M939 transfer case air lines installed wrong

[Suprman]

If you don't have any air in cab and you are building pressure then you have a bad pressure protective valve. Are your guages going to around 150psi and you hear air venting out the bottom of the air dryer?

 

[Andyrv6av8r]

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.

Jack up the front axle with full air pressure on the truck. I had my shop air hooked to the rear with the engine off. Spin one of the tires by hand with the front engage switch " out ". The front driveshaft should turn. Now engage the front axle, " In " and the driveshaft should stop turning. If it is plumbed wrong, the front driveshaft won't turn regardless of the position of the air switch.

 

[wheelspinner]

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

Do a search for the pressure protection valve. You will most likely find the answer to your specific problem there. As a new member, please post up in a new thread to introduce yourself.

 

[two.fiveM35]

Thanks guys for all your tips and knowledge. I have similar issues with my M923a1 but know nothing about all the air plumbing. I know have a little knowledge to go picking around. Sounds like very little to no quality control when they rebuild these trucks.

 

[ichudov]

Jack up the front axle with full air pressure on the truck. I had my shop air hooked to the rear with the engine off. Spin one of the tires by hand with the front engage switch " out ". The front driveshaft should turn. Now engage the front axle, " In " and the driveshaft should stop turning. If it is plumbed wrong, the front driveshaft won't turn regardless of the position of the air switch.

Thanks. So, in a good, normal case, with air pressure, I should see:

1) Engage off -- tire and shaft turn easily
2) Engage on -- tire and shaft cannot be turned by hand

If the switch is plumbed improperly, the tire and shaft should not turn by hand regardless of the engage switch position.

Right?

 

[Andyrv6av8r]

Thanks. So, in a good, normal case, with air pressure, I should see:

1) Engage off -- tire and shaft turn easily
2) Engage on -- tire and shaft cannot be turned by hand

If the switch is plumbed improperly, the tire and shaft should not turn by hand regardless of the engage switch position.

Right?

Not really. Either tire will turn whether engaged or not. Limited slip. Front tires turn opposite of each other. Only the driveshaft will stop turning when engaged properly.

 

[ichudov]

Not really. Either tire will turn whether engaged or not. Limited slip. Front tires turn opposite of each other. Only the driveshaft will stop turning when engaged properly.

OK, so I should turn my tire but look at driveshaft. Thanks a lot.

 

[camoyj7]

Just checked mine today and found it has been stuck in 6x6. It had full air pressure going to the front axle piston. I switched the lines at the valve that locks awd when in low range and now everything is good.

 

[madcroc1]

I think this is the problem that blew up my transfer case. Can anyone upload a picture of the correct air line routing ?

 

[madcroc1]

Yes mine came from RRAD and I think it also has the reversed air lines. I'll double check it tomorrow.

 

[madcroc1]

Ok, after a quick analysis and inspection. My lines are installed correctly. Air lines from the switch are routed to the frond axle activation valve on the back of the transfer case. Im still wondering if there is anything wrong by removing the washer. Wondering why the military would design it in such a way that when shifting into low, the front axles is also engaged.

 

[Suprman]

So the operator would have the best possible traction when low is engaged. You would need it if you were stuck with a heavy load or pulling something heavy. I'm sure the low reverse issue was determined later down the line long after the design was in full production. If there was no low gear reverse issue then no one would think twice about the front axle engaging in low. I don't know why they didn't do some type of redesign though when BMY started making the A2 trucks. I wonder if they just figured the turbo motor makes less torque at lower rpms before the turbo is spooled and that would lessen the issue.

 

[madcroc1]

(continued) instead of allowing the driver to select when to engage front axles. Could a problem develop when driving in low gear without front axles engaged ? and then engage the front axles on the fly with the flip lever ? Could damage result ?

 

[Suprman]

The issue is high torque in low gear reverse can damage the transfer case. If you are backing up not under load you should be fine as long as you go easy on the gas. I don't see how engaging the front axle in motion and in low gear would cause any problems as long as you weren't going too fast. Not like you can go that fast in low gear anyways.

 

[madcroc1]

I understand why and when it is good to activate the front axles but the design doesn't give the operator a choice on the front axles when low is engaged. So i'm trying to understand if there may have been a reason for that design. When in low, without the front axles engaged, is there no backup problem ?

 

[Suprman]

It is my understanding that with low gear engaged, under heavy load, if the operator gave it too much gas from standstill while in reverse then damage could occur to the transfer case that it couldn't handle the high torque of low gear. The transfer case only controls front axle engagement.

 

[madcroc1]

The transfer case controls front axle engagement and the selection of low-N-high. So this thread is claiming that it has a solution by removing the washer from the engagement lever. I'm wondering if the reverse problem in low gear is only with the front axles engaged. I assume yes or that solution wouldnt be a solution. This should mean that when operating in low gear under a heave load and the front axles are not engaged, going in reverse shouldn't be a problem. Is this correct logic ?

 

[Suprman]

Yes if you remove the washer then the front axle is not engaged automatically and you can reverse under load in low range

 

[madcroc1]

This is another way to protect the transfer case. (THEORY) One can get another air solenoid which activates upon any forward drive selection on the transmission shift lever (Just splice into that electrical connector). When in forward, the solenoid activates and supplies air to the front axle engagement switch on the dash This way, when in reverse or neutral the solenoid is not activated and will dump to the dump line, releasing the front axle engagement valve regardless of where the operator puts the switch.

 

[ichudov]

Truth is, you almost never need to use the low gear. It is only for pulling something very heavy that is stuck. Such as a stuck forklift or some such. It is not really for any kind of driving. Just do not use low gear and you will not accidentally reverse in low gear.