M939 Transfercase shifting

[nf6x]

I moved the valve to the other side of its mounting bracket after temporarily disconnecting one of the air lines to reroute it a bit. With the new valve location, the round disc on the transfer shift linkage doesn't contact the valve's roller arm any more.

Several drops of oil smelling of gear lube came out of the disconnected line, and that concerns me.

 

[Csm Davis]

Nf6x have you looked at the tc pto lever? That's the one that is just a piece of flat stock that bends way to easy, think I am going to reinforce it some.

 

[nf6x]

The TC PTO lever is the one at the left rear corner of the cab, that engages the crane pump, right? I haven't looked at the mechanism underneath at all yet. Up on top, I've found that the driver's seatbelt retractor is in just the right spot to interfere with grabbing that PTO lever. The safety catch is also hard to get fingers around, and I've had to resort to pulling it up with the seatbelt buckle a few times.

I haven't come up with a plan yet, but that PTO lever is one of the things I may want to improve on my wrecker. Another is adding remote control for the crane and winches.

 

[jrod66]

I did mine earlier today, Just loosened the set screw for the actuating arm and moved it 90*, then tightened the set screw back down and presto problem solved, my screw was an allen so it might have been replaced, but it was really easy

 

[nf6x]

I did mine earlier today, Just loosened the set screw for the actuating arm and moved it 90*, then tightened the set screw back down and presto problem solved, my screw was an allen so it might have been replaced, but it was really easy

I think my valve is different than yours, then. I thought that I would be able to just rotate the arm like you did, until I found it pinned onto the shaft with a roll pin in a blind hole. On mine, that phillips head screw draws a rectangular wedge against the shaft. The hole that the wedge goes into is visible in the picture where I'm holding the arm in its actuated position with a screwdriver.

The spring air brake valve behind the parking brake lever is the same style of valve, but its arm is constructed differently than the one down on my transfer case. I think it's standard style of valve, but different manufacturers construct them differently.

I like your solution better than mine, and I'll do it that way on my wrecker if it has a cooperative valve. On my cargo truck, moving the valve was a fall-back plan after I couldn't rotate the arm.

 

[m16ty]

I wonder if disabling the low range 6X6 switch is such a good idea. I don't know this but I wonder if the switch is to make sure the torque is divided between all axles. As we already know, the t-case is already a little on the weak side. I just know that on some civilian vehicles this is the case so you won't destroy the t-case by sending all the torque through the rear shaft.

 

[nf6x]

Hmm, good point. Maybe I'll be glad that I did the mod in an easily reversible way...

 

[spicergear]

I added a post a while ago about this. It not the low and reverse that can hurt the case, it's low reverse AND the automatic engagement of the front axle. The rear tandems will spin since they share so much weight and are effectivlely 'lighter' than the front. The friggin front axle is over laiden with traction with the whole nose weight on those two tire. Ask people who have had a case break, it breaks the front drop for the front axle drive.

Take a torch and cut off the flat washer on the linkage or simply pull one of the clevis pins in the linkage, unscrew the clevis end, unscrew the nut that holds that dumb big washer on and pull it. Then YOU ALWAYS have control of the front axle engagement from the cab. Bolbich (sp?) made a post about some 939 T-case mods and I posted pull the large flat washer there.

I may try the mod of pulling the air line off the lock cylinder, but I've found that once you learn the case a little, you can feel it going into gear or not, sometimes bump the trans into a gear to spin things then back to neutral and you and pop it right in.

 

[Vette427]

I worked on mine this weekend. I disconnected the linkage for the handle at the t-case. At that point I tried to move the linkage by itself, not connected to the T-case. It was very difficult to move, so my linkage needs to be cleaned, lubed, and adjusted so that it moves freely on it's own. This will certainly cause difficulty in shifting the T-case. While I've got it disconnected, I'll also remove the washer that engages the switch for auto FWD in Low range. I'll reassemble everything and check the shifting again. If I'm still having trouble, I'll disconnect/plug the lockout actuator on the T-case.

Also, and please correct me if I'm wrong, the T-case handle only moves to about a 45 deg. position when the T-case is in Low range. It does NOT go all the way up. Correct? It doesn't look like there is enough clearance at the back of the opening behind the lever to allow any more travel than 45 deg.

I just wanted to add the info about the linkage, since I haven't seen this mentioned in the thread. This is a common ailment it will be nice to have everything in one thread.

Also, any info on the plug size required to plug the line going to the lockout actuator? I'm assuming 5/16" JIC just by looking at the size of the line?

 

[nf6x]

Also, and please correct me if I'm wrong, the T-case handle only moves to about a 45 deg. position when the T-case is in Low range. It does NOT go all the way up. Correct?

That is correct if my memory has not failed. Of course, it's been over a year since the last time I fired up one of my 5-tons…

 

[Vette427]

It's a simple thing, but not listed in the TM's that I'm aware. I'm new to the hobby and my truck and it was one of my first questions. I'll post more info and pics as I work through my particular issue.

 

[Vette427]

Update: I was able to work a little more yesterday evening thanks to the extra daylight. I flushed all of the linkage points liberally with WD-40, while a friend worked the t-case shift handle (disconnected from the T-case). Then I applied a good coat of white lithium spray grease to all the pivots, while working the T-case shift handle. The effort to move the lever was reduced about 50%. It's still firm, but it doesn't bind or stick. It's an even resistance all the way through the range of travel, which should be normal. I also removed the washer that engages the auto FWD switch since the linkage was disconnected anyway. Now, I can control front axle engagement manually in Low range.

It looks like these trucks were painted underneath and not shifted into Low with any regularity, so paint on/in the linkage pivots may also be an issue. This is helped by the lube procedure above and simply working the shift handle through it's full range of motion repeatedly. You'll feel it start to work itself free.

Unfortunately, it still won't shift into Low with full air pressure. My next step is to plug the line going to the lockout actuator on the T-case. There's plenty of resistance in the shift handle itself, so there's no worry of accidentally bumping the shift handle out of position even with the lockout disabled.

 

[Vette427]

I was able to bypass the lockout actuator. Since there's not much here on what's involved, I though I would post what I did. There's an adapter between the actuator body and the air line. This adapter is compression male (for the air line fitting) to 1/8" NPT male (for the actuator). I bypassed the actuator by capping the NPT side of air line adaptor and installing a plug into the actuator (both 1/8" NPT) using a little pipe thread sealant. Do not use Teflon tape.

I tried to cap the line the cheap way by using a rubber vacuum cap and a hose clamp, but 120 psi air line pressure popped the rubber cap like a balloon. You will need to use a proper 1/8" NPT cap and sealant to ensure reliability and leak-free operation. I recommend brass fittings for corrosion resistance.

The transfer case now shifts easily in neutral, although you may need to move the truck slightly to get the T-case to shift all the way into Low or back into High. This is similar to a motorcycle or ATV that has to be "rocked" a little back-and-forth to get it into neutral.

Also, there's much less lever movement than I would have thought shifting from High to Low range. The lever only moves about 30 degrees from horizontal when shifting from High to Low.

So to sum up, my problem was T-case wouldn't shift into Low at all. I disconnected the linkage, lubed all the pivot points, and worked the T-case lever until it was easier to move and I could move it through the entire range of travel with even resistance. I also removed the washer which engages the FWD in low, which now allows me to select FWD manually in either High or Low range. After these repairs, the T-case wouldn't shift with full air pressure but would shift with no air pressure, so I bypassed the lockout actuator by capping the line and plugging the actuator. It's not necessary to plug the actuator, but it does keep water trash from getting inside the actuator should you choose to reconnect it later.

Hope this helps.

 

[DIndFab]

The Transfercase shift linkage has a bad weak link and in my opinion a unneeded locking mechanism. Both of these combine to make for a hard shifting tc.

First the weak link is the flat stock piece at the tc pto it flexes and bends I am looking at adding some reinforcement to mine.

Second problem is the air lockout on the tc I do not believe it is needed. If you will try to shift your tc without the truck on and no air pressure you will find that it probably shifts much easier, and unlike the 809 series trucks this tc lever is not likely to get knocked out of gear.

What does the air lockout do? It's easier to shift when the truck is off?

 

[Vette427]

Air lockout is an air-operated actuator on the T-case that prevents you from shifting into Low range without first pressing the button on the front of the T-case shifter handle. This is designed to prevent accidental shifting of the T-case.

This is more important on 9-series wreckers or dump trucks with PTO's that operate when the T-Case is in Neutral and the engine at higher RPM to drive the hydraulics. It prevents accidentally knocking the T-Case into either Low or High range which could cause the vehicle to move, AKA A Very Bad Day.

With regular cargo trucks, this isn't much of an issue. The shifter is very firm to move without the lockout, so there's very little chance at all of accidentally knocking the T-case out of the selected range. Also, unless being towed, there's not much reason to have the T-Case in Neutral in a cargo truck. Remember that these trucks may be driven by an 18 year old that may not have driven anything larger than a Honda Accord before entering the military so you have to make them "grunt proof" thus things like T-case lockout.

To complicate the matter it's pretty common that these are connected incorrectly which prevents shifting the T-case at all when the air pressure is up. Unless you want to trouble shoot both the electrical solenoid that controls the actuator AND the air lines themselves, the easy fix if you can't shift your t-case with full air is to disconnect the actuator.

if you want to troubleshoot the solenoid and lines, there are some excellent articles here on the site and also the service manuals are free to download as well.

 

[engineer233]

My 936 shifts easily just by pushing the button on the shifter. I'm sure I will have to disable the lockout later. My problem is the shifter shifts the tc into the low position but the truck won't pull out of its tracks. It acts as if a shifter fork inside isn't moving something into place. Any thoughts???

 

[Vette427]

Has the truck always had this problem or do you suspect you broke something? If this has always been the case, you may want to have a friend work the T-Case lever while you observe the linkage. It may just need a little adjustment to get into Low. You'll want to shorten up on the linkage to get more movement at the t-case.

Also, I need to shift the truck into Neutral to shift from high to neutral on the t-case, often I have to put the trans in 1-5 and move forward just slightly to get the t-case into low. You can feel it shift into place after everything in the t-case lines up. There are fairly big gears in that case and no synchros, so it can be tricky at first to develop a "feel". Also, from reading on here, all the trucks are a little different on how they act and work.

I'm assuming you have High and Neutral, but no Low, correct?

if this doesn't work, disconnect the linkage and pull the t-case pin by hand and see if it feels like something's broken.

 

[engineer233]

This truck has had this problem since I got it. I have adjusted the linkage with no luck. With the truck shut down and 0 air pressure it shifted easily by hand to the low position. Almost felt to easy. Started it up built air and no luck. The high range and neutral are fine. It just feels like to me maybe a shifter fork isn't pulling the gear forward .

 

[73m819]

Has the truck always had this problem or do you suspect you broke something? If this has always been the case, you may want to have a friend work the T-Case lever while you observe the linkage. It may just need a little adjustment to get into Low. You'll want to shorten up on the linkage to get more movement at the t-case.

Also, I need to shift the truck into Neutral to shift from high to neutral on the t-case, often I have to put the trans in 1-5 and move forward just slightly to get the t-case into low. You can feel it shift into place after everything in the t-case lines up. There are fairly big gears in that case and no synchros, so it can be tricky at first to develop a "feel". Also, from reading on here, all the trucks are a little different on how they act and work.

I'm assuming you have High and Neutral, but no Low, correct?

if this doesn't work, disconnect the linkage and pull the t-case pin by hand and see if it feels like something's broken.

Just a slight correction to this GREAT write up, Thank You, The correction is this, the 939 transfer case HAS a synchro for the H/L shaft, that being said and these transfers can shift on the fly, it is a lot better to treat it as a NON synchro, come to a stop to shift transfer, it WILL last a lot longer (shifting on the fly is really hard on it and not really designed for it).

 

[engineer233]

Can anyone point me in the direction of the correct TM for the teardown of a transfer case ? Thanks