M939 5 Ton Emergency Air Brake Release Pressure Switch

[Jersey4x4]

On the M939 trucks attached to the Emergency Air Brake Release valve mounted on the instrument panel is a pressure switch that is connected to the safety control module. It controls the light on the panel in front of the driver to tell the driver the emergency brakes have been disabled. I need a new pressure switch and have exhausted all my vendor options and also search options, and TM searching has been futile. Can someone point out on exactly what page in the TM this pressure switch is located and/or a part number for the switch. It is a very low pressure switch so it is not the same as the low air warning switches(Already tried them) Any help would be great. Thanks

 

[Ford Mechanic]

I've had good luck just taking it out and taking it to my local big truck shop. They have a catalog that gives all the specs for all sorts of different air switches.

 

[Castle Bravo]

I have this for sale -



M939 Spring brake override pressure switch - $15.00 + shipping
Used, in good condition. This pressure switch activates the "Spring brake override" light on M939/A1/A2 trucks. It could be used for other applications as well. It is a 2 pin, Normally open 60 psi pressure switch. P/N 11669414, PS4099. NSN 5930-01-090-7634.

Among other things listed here - http://www.steelsoldiers.com/showth...ab-electrical-switches-air-system-brake-parts

 

[Jersey4x4]

Castle, That is actually the low air pressure switch. The spring brake override pressure switch operates at a much lower pressure then the 60psi rating. I am looking to find it in the TM so I can get the correct switch. I have 5 trucks here and every one has a different switch in it, 2 of which don't work.

 

[Gunzy]

The part # is 12375453.

 

[Jersey4x4]

The part # is 12375453.

Where did you find that number???

 

[M35A2-AZ]

Where did you find that number???

Do a search with the P/N on the T/M and you will find it.
I found it in my tm 9-2320-272-24P-1 on page 132-1

 

[Jersey4x4]

Do a search with the P/N on the T/M and you will find it.
I found it in my tm 9-2320-272-24P-1 on page 132-1

Bingo!!! That's it. I did not see it in the 27 times I looked and none of the vendors saw it in the books either. I will pass the word to them as well. Thanks

 

[Castle Bravo]

Castle, That is actually the low air pressure switch. The spring brake override pressure switch operates at a much lower pressure then the 60psi rating. I am looking to find it in the TM so I can get the correct switch. I have 5 trucks here and every one has a different switch in it, 2 of which don't work.

It is the same as the low air pressure switch, according to the NSN. This switch was attached to the spring brake override air valve through a reducer and 90 degree street elbow. It functioned correctly and made the light on the dash turn on when the spring brake override was engaged.

If you look here - http://www.parttarget.com/5930-01-1...-1.html/-a764634b-4be6-4464-8d90-977537967b0a - The P/N 12375453 and P/N 11669414 point to the same NSN and it is a 65 psi NO switch.

Also, look at Erik's Military Surplus P/Ns for it - http://www.eriksmilitarysurplus.com/lowairprwasw.html

 

[Jersey4x4]

It is the same as the low air pressure switch, according to the NSN. This switch was attached to the spring brake override air valve through a reducer and 90 degree street elbow. It functioned correctly and made the light on the dash turn on when the spring brake override was engaged.

If you look here - http://www.parttarget.com/5930-01-1...-1.html/-a764634b-4be6-4464-8d90-977537967b0a - The P/N 12375453 and P/N 11669414 point to the same NSN and it is a 65 psi NO switch.

Also, look at Erik's Military Surplus P/Ns for it - http://www.eriksmilitarysurplus.com/lowairprwasw.html

With the help from above I found it. The TM only lists a quantity of 2 of these switches on the truck when in fact 3 are used. The one truck I have the light will operate at well below 60psi, as it will function almost down to zero air in the tank. After finding it in the book, I decided to order a lot of them to replace them in all my trucks. I am tired of chasing electrical gremlins in the trucks from having the wrong parts installed, so I like seeing it in the TM to get the correct part, Not a part that will work. Thanks

 

[Ssgt cornbread]

Would this be the problem if it was pressed in and won't release (pull out)

 

[simp5782]

If it is pushed in then your spring brakes will not apply when the system has air pressure when you pull up on your emergency brake and listen for the "honk". Your spring brakes will apply when the system drops below 40 psi or you drain the tanks. The valve only overrides the spring brake system if the truck has air. . It allows air to pass thru it bypassing the linear valve that is hooked to the emergency brake handle assembly.

 

[Ssgt cornbread]

My kids pressed it the other day now it wont pull out it also will not build any air pressure

 

[Ssgt cornbread]

And the air dryer in the back is psss popping soft air

 

[Ssgt cornbread]

Barely or no air going to tanks

 

[simp5782]

And the air dryer in the back is psss popping soft air

Take a screwdriver and push in on the purge valve on the bottom of the air dryer. Up inside that little dust cover. It can cause a no air pressure building problem if its stuck out. If you need a replacement dash valve just send me your shipping address and I will send you one free of charge. Or you can take the dash valve loose. Remove the charging air line to it. Remove the 3/8" DOT fitting with the 1/8" male pipe fitting on the other side and install a cap on it. This will prevent air from passing thru it. That is a fix if you don't plan on ever using it. I don't run one of those valve on my truck at all.

 

[Ssgt cornbread]

Thank you will try after work

 

[snowtrac nome]

the spring brake over ride is only for testing the parking brakes ability to hold during pmcs.

 

[simp5782]

the spring brake over ride is only for testing the parking brakes ability to hold during pmcs.

Or in the event of a linear valve failure as they do like to fail in the closed position.

 

[Ssgt cornbread]

Thank you, any suggestions on whyits not bulding air i checked the dryer its not open.