M923 Spring brake overide

[engineer233]

I have a couple of questions. First what is the function of the spring brake override valve in the cab and , when should it be used ? The other question I have is when my truck is running and the air tanks are completely charged you can't shift the transfer case into low range even if you hold the button on the shifter down. If you bleed you main air res down to around 50 psi. it shifts great. Why is this ?

 

[wsucougarx]

Hmm good questions. Anyone know?

 

[AaronW]

-10 manual page 1-64/1-71 for spring brake release control valve info.

 

[Krink545]

Transfer case info here.

http://www.steelsoldiers.com/5-ton-up/79779-m939-series-transfer-case-low-range-front-axle-engagement.html

 

[engineer233]

Thanks. That is exactly what I needed to know about the t-case. Anyone know how to test the interlock solenoid or where to buy one ? Do these trucks have any fuses? If so where are they ?

 

[Over]

-10 manual page 1-64/1-71 for spring brake release control valve info.

Still didn't tell me anything. When the over ride is activated you have the equivalent of your parking brake? The reason I'd like to know is I picked up a M923 the other day and after going down the road a couple of miles I noticed the knob for the spring brake was activated and the light on. Would this be the equivalent of driving with your parking brake on? I noticed this trucks brakes are not as touchy as my other one at low speeds. Thanks.

Still don't understand why a straight answer plus where it came from is such a big deal.

Thanks again for the info.

 

[Coffey1]

Over the over ride does take out the air parking brake as long as there's air to engage it and if the air don't leak leak out of the spring brake cans you have done away with that Axel brake.

You would still have a parking brake with the handle with a set of brake shoes on drive shaft.

 

[silverstate55]

Still didn't tell me anything. When the over ride is activated you have the equivalent of your parking brake? The reason I'd like to know is I picked up a M923 the other day and after going down the road a couple of miles I noticed the knob for the spring brake was activated and the light on. Would this be the equivalent of driving with your parking brake on? I noticed this trucks brakes are not as touchy as my other one at low speeds. Thanks.

The knob should NOT be pushed in under normal operating conditions, it should be pulled out. If you had a primary/secondary air system failure, and pressure dropped under 70 psi, then your spring brakes would engage on the rear axles & you wouldn't be able to move the truck. Then you would push in the Spring Brake Release/Override knob; there should be enough air pressure in air tanks (via the one-way check valve) to override the spring brakes so you could move the truck off the road to somewhere safe. Under normal operation you shouldn't have that knob pushed in.

Your hand-operated parking brake (next to driver's seat & transfer case shift lever) is a mechanical parking brake; if you needed to adjust this separately & test it, you could also push in the override knob to disengage the spring brakes...that way you'd truly know if the mechanical parking brake is working properly or not!

In summary, if you push in (engage) the Spring Brake Release/Override knob, disengage your mechanical parking brake, and leave the transmission in Neutral, the truck should roll freely...pull OUT the Spring Brake Release/Override knob, the spring brakes should engage on the rear axles (when air pressure drops below roughly 70psi) and the truck should NOT roll freely.

HTH.

 

[Over]

The knob should NOT be pushed in under normal operating conditions, it should be pulled out. If you had a primary/secondary air system failure, and pressure dropped under 70 psi, then your spring brakes would engage on the rear axles & you wouldn't be able to move the truck. Then you would push in the Spring Brake Release/Override knob; there should be enough air pressure in air tanks (via the one-way check valve) to override the spring brakes so you could move the truck off the road to somewhere safe. Under normal operation you shouldn't have that knob pushed in.

Your hand-operated parking brake (next to driver's seat & transfer case shift lever) is a mechanical parking brake; if you needed to adjust this separately & test it, you could also push in the override knob to disengage the spring brakes...that way you'd truly know if the mechanical parking brake is working properly or not!

In summary, if you push in (engage) the Spring Brake Release/Override knob, disengage your mechanical parking brake, and leave the transmission in Neutral, the truck should roll freely...pull OUT the Spring Brake Release/Override knob, the spring brakes should engage on the rear axles (when air pressure drops below roughly 70psi) and the truck should NOT roll freely.

HTH.

THANKYOU!!! That explanation will be appreciated by a whole lot of folks that may not be wise in the way of the M923 trucks! This all by itself should be a sticky. Thanks again.

 

[doghead]

You have the option to make anything you want, a personal sticky.

 

[Artisan]

The way I understand the spring brake override is this.

If you are in a convoy and your trucks brakes go bad
or engine stops and no compressor, you hit that button and
it releases the brakes just long enough to push the truck off the road
and out of the way of the convoy behind you, this is a war machine .
remember, there is no time to wait for a tow truck, move it, if necessary
push it off a cliff but do not hold up the convoy.

It will only last
a short time too, and then the brakes will set on again and you will
have to cage the brakes to move it this time.

There are valves underneath the truck too, make sure they are all ON,
well, read the manuals and do what they say...

 

[TEAMJENSEN]

I'm new to this and just purchased a m939a2. Should the green spring brake override be lit when driving the truck?

Sent from my SM-G950U using Tapatalk

 

[simp5782]

I'm new to this and just purchased a m939a2. Should the green spring brake override be lit when driving the truck?

Sent from my SM-G950U using Tapatalk

No, it is only lit when it is in override mode. Pull the knob on the far right of the instrument cluster back out. If it not depressed then you have a bad switch on that PP1 valve behind the dashboard.

 

[TEAMJENSEN]

Thank you, I'll check it out tomorrow.

Sent from my SM-G950U using Tapatalk

 

[TEAMJENSEN]

I guess I just don't understand its purpose or what it is indicating

Sent from my SM-G950U using Tapatalk

 

[simp5782]

I guess I just don't understand its purpose or what it is indicating

Sent from my SM-G950U using Tapatalk

Its purpose is to override the spring brakes in case of a linear valve malfunction. The linear valve is the little wheel valve on your parking brake that gets pushed up when the parking brake is set. This valve allows air to pass by it and flow to the spring brakes to release them when you release the parking brake. In the event the linear brake valve there fails or gets stopped up or neglects to let air pass thru it to the spring brakes the override on the dashboard takes air that is in the spring brake tank and diverts it thru double check valve #4 then sends the air to the spring brakes to be released. It is a double override in case of a failure and the truck still needs to move.

Think of the linear parking brake valve as the same valve a semi truck would use for parking. The military just used that as a double fail safe so the driveline brake is set along with the spring brakes.

 

[TEAMJENSEN]

Thank you for the very good explanation

Sent from my SM-G950U using Tapatalk

 

[TEAMJENSEN]

No, it is only lit when it is in override mode. Pull the knob on the far right of the instrument cluster back out. If it not depressed then you have a bad switch on that PP1 valve behind the dashboard.

So this knob on the far right should not be pushed in unless you have a brake problem. In other words under normal conditions it should always be pulled out. Thank you again!

Sent from my SM-G950U using Tapatalk

 

[74M35A2]

No, having nothing to do if you have a brake problem. More-so if you are having a rapid air loss problem. This button does one thing only, it prevents the spring parking brake from setting when the truck air pressure is bleeding down. It takes a very good understanding of that single sentence to know if you want to use that button or not. Here are two examples, one has dire consequences and one does not:

1. Driving truck at my dad's farm on flat ground, truck suddenly losing air quickly, don't want the spring parking brake to set and lock the truck in place, as would normally happen. Want to be able to push it with dad's tractor into the barn to fix the air leak. Then activate this button while truck air pressure is still over 60psi. Once air pressure gone, spring park brakes will not be set, and truck will easily roll. This is effectively the same as "caging" the brakes, but without having to climb under and do so.

2. Driving down the road in traffic, sudden loss of truck air pressure. Normally, as the truck air pressure is falling, once dropping below 60psi, the spring parking brakes will automatically come on and bring the truck to a stop, having nothing to do with the foot pedal brake. If you activate this button during this time, the automatic spring parking brake will not set, and as the air pressure continues to drop, your service (foot pedal) brake effectiveness will diminish quickly. You now have an unstoppable rolling 20,000lb all metal 10' tall vehicle, in traffic. Very bad. You have deactivated the "emergency" brake portion of this system which would normally slow this vehicle in a gradual yet steadily increasing manner.

I really suggest to simply leave this button alone, in the normal position, unless 100% understood what it does, and therefore when to use it. I believe it is only present on mil air brake vehicles, to "get away" when needed, even if the vehicle is losing air pressure, or if the vehicle needs to be towed by something that does not have an air system on it. It has nothing to do with the park brake dash button you see on conventional civilian 18 wheel tractor trucks. It is simply a spring brake disable button. That's why it says spring brake over-ride. Can be convenience, or disaster. One of the most mis-understood parts of these vehicles. If it said "parking and emergency brake disable", it would probably be better understood.

 

[TEAMJENSEN]

Thank you for the very good explanation, I finally get it....sorry, I was a little slow, but it was new to me

Sent from my SM-G950U using Tapatalk