M939 series transfer case low range / front axle engagement

[emr]

Fast case break answer, from AM General,US Government,Allison trans, all 3 did tests after the cracked/broken case became a reoccurring problem, All 3 came up with driver error, The ratio in reverse is lower or as low as a D 6 cat I was told, that is the part I dont have the true facts on , all else I read from publications on this, It is so low it is a problem when in low reverse, it is amazingly low, It is also what the Military ordered, and wanted, so here is why it proves to be driver/lack of training error, After the training of how not to use low reverse and proper shifting, meaning do not ever shift this truck unless the huge trans has time to: finish" from forward to reverse! the shifting the problem all but went away, I know for a fact the unit here put more miles on these 939 trucks mostly ...during the first gulf conflict, and have the citations to prove it, all soldiers i know said these trucks out performed all others,anywhere anytime,, and had minimal break down issues, and the cracked case was not a noticeable on its own issue anymore, much different that the beginning use of these trucks, so as hard as it is, these trucks are what they are designed to be, and like 99% of all issues they are driver preventable. and were human error not the truck, I dont think U will find a truck with a low low as these anywhere, but that is why I would rather have one, I have used and seen low reverse used alot, its up to the driver not to slam the case, slam the direction Forward to reverse , patiense with these beasts in that area will show just how much more awesome these trucks are than we even think they are, could something be better, today the answer is yes, but then the answer was an auto that would be as low or lower than the standard shift, and the technology was this, and it worked and is working

 

[jrod66]

^THIS answer is the best one I have seen yet for this issue. I was thinking along the same lines, as in if you take care of it and use it with care then there shouldnt be an issue.

As far as the lockout for the transfer case goes, I went outside and looked at mine and it seems that you could just modify the large strike washer on the shift linkage with a slot, if you want it to be bypassed then move to the slot, if you want it to act normally then rotate the washer to the unmodified portion. Just a thought, another solution could be to just remove the small actuating arm from the solenoid and there will be no more low engagement, with this mod you wouldnt have to plug a line or mess with any of that, and could be easily put back to stock on the trail or road, again this is just my .02

 

[kennys@wi.rr.com]

I have heard and read about the mod done to the transfer case. However, I have yet to find a detailed thread about how to do it. Is there any where that I should be looking or different search terms I should be using? I will gladly make a detailed thread complete with pictures if someone can walk me through this with mine.

 

[jrod66]

super easy, read the other thread about the transfer case, its pretty straight forward

 

[Beerslayer]

I was crawling around under an M925 this evening that was stuck in low range. While getting a 2x4 into position to get that thing shifted back into high range I noticed the actuator for automatically shifting in the front end.

It would be easiest to just take off the lever arm on the valve or "adjust it" out of the way so it can't automatically engage the front end. No need to take off hoses and plug things.

 

[SLOrazorsedge]

We were using my M936A1 wrecker today to sling load a shed. We couldn't get the transfer case into neutral no matter how many times we pushed the button and applied pressure to the lever. We ended up adjusting the linkage as the lever lays on the floor and my other two M923's lever is elevated off the floor about 3-4" while engaged in high. After the adjustment, voila we could shift into neutral and back like butter multiple times. We fired it up, engaged the crane and set out tackle. We were a bit off where we needed the truck centered so we disengaged the crane and attempted to move the transfer-case back into high. No luck, again no matter how many times. I read this thread and the trick of bleeding off the air pressure and trying again. As soon as I was at "0" air pressure, I was able to shift the transfer-case back into high.

So what do you think, is my interlock valve bad, is it simply an adjustment or what? Its always been a bit hard to get it into neutral an then back into high when we use the crane/winches. Any directions from you guys is appreciated.

 

[SLOrazorsedge]

We finished lifting and moving the shed this past weekend with our M936A1. My problem continues, can't get the transfer case into neutral unless I pump the brakes down and bleed the air pressure off to about 30 psi. Same thing goes when its time to put it back into high range after using the crane. Anyone know the "why" and a idea of how to fix it? Nothing I've read in the PM points me in the right direction.

 

[Floridianson]

We finished lifting and moving the shed this past weekend with our M936A1. My problem continues, can't get the transfer case into neutral unless I pump the brakes down and bleed the air pressure off to about 30 psi. Same thing goes when its time to put it back into high range after using the crane. Anyone know the "why" and a idea of how to fix it? Nothing I've read in the PM points me in the right direction.

Did you start with checking if you had power to the release buttton on the transfer case lever? if you have power to it then check if the power is comming through by checking the outher wire with the button depressed.

 

[M35A2-AZ]

We finished lifting and moving the shed this past weekend with our M936A1. My problem continues, can't get the transfer case into neutral unless I pump the brakes down and bleed the air pressure off to about 30 psi. Same thing goes when its time to put it back into high range after using the crane. Anyone know the "why" and a idea of how to fix it? Nothing I've read in the PM points me in the right direction.

Sound like your air solenoid valve is not disengaging, I would look at the wiring from the button all the way back to the transfer case.
Make sure your getting power to the solenoid. If the truck is turn off and the push the button on the shifter you should be able to hear
a click from the transfer area.
When I got my wrecker there was a plug that was unplugged and not disengaging the solenoid. It is a pain in the b#@*& to trace.

 

[SLOrazorsedge]

Thanks Albert, when the issue started on Saturday we could hear the resounding click while pushing the button with engine off but still had the problem. Sunday I don't think we could hear it but we were rushed getting the job done. We played with the 2 wires from the transfer stick back down the floorboard hole, you are right it will be a **&^%$# to trace them up under the floorboard and on top of the transfer case. Its a bummer as this trucks cab with all electrics was totally replaced during the rebuild, everything is new, bright w/o CARC sprayed all over the wiring loom and connections. We'll try to trace it and confirm we're getting power to the button. Why is it we can actually engage the transfer case w/o air pressure? Is it just lucky hair of the dog manual exertion making engagement?

 

[Floridianson]

Yea im just learning about my 931 but the TM's do show the lock out. You need power to the transfer case button then power out of the button to the solenoid on the transfer case #B that opens and lets the air out of interlock air cyl. #A.
If all the airlines are correct and you get power to #B the only thing I can think of is Solenoid B is bad.

 

[Floridianson]

[QUOTE=SLOrazorsedge; Why is it we can actually engage the transfer case w/o air pressure? Is it just lucky hair of the dog manual exertion making engagement?[/

I would think interlock cyl. is spring loaded in the disengage mode without air.

 

[SLOrazorsedge]

Thanks brother! That makes total sense to me now and explains the "why" the transfer case lever can be engaged/disengaged with the air pressure bled off. Its always been tough to pull it into neutral and again to return to high range. Its actually good to know now if a guy gets in trouble trying to shift from high range into neutral or low range and the lever won't move, just bleed off your air pressure by pumping the brake pedal, unless your on a 26% grade of course!

Also, a very Merry Christmas and prosperous New Years to all of you and yours here on SS! I've truly enjoyed the brotherhood and friendship of everyone here that I've interacted with.

 

[M35A2-AZ]

You have a good one also!!!!

 

[Hoefler]

My m932a2 easily shifts into low range. However, once in low, tower shifter is not allowed to go into neutral or reverse. There is no way to get transfer case into high without draining the air tanks. Pumping the brakes down to what pressure will allow the shift? Has anybody by passed this?
pete

 

[helomech]

My m932a2 easily shifts into low range. However, once in low, tower shifter is not allowed to go into neutral or reverse. There is no way to get transfer case into high without draining the air tanks. Pumping the brakes down to what pressure will allow the shift? Has anybody by passed this?
pete

It should not work that way. My 932A2 shifts easy back and forth from low to high.

 

[webbk24]

I have had this problem with one of our M939's and was at a fire down in a canyon and couldn't get in to low range I loosened up the air line after the solenoid to release pressure when ever I needed to shift in too low/high range, tomorrow I am going to see if I can either get another solenoid or find another switch I can wire myself to bypass these solenoids, We use a air switch kind of like an air horn switch to work some of our nozzles on our fire truck so I think I want to try this to see if it will fix this problem fairly cheap.

 

[plode]

I disconnected the air supply to that interlock valve on the transfer case completely. I have not had an issue with the transfer case slipping in or out of gear.
I also disconnected the automatic front axle engagement for low range selection as well. Now the only way the truck can lock the front axle is via the front axle lock control in the cab, so I have complete control over it.

 

[webbk24]

So i started trying to figure this out today and came up with inside the shifter there is a neutral switch which allows power to the button on the transfer case shifter. I found that the piece that pushes the switch in was worn and wasn't pushing the switch in far enough to make contact inside. It's a square like bolt that has a retaining clip holding it in place so all i did was remove the clip pull it out and rotate it so the surface that makes contact with the switch is fresh, This worked perfectly after i did this. I also removed the arm on the shifter linkage that engaged the front axle when you put the truck in low.

 

[Csm Davis]

I disconnected the air supply to that interlock valve on the transfer case completely. I have not had an issue with the transfer case slipping in or out of gear.
I also disconnected the automatic front axle engagement for low range selection as well. Now the only way the truck can lock the front axle is via the front axle lock control in the cab, so I have complete control over it.

I do this to my trucks and it takes care of all my TC problems, these lockouts are not needed by trained drivers and will help prevent problems in my opinion.