M939 series transfer case low range / front axle engagement

[rrrr]

When you manually engage the front axle the "Front Axle Engaged" light comes on above the steering wheel. When you shift the transfer into low that also automatically engages the front axle.

Is the Front axle engaged light suppose to come on with the transfer case in low without using the air locker for the front axle ?

Mine doesn't turn on the light with the transfer case in low but will if I manually engauge the front axle.


TM's no help on this question.

 

[Krink545]

Just got mine home less than 24 hours ago. Messed with it a little today and mine does the same.
My biddies is almost impossible to get into low. Were able to get it in once and could never get it in again.

 

[fsearls92]

Mine is the same way, light isn't on in low but is engaged. For the hard shifting into low do it while the truck is rolling and it goes in alot easier than when stopped.

 

[wsucougarx]

From what I've been hearing, it can be downright a chore shifting into low gear. Something about the shift linkage and not being used too often.

 

[Krink545]

fsearls92 Thanks for the info on my buddies truck. Had a bypass line ready to temporally bypass the drier.

Do you shift into low while in gear or do you pop it into neutral?

Ill post the epic recovery later. 590 miles in one day

 

[fsearls92]

Awesome, great to hear you guys made it back fine! that is a heck of a drive in one day! you have put more miles on it in one trip than I have in two weeks! I hope you guys like your new trucks, all of them in that auction were nice trucks. Yeah put it in nuetral and shift into low, otherwise it will be a horrible grinding and whinning noise. I tried shifting mine stopped and it was a two hand hard pull, and when it was moving it was a good one handed pull, but still substantially easier.

 

[Bolkbich]

m923 transfer case shifts like butter my m932 shifts a little stiffer but good my m932 w air ride is basically stuck i havent looked into it yet i would say its a lube issue

 

[Bolkbich]

Well my truck wouldnt engage the front axle. I ended up bypassing the air valve on the linkage to the transfer case. Now i can lock the front axle in. I got it into low range now i cant get it out. is there some sort of air lock. What is the button in the end of the t case shifter for. Any ideas ?

 

[Krink545]

f. Transfer Case Control System Operation.
The transfer case control system converts four-wheel driving power into six-wheel
driving power, provides smooth shifting of transfer case into high or low driving
rangs while vehicle is in motion, prevents transfer case from being shifted with
transmission in gear, and provides hydraulic power for auxiliary equipment through
a Power Takeoff (PTO).
(1) Six-wheel drive is achieved two different ways depending on the drive
range (high or low) desired. In low range, the transfer case shift linkage
automatically moves a cam-actuated valve which dumps air into the front
drive cylinder. This forces a piston against the transfer case clutch to
engage front-wheel drive. In high range, front-wheel drive is engaged in
the same manner except that the front-wheel drive valve is manually
actuated by the front-wheel drive lock-in switch on the instrument panel.
(2) In order to shift the transfer case from one driving range to another, an
interlock system working in conjunction with the 5th-gear lock up solenoid
is used. This system prevents the transfer case form being shifted unless
the transmission is in neutral.
(3) With the automatic transmission, several actions must occur in order to
shift the transfer case from one driving range to another. Because of the
interlock system, the transmission must be placed in neutral. The transfer
case shift lever switch must also be depressed.
(4) The transfer case control system, through the use of a PTO driven by the
transfer case, also provides hydraulic power to operate the crane and rear
winch on the M936/A1/A2 wreckers.
(5) Major components of this system are:
1-17. CONTROL SYSTEM OPERATION (Contd)
TRANSFER CASE SHIFT LEVER SWITCH – When depressed with
transmission in neutral, signals interlock solenoid valve to exhaust air
pressure from interlock air cylinder and actuates lock-up solenoid.
TRANSFER CASE SHIFT LEVER – Is pushed down to high for light load
operations, and up to low for heavy load operations. Six-wheel drive is
achieved automatically when transfer case shift lever is placed in low.
TRANSFER CASE POWER TAKEOFF CONTROL LEVER – Manual
control for engaging power takeoff.
TRANSFER CASE POWER TAKEOFF – Mounted and mechanically driven
at rear of transfer case, the PTO drives a pump to supply hydraulic pressure
to power the rear winch and crane on the M936/A1/A2 wreckers.
FRONT-WHEEL DRIVE LOCK-IN SWITCH – Manual control for
activating front-wheel drive valve to provide front-wheel drive with transfer
case in high drive range.
E
D
C
B
A
INTERLOCK AIR CYLINDER – Under air pressure, a piston in the
interlock air cylinder forces a shaft against one of three grooves in transfer
case shift lever. This prevents transfer case from being shifted with
transmission in gear.
INTERLOCK SOLENOID VALVE – Releases air from interlock air cylinder
when transmission is in neutral and transfer case shift lever switch is
depressed. This permits the transfer case high/low shift shaft to move.
FRONT-WHEEL DRIVE AIR CYLINDER – When under pressure, it moves
transfer case clutch forward to engage front-wheel drive.
FRONT-WHEEL DRIVE VALVE – When tripped by transfer case shift
shaft, the front wheel drive valve routes air to front-wheel drive air cylinder.

It didn't copy and paste well Operating manual page 1-34 and 1-35

 

[OPCOM]

If you want a really hard time shifting the case, try installing it in an M818 and using the short throw shifter. It is almost impossible. I'd like to know what to do to 'fix' the case as it is super-hard to shift.

 

[Bolkbich]

Krink545 Thank You for the information...Im sure my problem has to do with the interlock solenoid valve. When i first got the truck i could get it into low and back out , not easy but i could do it. The truck had so many air leaks it wouldnt hold enough air unless you were over 2000 rpm so thinking back i had no air at that point so i could shift it. Now i fixed the air leaks and it wont shift. Im going to disconnect the air to the interlock air cylinder and try that.

 

[Bolkbich]

If you want a really hard time shifting the case, try installing it in an M818 and using the short throw shifter. It is almost impossible. I'd like to know what to do to 'fix' the case as it is super-hard to shift.

OPCOM are you using the interlock air cylinder and solenoid valve? Which ports on the t case did you run air to ?

 

[Bolkbich]

Sure enough, pump the air down all the way and it shifts like butter.

 

[spicergear]

TRANSFER CASE SHIFT LEVER – Is pushed down to high for light load
operations, and up to low for heavy load operations. Six-wheel drive is
achieved automatically when transfer case shift lever is placed in low.

Wait a minute...REALLY?! Low range automatically locks in the front axle?!

If this is right, well no friggin wonder they're breaking T-cases in reverse in low range because you won't just spin the tandems, the locked in front axle will have all the traction in the world! You'll have enough traction to throw you through the windshield if you goosed it. Yes, I know the Allisons low gears and torque converter multiply for a lot of force, but it IS a reverse low range lock out.

 

[spicergear]

Okay...I kinda think I may be onto something here and would you guys that think on things to really re-read the above. I also think a set of lock out for the M939 Series trucks would reduce the T-case breakage issue even further as the only time you would have all that super heavy front traction would be if you had the lockouts engaged. At a possible grand a piece (hopefully WHEN they get made) it may help as added insurance for the T-case. Obviously not hammering the truck in low range and reverse is key but but the absence of traction from the front axle so the power could be spun out of the lighter tractioned rear tires may really be best! With lockouts disengaged, the power to the front axle would just spin in the hubs instead of super loading the T-case. Once I get more done on my M931A2 I'm going to see if I can eliminate the automatic LOW range lock in of the front axle and select it when I want it like the deuce dash mounted air switch.

 

[Bolkbich]

Spicergear if you go under the truck and look at the valve thats connected to the transfer case linkage,it gets air fron 2 sources.one is the switch on the dash ,the other is full line pressure going into the valve with the wheel on it. Just disconnect and plug the fitting with full line pressure to it. on my truck it was a fairly short airline that went directly to a t on the inside of the drivers side frame. no more lowrange all wheel drive unless you flip the switch on the dash.

 

[spicergear]

I was under over the weekend and saw the switch you're talking about. Yeah, that sucker is out of there for sure. I'LL pull the front axle in when I want it in.

Any thoughts on the reverse gear, front drive axle engaged, medievil driving, busted case VS reverse gear, front axle NOT engage, medievil driving...tandems spin- no busted case. ...or at lease a SIGNIFICANT reduction of odds??

 

[Bolkbich]

The exploded t-case i saw had the whole bottom of the case by the front driveshaft broken out. Looking at that t-case i would say that is the weak area,so not having the front axle engaged would greatly reduce the odds of breaking it. Reverse high range is low enough to get you out of just about anything. If you think that low range is needed to get you out ..consider yourself stuck. On a regular basis i bring my unloaded m923 down in the woods, in low range creeping up the trails flipping the front axle switch when needed. Backing up a steep dirt hill if the rear tires spin i'll engage the axle but i dont beat it up.

 

[spicergear]

The exploded t-case i saw had the whole bottom of the case by the front driveshaft broken out. Looking at that t-case i would say that is the weak area...

That is EXACTLY what I wanted to hear. I haven't seen one of the broken cases but that pretty much sums up my suspicions. With the low range, the low trans reverse gear, plus the general rule of thumb is a 2:1 ratio reduction for the torque converter...that's one HECK of a lot of torque there especially where the front tires are inherently going to have SO much traction with the whole nose of the truck on them. I bet those trucks would have served much better had that automatic low range front axle engagement option NOT have been opted for. I know just backing my tractor up a wet hill one day and the tandems spun. Flicking in the front axle and it never even thought about breaking traction again.

I'm DEFINITELY going to eliminate having the auto front axle engagement for low range and just pull it in when I want.

Bolkbich, thanks for the info on the busted case and airing down the system!!!

 

[GabrielDuce]

Warning label data plate

My m934a1 has a warning label that says "DO NOT USE REVERSE IN LOW RANGE."

I bought my truck in kansas at fort riley and i live charlotte nc. Im on my way home right now. I drove the truck 800 miles today and i have 400 miles to go tomorrow. im almost there my truck has a refrigerator and air conditioner in it .... ha ha ha im loving it... o yeah and a bank of batteries that recharge while you drive it to run the expansible box lights and electronics in the back