M925 transfer case posessed ? Will not shift above 65 psi in air system!!!

[61sleepercab]

After reading the manual me and my mechanic/driver friend are both scratching our heads:

My M925 transfer case will not shift at all if air system pressure is above 60-65 psi .
At 120 normal working pressure I cannot shift transfer, use brakes to drain air to 60 psi, transfer will shift.

All engine start up lockouts seem to work, Neutral safety switch , low air warning horn, parking brake warning.
Driving in low range reverse lock out prevents shifting into reverse as normal.
I got the winch pto in gear but the cable control for the drum travel only wanted to feed out and not in. I do not think that the winch line has ever been spooled out .

My truck has 48,000 miles and 30 years old and some things are tight to move or shift.

Is the transfer case smarter than the operator?? What is going on?? Thanks Mark

 

[patracy]

Air lines plumbed correctly? I seem to recall there's been a lot of trucks that have been plumbed backwards.

 

[Coffey1]

They are a bear to shift sitting still mine will do it now, but have to shift to drive then neutral a couple of times. It goes in easier moving. Stay below 20 mph and shift into neutral and shift transfer case.

 

[wsucougarx]

Ok, I've got something for you to try. Start up your truck and get your air pressure up far enough for the buzzer to go off. Turn off the truck. Turn on the battery switch only (make sure anything like the parking brake is not on so the buzzers are quiet). Now open your driver side door. Reach down to the transfer case lever and just push in the button. You should hear a click to the back of the truck (transfer case). If you don't hear it, then the wires under the cab could be connected incorrectly or the push button switch wires could be crossed. My M923's transfer case lever could be operated as long as the air pressure didn't go over 60. Once over 60 it wouldn't budge. Found out the wires to my pushbutton were crossed and the wires under the cab were hooked up incorrectly.

 

[wreckerman893]

I have the same problem with the M927.

 

[Scar59]

I jumping in, just picked up a low mileage M925 and have not exercised the transfer case yet, might give it a test this weekend.

 

[Coffey1]

Mine refuse to work at first. I think this was due to long term not exercise. now that I engage it once in a while it goes in a lot easier.

 

[rangereter]

The transfer HNL (high neutral low) range shifter uses air pressure to "keep" the shifter lever/shaft located in position until the button/switch on the lever is activated. This arrangement keeps the transfer in the selected range when bouncing around on rough terrain (a more positive lock than a detent ball and spring). When you push the button on the lever, you are activating an electric over air valve (located on top of the transfer) that will allow the air pressure in the transfer range engagement cylinder to momentarily "vent" and enable the range lever and shaft to be moved into a different range. This is why the lever can move easily with lower air pressure (too low for cylinder to "pin" the shift shaft in position.
As was mentioned (kind-of), with the engine not running, you can usually hear the cylinder vent in the air intake bonnet when the button is pressed. Too much moisture in the air system (especially frozen) can prevent the air cylinder from venting properly and make it difficult to shift even when the electric portion of the shifter is operating correctly. As the TM should show, the electrically activated air valve has three 3/8 air lines to it...one is constant/supply air pressure in, one for pressure out to pressurize shift shaft cylinder and hold shaft in position, and the third line is to vent (air pressure out of cylinder) when button/switch is activated.
Hope this helps a little.
Regards, Bob

 

[Scar59]

Bob,
Good information, thanks for the description/ operation of the transfer case selector, going to check out my latest purchase.
JC