PTO spring brake override vs. spring brake override on an M936a1

[gstirling]

Experts on Wrecker operation (M936A1)
Have been going over the TM's, and realized i don't know the difference between the spring brake override switch (the one on the far right of the pnl) and the PTO spring brake override switch (located on the right side and just under the steering wheel). I know and understand the spring brake override switch and what it does and check it when starting the truck etc.. but have never used or checked the PTO spring brake override switch. Not sure what is different about the PTO spring brake override switch and when i would use it instead of the "normal" spring brake override? if given the name, they both override the spring brakes. The TM (at least the sections i've found doesn't give any use or reason for the PTO switch, just a description). so what is different in there operation or affect? and when would i want to use the PTO one instead of the "normal" override? (i realize the PTO is push and hold and normal one stays overridden when pushed till pulled back out). and i have never tried either when the wrecker PTO is engaged.
thanks

 

[M35A2-AZ]

OK here is what I have found out. When you are using the PTO for the wrecker bed use you can push and hold the PTO spring brake override and the brakes will release. Like you can pull your self out with the rear winch. (Will take two soldiers) When you engage the PTO for the wrecker bed the brakes are put on.
Now for the spring brake override on the right side, I have not found a good use for that. Could be for if some thing gets damaged.

 

[gstirling]

sounds right, the one on the right (i call it the "normal") is the one for primary and secondary air failure, once the spring brakes engage, you hit it to unlock them so you can be towed is what i thought it is for. so it sounds like they do exactly the same thing? i was thinking with the PTO engaged maybe the "normal" spring brake override would not work, hence the other one....??? the normal spring brake override is the one you use during your CLD test to show the parking brake is working.

 

[silverstate55]

The “normal” spring brake over-ride is for if you are operating the truck and suffer a loss of air pressure, you can use this to keep the spring brakes from activating (from loss of air pressure) in order to get the truck off the highway to somewhere safe.

 

[M35A2-AZ]

I think for the CLD test I would use the parking brake down by the driver seat, not the override.

 

[gstirling]

agreed, so why two switches, if both override the spring brake? the normal one for loss of air (an only a limited number of uses or time till the Emergency air-tank is empty), one when you need to let the brakes off when recovering, but why not just use the "normal" then too? does the one under the stearing wheel use primary/secondary air to release, where as the "normal" uses Emergency tank air? could that be the difference???

 

[snowtrac nome]

ps magazine put out a deal on the spring brake override the only use for it is to check the mechanical parking brake during pmcs. if the truck suffers a los of air the air control knob for the spring brake override isn't going to do anything.

 

[Csm Davis]

Yes! Finally a wrecker question worth digging into, that I don't know the answer to. Also not all wreckers had the brake release under the steering wheel.

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[gstirling]

Davis - i look forward to your research... since i have not tested my theory, i am inclined to think (based on no facts) it has to do with the source of air used to release the spring brake, the one under the steering wheel "might" use 2nd air rather than E-tank air. but that just a guess, i have not run down the air lines either on the truck or in the TMs to prove it. I was just reviewing my knowledge of the controls when it hit me i had no clue and had not read anything - so why the second control?? by the way in the very near future i am going to hook up the lodar remote controls on the rear winch so one person can use it. will post pics as i do it. it has been done by one other SS member some time ago.

 

[simp5782]

From the TM listing the description of major components

TRANSFER CASE POWER TAKEOFF (PTO) - Used on M936/A1/A2 to control the hydraulic pump that delivers hydraulic pressure to the rear winch and crane. A spring brake air dump switch
is installed on the lever when used on these vehicles, which engages the spring brakes. To override this feature during self-recovery operation with the rear winch, the driver must depress and hold the PTO spring brake override air switch on the instrument panel.


The air dump switch would be before the dash override switch for regular spring brakes so it would not matter to push it as air would not get to the spring brake cans via it with the pto engaged
Also

https://www.steelsoldiers.com/showthread.php?t=167527

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[gstirling]

Simp5782 - thanks for the info, ok will go check this out, so if i understand it. with the PTO engaged, if i push the "regular" spring brake override, they should NOT dis-engage. i have to push the spring brake override under the steering wheel while the PTO is engaged to get them to release (hence the name on the button). so it is needed due to the order they plumbed in the auto engage of the spring brakes with the PTO. excellent - love it when i know how it works and understand why it works...LOL

and i like your idea of routing air back around to the service brakes... sneaky...LOL.. you da man!!