M939 transfer case air lines installed wrong

3 out of 5 m939 trucks i bought have the air lines to the t-case routed to the wrong place. 2 out of the 3 were that way right from the military. 2 of them had the air lines fom the low range shifter linkage valve( the valve that engages awd when you engage low range) going to the t-case shifter lock... not the axle engagement air piston. 2 of them had the air lines going to the low range shifter linkage valve crossed up so the truck had full line air pressure going to the front axle engagement air piston all the time so the truck was locked in all wheel drive (the parts truck i bought with a blown t-case was locked in all wheel drive..im sure thats why its blown) the other truck had bad cupping on the front tires(pretty good sign its locked in awd) luckily the t-case is ok.... Just a heads up to see if your front axle is locked in. If you cant shift your t-case this could be your issue.(Ive also seen the shaft up in the t-case linkage seized up).

Good to know. Thank you

So are you thinking these came out of Red River this way? What can you tell of the history of the trucks.

And Better Yet? How about some pictures of what the hookups look like.

I'm curious as well. I will have to double check my truck when I go to recover it. Great info!

The first thing to do is build up FULL AIR PRESSURE then jack the front wheels off the ground just to make sure the front axle isnt locked in. The rest you can deal with as you get a chance. The parts truck destroyed itself... when the t-case blew apart the whole bottom cracked right off...the front driveshaft tore a hole in the trans pan ,beat the oil pan on the engine pretty good, tore out the hyd and air lines. Its pretty ugly under there. My trucks came from all over ,the one that came from red river is beautiful.

Heres some pics. The air piston at the top of the t-case is the lock for the shifter. The air/electric solenoid down to the left is the unit that bleeds the air off the shift lock piston. When you push the button on your t-case shifter it makes the solenoid dump the air to the shift lock so you can change ranges. The piston down on the back side of the t-case is the front axle engagement piston. That piston should get its air from the valve on the front of the t-case that comes in contact with the shift linkage when you put it in low range(this valve gets air from the switch on the dash and full line pressure as there are 2 inlets on this valve i recommend doing away with the full line pressure supply hose so it wont engage the front axle when you put the t-case in low range). You can see in the pics that the air lines are routed wrong. Im sure thats why this t-case blew up.

My buddy got his truck this morning. The first thing i did was check all the fluids,then the all wheel drive. Sure enough it was locked in awd. Thats 4 out of 6 trucks with the air lines installed wrong and 3 out of 6 locked in awd. I removed his valve that locks awd when in low range and ran the air line from the dash switch right to the front axle piston on the t-case. If you want to keep the valve in place heres a pic of the valve telling what each port does

Too bad the "THANK" button goes away after you click it or I'd have about 10 of them after these posts!

Yeah, these transfer cases MUST be garbage!!! Geeze...and I thought just automatically pulling the front axle in with low range was a problem for reverse; now seeing this, WOW. I bet it would be possible to blow up the 800 series T-case with the Allison and always being locked in with the front axle.

AWESOME post Bolkbich!!!

Thanks very much

I'm going to hook the dash switch to the front axle piston as soon as I get to work on the truck again. I still can't believe it's an automatic engagement in low and can hardly believe (but do, you know...) that 4 of 6 random trucks had the t-case plumbed wrong. WOW. Again, great post!

After all this BS with these trucks breaking the t-cases, learning that when they're put into low range they AUTOMATICALLY (via tripped air solenoid) bring in the front axle, AND Bolkbich's finding of multiple trucks with the plumbing wrong on the T-case I had to do something on my M931A2. I still am confident in saying that the automatic front axle engage has had a LOT, LOT, LOT to do with the busted T-case especially with the Allison and it's ability to gear deep...plus most of the time in a gear calculator they consider the torque converter as another 2:1 reduction. All that gearing and now all that massive traction from the front axle and it's corresponding nose weight, and a lot of time 1400x20's...something's gotta give if someone horses around hard.

Also, Bolkbich said that one of the cases he saw that was busted had broken the case at the front drop. It all falls in line...

Anyway, I sent him a couple of PM's to verify a quick idea I had and over the weekend spent about 5 minutes to take control of the front axle engagement ALL THE TIME. The picture is a top view of the linkage at the transfer case and you can see the air solenoid with the little arm and roller wheel in the left side of the picture. Take notice to the clevis at the end of the likage. You have going from bottom of picture to top, clevis, jam nut, then a gap, then a second nut, then just resting on the linkage is a large heavy washer. THAT WASHER was in the gap between the two nuts. The transfer case is currently in HI. When shifted to LOW range the linkage pulls forward and that large washer would trip the roller arm on the solenoid and send a shot of air to the front axle engagement and there you have your LOW range auto engage.

When in HI range, YOU in the cab, have a manual operated soleniod to lock in the front axle. The air still plumbs into the bottom of the roller arm solenoid but bypasses though. I was going to change the plumbing but then realized that as long as the roller arm solenoid -THINKS- the T-case is in high range, the driver will always have control of the front axle engagement. Simply taking out that big washer so nothing trips that auto engage solenoid is the ticket. I've not tested it yet but there's really nothing to test...it's just straight mechanics on how these couple things work.

Seriously, that's gotta be the most expensive washer on the trucks that have busted cases.

I have a couple that are not shifting into low when the air pressure is up, obviously they are probably routed incorrectly, does anybody have a schematic showing the proper routing of the air lines? Thanks, Glen

91W350 said:

I have a couple that are not shifting into low when the air pressure is up, obviously they are probably routed incorrectly, does anybody have a schematic showing the proper routing of the air lines? Thanks, Glen

Click to expand...

Here you go Glen. Now how about getting a pallet under that top and getting a weight on it?

91W350 said:

I have a couple that are not shifting into low when the air pressure is up, obviously they are probably routed incorrectly, does anybody have a schematic showing the proper routing of the air lines? Thanks, Glen

Click to expand...

Glen, are you having trouble moving the shift lever when you have air pressure in your system? My issue was two wires were not connected correctly. The wires coming out of the capacitor (wires from the transfer case lever switch) were connected incorrectly. This caused the air not to dump out of the lockout in the transfer case. If the air cannot dump then you cannot shift with air pressure in the system. Highly recommend you crawl under your truck and look at the capacitor (circular black rubber looking thing with 5 wires coming out of it. 3 wires from one end and 2 from the other end).Th capacitor is bolted to the underside of your cab just above the tranny.

That is good to know

I was looking at one recently that had severe cupping on the front tires.

Reversed transfer case air lines seem to be a common issue.

The reason for not back in low is it's way too much torque for the transfer case to handle.

I'm new to this forum and looking for help to with this same issue. Where can I find the actual diagrams showing the proper routing for the air lines on the transfer case? I work for a county government and we recently received several of these vehicles but without any documentation.

I have searched this site and the TM's that came with the truck, and herein, with no success. Do you think crossed up air lines would cause an in-the-cab, axel in/out valve to start leaking, just from actuation of the Hi/Lo shifter? It did not leak the first 160 miles I drove it. I picked up my truck from Fort Gordon last week and did not have any problems until I was within 20 miles from home. I stopped for a break and a refuel of diesel. While I sat waiting for my chase vehicle driver, to eat her lunch, I mistakenly started playing with the Hi/N/Lo range handle. With the engine on, in neutral, I pulled the handle into Lo range and back into Hi/Rg. From that time on, the axel in/out valve to leaked air, when air pressure climbed to 70. Prior to the leaking valve the air pressure would build up to 125. After the leak, it never rose above 80 psi. I have resealed the valve with no change in the leak. I took that auto lock-in valve (the one the washer activates) apart and cleaned it. No difference. During my TS, I discovered air pressure coming out of both lines that are connected to the axel in/out valve. I like your idea of eliminating the auto valve. My truck was overhauled in 2010, and received a heavy coating of OD green on everything-everywhere.
Do you think rerouting the lines could fix my problem?
If so, where do you recommend I get (A) 120 psi in, from?