly correct when describing the sprag transfer case found in the M39 and M809 series trucks. The original poster has an M939 series truck which does have a transfer that operates similarly to a deuce, direct engagement when air pressure is applied.

R/S,
Joseph

 

[M543A2]

Thanks Sandcobra164. I only saw his thread title as an "A2" truck. Many of my M series trucks are A2 models, so that is why I assumed what I did. I missed it was a 900 series. So they finally got smart and went away from the sprags! Too bad Memphis Equipment has not come out with a TC modification that would make trucks like mine direct shift drive. Maybe I will suggest that to them---
Regards Martin

 

[Julian]

I am not sure how the front drive shaft is engaged, but I recall it can be manually engaged at any speed. That makes me think it is an actual clutch disc rather than a gear engagement. If it is a clutch with friction surfaces, it would rely on hundreds of pounds of force supplied by the piston to lock in the front axle. Simply moving the rod in until it stopped would not lock the front axle in that case. Make sure the piston was not in the cylinder backwards as has happened several times- the shallow recess in the piston is what should push on the rod. Your assessment is correct about being engaged- if front drive shaft turns and truck doesn't move, transfer case is not locked in. Maybe someone else can comment about whether or not high pressure on the rod is required.

 

[Floridianson]

Martin,
Your description is perfectly correct when describing the sprag transfer case found in the M39 and M809 series trucks. The original poster has an M939 series truck which does have a transfer that operates similarly to a deuce, direct engagement when air pressure is applied.

Yep no sprag. Hope the set screw did not let go on the engagement fork. Also thinking when you took off the lower air cylinder not the top interlock did you do this alone or with help. The front engagement would take a lot of pressure and while holding that pressure to over come the spring that disengages when no air is supplied. So did you hold the shaft in hard while someone else tried to turn the driveshaft until the two cogs meshed?

 

[Julian]

Floridianson- what is the manual number that diagram came from?
I am obviously wrong with my guess about how the innards of the transfer case work.

 

[Csm Davis]

Smokinyoda leave the interlock disabled and the auto engagement switch is probably disabled also,the only air you need is coming straight from the dash switch so find it and hook it up then check engagement. You may need to rotate the rear shaft as well as the front to get the clutch to engage. If that doesn't work then you are going to have to open the case to find the problem. As to the removal I would slide the bed back and ease it up and down from above, I take the spare holder and exhaust mount off and come out the top, there is a inspection cover under the interlock and your vent screws into it. Sorry about the bad week bud but maybe it will get better.

 

[tobyS]

Taking out the 817 TC required a truck tranny jack on hard surface and the front wheels to be put up on blocks to get the whole thing out. We disconnected the axle drive-shafts and ran it prior to pulling it...which I highly suggest.