M939 Transfercase shifting

[Csm Davis]

M939 Transfercase shifting and front axle engagement

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.

 

[m16ty]

On both M939 trucks I have, I disconnected the lock cylinder and installed plugs in each end. Both shift 100% better with the lock disabled.

 

[kennys@wi.rr.com]

Does anyone have pictures of this mod or a thread on how to do it? I can't seem to find anything other than suggesting to do it.

 

[m16ty]

Does anyone have pictures of this mod or a thread on how to do it? I can't seem to find anything other than suggesting to do it.

Look on the top of the t-case. You'll see the end of a cylinder sticking out of the case with a air line going to it. Disconnect the air line, plug the line, and plug the cylinder. There's no operational reason to plug the cylinder but it keeps trash and water out so water won't eventually get into the case and it can be hooked back up if needed.

 

[M35A2-AZ]

I have three M939 trucks and two of them are hard to shift also.

 

[kennys@wi.rr.com]

That's all that's needed to be done so it will no longer shift automatically when putting it into reverse?

 

[M35A2-AZ]

That's all that's needed to be done so it will no longer shift automatically when putting it into reverse?

Does it lock in, in reverse or low range?

 

[nf6x]

I don't think that the locking cylinder being discussed here has anything to do with reverse gear. There is an add-on kit to interlock the transmission/transfer to prevent using reverse in low range, but that's not present on all trucks, and I think it's entirely mechanical.

I think that the locking cylinder discussed here is just actuated by the button on the end of the transfer shift lever.

 

[Csm Davis]

No this has nothing to do with reverse and low range which you should not do!

 

[Csm Davis]

I think I am going to disable the front axle engagement for low range if possible I haven't looked closely at if it can be done. There are lots of times I use low range and don't need the front axle pulling.

 

[M35A2-AZ]

I think I am going to disable the front axle engagement for low range if possible I haven't looked closely at if it can be done. There are lots of times I use low range and don't need the front axle pulling.

I understand now!!!

I was thinking about removing the engagement plate on the shifter linkage
to disable the front axle lock up. Or just move it all the way to the end of the linkage.
I think I saw a post that someone cut a slot in it and turned it.

 

[nf6x]

I think I am going to disable the front axle engagement for low range if possible I haven't looked closely at if it can be done. There are lots of times I use low range and don't need the front axle pulling.

I think that should be easy to do. I discovered (after some wheel-spinning...) that my M923 front axle wasn't automatically engaging in low range. The cause appears to be that the transfer shift linkage isn't pressing the lever of an air valve under the truck quite far enough. I could fix it by readjusting the lever, but instead I think I'll deliberately rotate the lever so it doesn't engage the shift linkage at all, on both of my M939-series trucks. Looks like it should be a quick, easy and reversible job.

 

[nf6x]

I understand now!!!

I was thinking about removing the engagement plate on the shifter linkage
to disable the front axle lock up. Or just move it all the way to the end of the linkage.
I think I saw a post that someone cut a slot in it and turned it.

I think it would be easier and less destructive to rotate or remove the lever arm on the air valve that the engagement plate bumps into. I haven't tried it yet, so I don't know if that little arm is keyed to only go on the valve's shaft one way. I might try it today...

 

[Csm Davis]

Yeah why not just unhook it? The air or linkage see no need to cut anything!

 

[jrod66]

I just commented in the other thread like this one, I like the strike plate or actuating lever mod idea alot better than removing/plugging air lines

 

[nf6x]

I just tried repositioning the arm on the valve which engages the front axle in low transfer range on my M923. No success yet. Click the pictures below to enlarge them.

Here's a wide shot of the valve and the front of the transfer case:

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Here's a close-up of the valve in high range:

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Here it is in low range: Notice that the arm hasn't been moved very far by the round plate on the transfer shift linkage, causing my truck to not automatically engage the front axle in low transfer range. I can still manually engage it with the switch on the dashboard.

​

After removing the setscrew and clamping wedge from the arm, it'll wiggle but it still won't come off. It looks like there's a blind pin holding it in place, just to the left of the hole that the clamping wedge goes into:

​

 

[kennys@wi.rr.com]

So that is the switch that needs to be dissabled in order to shut off the auto engagement of the front axle? Should be easy enough to bypass.

 

[nf6x]

So that is the switch that needs to be dissabled in order to shut off the auto engagement of the front axle? Should be easy enough to bypass.

I'm about 90% sure that it is, so proceed with caution. The air lines could be removed and plugged, but you'd need to study the TM/PSP carefully to make sure the manual front axle engagement will still work. Anything that prevents the round disc from pressing the lever on the valve should work. You could simply cut either the round disc or the arm on the valve, but I'd like to do it in a reversible, non-destructive way on my trucks. I'd prefer to leave the plumbing alone.

I think I'll try unscrewing the valve next so I can get a closer look at the arm and shaft. The screws didn't want to move because they have so much paint on them, but I think my little Bosch electric impact driver will probably zip them right out.

 

[kennys@wi.rr.com]

I think a valve in the air line would be a good way to go. Make it so it's shut off unless you "flip the switch" in the cab. When I get back to WI I will start crawling around and under mine and see what I can figure out. I fully agree on making it fully reversable. I do not like making any changes I can't go back from.

 

[nf6x]

I got the valve mounting screws out with difficulty, and impact driver, and vice grip pliers. If one of the air lines was just a tiny bit longer, then I think I could just move the valve to the other side of its mounting bracket so that the disc on the shift linkage would no longer hit it. So now I'm in the middle of cutting away zip ties to see if I can find another inch of slack in that air line...

Also, with the valve off I can now see that the arm is held on with a roll pin in a blind hole.