Possible solution for dreaded engine stall when brakes are locked up.

[mdainsd]

I've been reading as much as I can find on the fatal (literally) fault of the M939 series. This of course being the issue that an over-zealous application of the brakes stalls the engine and in doing so kills the power steering resulting in skids, crashes and roll-overs.

I work in the field of high pressure connectors, systems etc for offshore oil production.

Here is what I am thinking of: Much like the air brake system which has storage tanks to supply several stops without engine power, the addition of a hydraulic accumulator would serve the same purpose for the power steering.

How do I propose doing this? First measure operating pressure. I'd guess than no more than 3000psi which is low pressure in my world. A pressure gauge Tee'd into the system will tell this. Next measure volume of power steering fluid required for one lock to lock cycle. Then decide how many lock to lock cycles I want to support. Size the accumulator accordingly. A check valve needs to be added at the pump out put. then a Tee to tie the accumulator into the high pressure line. The remaining task is increasing the PS pump reservoir to add enough capacity to charge the accumulator.


I see no reason why this wont work.

 

[simp5782]

I've been reading as much as I can find on the fatal (literally) fault of the M939 series. This of course being the issue that an over-zealous application of the brakes stalls the engine and in doing so kills the power steering resulting in skids, crashes and roll-overs.

I work in the field of high pressure connectors, systems etc for offshore oil production.

Here is what I am thinking of: Much like the air brake system which has storage tanks to supply several stops without engine power, the addition of a hydraulic accumulator would serve the same purpose for the power steering.

How do I propose doing this? First measure operating pressure. I'd guess than no more than 3000psi which is low pressure in my world. A pressure gauge Tee'd into the system will tell this. Next measure volume of power steering fluid required for one lock to lock cycle. Then decide how many lock to lock cycles I want to support. Size the accumulator accordingly. A check valve needs to be added at the pump out put. then a Tee to tie the accumulator into the high pressure line. The remaining task is increasing the PS pump reservoir to add enough capacity to charge the accumulator.


I see no reason why this wont work.

You must be an engineer cause you are overthinking this.
If your ABS is working properly you need not worry about this. Even then, if you are not familiar with the air brake systems especially wedge brakes on their application while stopping empty vs loaded then you need to familiarize yourself. If your truck does not have ABS, like mine, then brake application should be pressure,slight release, pressure. You should never slam on your brakes as you are not going to be able to stop anyway as you are trying to stop a 21,000lb truck in a small amount of distance. You can stop faster by the pressure,release, pressure method, especially loaded heavy, than you can by simply applying the brake pedal.

try it when you drive. apply brakes, let off till you hear the air release burst, then reapply. you will feel a harder grab. If not you should inspect your brakes if you have not because sometimes they come apart or the springs are not installed right and not all your brakes are working.

Oh and if you wanted to go down your road. it would be easier to put some type of valve in the transmission that would simulate neutral. Then again when I had that stock shifter setup whenever i went to stop at a stoplight or in traffic i always threw my truck into neutral just as habit. This is also good habit to do when backing your truck up or maneuvering tight areas over using your brakes so conserve air pressure for your brakes. CSM Davis will scold you for using all your air up.

 

[mdainsd]

Well, I am an Engineer!

Actually in my lifetime of driving I have locked the brakes a total of one time on pavement. Granted that's with hydraulic brakes. Ive driven air brakes a fair amount and there too have never locked them up. This is my 50th anniversary of driving, all without a single ticket or accident. Call it luck or whatever.

My truck doesn't have the MWO ABS installed.

Being an Engineer I like to correct deficiencies, it's in my blood.

Funny side note, the CA CDL handbook instructs emergency stops to be done by breaking to lockup, releasing and repeating as necessary.

 

[simp5782]

Well, I am an Engineer!

Actually in my lifetime of driving I have locked the brakes a total of one time on pavement. Granted that's with hydraulic brakes. Ive driven air brakes a fair amount and there too have never locked them up. This is my 50th anniversary of driving, all without a single ticket or accident. Call it luck or whatever.

My truck doesn't have the MWO ABS installed.

Being an Engineer I like to correct deficiencies, it's in my blood.

Funny side note, the CA CDL handbook instructs emergency stops to be done by breaking to lockup, releasing and repeating as necessary.

On lockup you can lose control causing the rear of the truck to shift. Especially on these trucks as and bust a torque rod insert by causing the tires to grab. With 1600s you can increase your rear brake braking by adding a piece of steel to the bed and securing it. This will keep the tires with some weight on them and less chance to hop on a hard brake. Only time you really have lockup issues is during a hard brake and you hit a good bump that causes the rear tires to jump and lock

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[74M35A2]

How do you know if somebody is an engineer? Don’t worry, they will tell you.

Pretty sure all the M939 trucks got the ABS rework. Case closed?

 

[98G]

How do you know if somebody is an engineer? Don’t worry, they will tell you.

Pretty sure all the M939 trucks got the ABS rework. Case closed?

Not all of them got it. There's a scant few that didnt. I've seen several go through auction without it.

 

[simp5782]

How do you know if somebody is an engineer? Don’t worry, they will tell you.

Pretty sure all the M939 trucks got the ABS rework. Case closed?

Not all. I have ran across a few that never got it. Mostly A0 trucks and were tractors.

A complete ABS kit for the truck is $600 on ebay.

 

[BKubu]

Not all of them got it. There's a scant few that didnt. I've seen several go through auction without it.

Especially the ones that came out early. I have had M939 series trucks since 2001 when it was almost unheard of to own one. My first truck (an M929) did NOT have ABS.

 

[M35A2-AZ]

I know of three trucks that do not have ABS or an air dryer, one is in my barn. The Navy and USMC trucks for the most part did not get the ABS upgrade.

 

[mdainsd]

Mine is ex USMC, No ABS, no air dryer.

 

[simp5782]

Mine is ex USMC, No ABS, no air dryer.

Well add an air dryer and add the abs kit.

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[mdainsd]

I read through the MWO up in downloads, quite an installation. I see that kit you mention on the epay. I will contact him and see if that is a complete kit. He states its for all the trucks, but reading the instructions there is some picking and choosing between sub-kits.

Thanks.

 

[simp5782]

I read through the MWO up in downloads, quite an installation. I see that kit you mention on the epay. I will contact him and see if that is a complete kit. He states its for all the trucks, but reading the instructions there is some picking and choosing between sub-kits.

Thanks.

Its description. sounds like everything and then some. heck the air dryer is worth $200 by itself. Should not take you more than about 10 hours as a newbie to do it.

[h=2]Item description[/h][FONT=&quot]Fits M939 Models ABS Brake Parts Kit. ABS Brake Parts Complete Kit. Includes Air Dryer with Heater. View more info on thisABS Brake Parts Kit. Oshkosh, WI 54902. 3664 Nekimi Avenue.[/FONT]

 

[mdainsd]

Ordered.

Probably cheaper than the "engineered" fix I was think of, LOL.

Be safer and peace of mind.

 

[74M35A2]

...or, swap out the slush box for a manual trans. Nice yummy 13-speed, double-over.

 

[mdainsd]

...or, swap out the slush box for a manual trans. Nice yummy 13-speed, double-over.

No way! One of my first military vehicles back in the 70s was a GMC deuce with a Hydramatic with the "Garrison" wheels. I loved that truck. Sound of that six and (mine at least) had incredibly crisp shifts. Probably something wrong with it but it always chirped going into second.

 

[Csm Davis]

...or, swap out the slush box for a manual trans. Nice yummy 13-speed, double-over.

No way! One of my first military vehicles back in the 70s was a GMC deuce with a Hydramatic with the "Garrison" wheels. I loved that truck. Sound of that six and (mine at least) had incredibly crisp shifts. Probably something wrong with it but it always chirped going into second.

Great 2 engineers can't agree on which way to proceed. To bad they don't have to, we could see a true monstrosity of a fix that would cost $100,000 and fill the cargo box completely and only work if the truck is running 55 mph or over.

 

[mdainsd]

Great 2 engineers can't agree on which way to proceed. To bad they don't have to, we could see a true monstrosity of a fix that would cost $100,000 and fill the cargo box completely and only work if the truck is running 55 mph or over.

wow, so negative.

 

[74M35A2]

I’m not an engineer, I work at the bowling alley. Davis is a good guy. I just drive him, and most others, nuts sometimes.

 

[simp5782]

I’m not an engineer, I work at the bowling alley. Davis is a good guy. I just drive him, and most others, nuts sometimes.

To the OP, call your local Allison dealer and explained what happened, some here have had tremendous success, and magical low cost fixes from just a few hundred dollar pan drop and technician tweak and tune at their facility. They go in broken, and come out saying it has never shifted better. Another option for you. Not sure many here have torn into the Allison and performed a successful repair/rebuild. Most are just swapped out with another used working unit. I doubt yours will suddenly fix itself though.

The OP wasnt referring to a broken transmission. His thread was in relation to the military mwo to add abs to the trucks. The engine stalled and steering is lost when the brakes locked up stalling the whole driveline. Hint as to why your truck has abs

https://youtu.be/PvW9ISZ6LlM

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