M939 transfer case air lines installed wrong

[jwesley74]

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.

I'm having same problem with M925 not shifting when air pressure is up. I tried switching the wires going to capacitor. Didn't make a difference. I even tried bypassing lever switch and just jumping the 2 connectors to get it to dump air. didn't make a difference. Any suggestions??

 

[jwesley74]

Well, don't know the true source of the lack of power to the capacitor/transfer case lever switch, but, found there was no power to any of this. Traced the main power lead to the main wire harness and lost source after that. So we disconnected at main wire harness and ran new switched (and fused) power supply to this and transfer case shifts normal now. So problem fixed, but still never could find why it lost power to begin with.

 

[jarhead1086]

I haven't tried shifting into low range yet. I have had plenty of entertainment with crossed air lines on my brakes and need to fix one thing at a time in proper order. I would never have figured any of this auto engage thing out without this post unless it was the hard way. This fix is on my list now, thanks guys. I have a lot of black electrical tape 'fixes' that I take as clue to a rig job. I am removing all of it and verifying its right before I give the connection a solder, clean, and polyolefin shrink tube. Anybody that knows what they are doing or cares at all would not twist wires together and black tape over the twist on a wire going to an alternator as I found on my 923. Its the alternator in a tough environment, not the horn, so why not a little more effort? Depot sabotage is harder to recognize than the often obvious PVT Pyle sabotage. Thanks again all.

 

[jhcattleco]

is it a problem to use reverse in low range

 

[martinwcox]

Yes a big problem, your truck may well have a warning label in the cab, you will destory the transfer case, a few example of busted cases on the site.............

 

[jhcattleco]

does the front axle auto engage when the truck is reversed in low

 

[jhcattleco]

what causes this problem

 

[martinwcox]

It auto engages when low is selected, that fuction can be bypassed, details on the site.

 

[martinwcox]

If you are asking about transmission damage, the torque exerted in low range reverse exceeds the design parameters, if you are very careful and don't goose the gas you MAY be ok but if in doubt, don't do it. If you do a search there are pictures of destoryed parts.

 

[jhcattleco]

so reguardless of whether you bypass the auto front axle engage valve the transfer case can be damaged if run in reverse low

 

[jw4x4]

The purpose of the bypass is to keep the truck in rear wheel drive until it is manually shifted into all wheel drive. That way, you can use reverse in low range, provided the auto-lock is bypassed and therefore the truck's front end is not engaged. The damage occurs when in reverse, low range, with the front axle engaged. At least that's the way I interpret all of this.

 

[doghead]

I thought the issue was the increased strain with the larger diameter tires.

I think you'd have that with or without the front engaged.

 

[ecostruction]

Can the bypass be accomplished with removing the big washer that trips the valve arm for the auto engage under the truck? Or, is the assumption that the case is plumbed wrong?

Anyone have a picture of the piston which engages the front axle? Need to know how to tell if the front is "in" AWD mode.

 

[Tow4]

Can the bypass be accomplished with removing the big washer that trips the valve arm for the auto engage under the truck? Or, is the assumption that the case is plumbed wrong?

Anyone have a picture of the piston which engages the front axle? Need to know how to tell if the front is "in" AWD mode.

Yes, you can remove the washer. That's what I did. I left the switch and associated hoses intact.

 

[Durango_USMC]

so reguardless of whether you bypass the auto front axle engage valve the transfer case can be damaged if run in reverse low

Yes.

 

[Rkrug]

I'm trying to get my T case on my 923A1 working, it will not shift once the truck has built up air pressure, it will shift sometimes when there is no air pressure. When I hit the button on the handle I can hear a click coming from the T case solenoid.
Thanks for any help

 

[Tinydragon2001]

Any body know where I can get a replacement seal for the front axle air engagement cylinder. Mines leaks air into the transfer case through the air cylinder when I engage the front axle either with the dash switch or when I pull the range selector.

 

[Andyrv6av8r]

I just picked up a 923 this Tuesday from Eastover that was a 2008 rebuild from RRAD. I noticed immediately that the front end was locked in AWD. As soon as I got home I found 2 air lines switched on the auto engage valve. Works as advertised now. It looks to be an easy mistake to make too.

 

[dpowell]

new to these trucks can someone talk me thru the air line routhing for the 6x6 to go to the front axel lock? i dont have any air in cab like the the horm to the 6x6 switch its self. like i said im new to this stuff so very detailed will be best

 

[ichudov]

I would like to test my M939 series trucks for this condition of all wheel drive being engaged at all times. I did see that I need to jack up the front (easy, I have a forklift), but I am not sure what exactly would be the test to perform once the front wheels are up in the air.