M931 Air Brakes (again)

[D6c]

I have an intermittent problem with the trailer air supply on my M931

Here are my symtoms:

When working normally:
I have about 125-130 psi on both air gauges and when I press the trailer valve it locks in like it should and the emergency trailer line has pretty much the same pressure as dash gauges....OK so far.
Service brakes on truck work normally.
*******************
When not working right:
Hard to pin down if all problems are happening together.
Some times the air pressure runs up to just over 150 psi, and then you hear a relief valve under the truck release the pressure. (unloader valve on compressor sticking?)

When trailer air supply button is pressed I get no pressure to emergency line and button won't stay in.

Hand trailer brake doesn't apply air (because button won't stay in)
Service brakes seem to still work normally.
********************

Normally the windshield wipers won't operate until system pressure is all the way up (priority valve somewhere?) but when it's showing the above symtoms the wipers don't operate at all....even when system pressure is 150 psi.


Thought perhaps the Tractor Protection Valve was sticking and not supplying trailer air. I bypassed the Emergency side of the Tractor prot. valve and got full pressure from the line.....thought that was the problem, but when I hooked it back up I couldn't get the trailer button to hold in and supply ANY air and it showed all the above symtoms.

Confusing to say the least. Anyone out there have suggestions about how to proceed in diagnosing the problems?

Would be nice if there was an air system diagram in the manuals but I haven't been able to find one.

 

[porkysplace]

Post # 12 of this thread has a link to diagrams
[h=1]Air Brake system troubleshooting[/h]

 

[D6c]

Note: I pulled the lines off the trailer supply button and I'm not getting any air to the valve. Problem is upstream somewhere.

 

[D6c]

Post # 12 of this thread has a link to diagrams
Air Brake system troubleshooting

Thanks porkysplace,
Downloaded the P2P program.....should help me follow the air circuits.

It's looking like part of the problem is a malfunctioning air governor which should limit the pressure to about 125 psi. The wet reservoir pressure relief valve is popping off at about 150 psi.


Now if I can just find where the governor is mounted.......

 

[Bandit02]

My M931 had an issue where it would build to 150 psi and the blow-off valve would kick in.

No air to accessories (horn, wiper, etc) or governor.

Turns out the priority valve was stuck closed and over pressurizing the system. Replaced with a civvy one that opens at 90 PSI, good to go.

 

[D6c]

My M931 had an issue where it would build to 150 psi and the blow-off valve would kick in.

No air to accessories (horn, wiper, etc) or governor.

Turns out the priority valve was stuck closed and over pressurizing the system. Replaced with a civvy one that opens at 90 PSI, good to go.

Sounds like your problem was almost exactly like mine but in studying the air circuit diagrams and I'm not sure which valve you mean. There are several and none are specifically called a "priority valve".

* Governor valve: 120-127 psi cut out
*Compressor Relief valve: 250 psi (unloader valve on compressor)
*Air Dryer 150 psi relief valve
*Wet reservoir 150 psi relief valve
*Pressure protection valve 65 psi (between reservoir and accessories such as wipers horn etc.)

Seems like it would be the Governor valve, which is supposed to limit system pressure.
Can you tell me more about which valve you replaced and where it was mounted?

 

[98G]

PPV, pressure protection valve.

Over pressuring , with horn, wipers, and other "nonessential" equipment not fed is the PPV for sure, especially if intermittent.

 

[Swamp Donkey]

*Pressure protection valve 65 psi (between reservoir and accessories such as wipers horn etc.)

This one. Mounted on the forward end of the wet tank.

 

[SLOrazorsedge]

Yup, PPV you can purchase at any Hvy Truck parts store. I had the same issue on my M932A2, solved it with a new PPV. I want to say it cost $20.

 

[D6c]

Thanks,
Not sure I understand the circuit exactly but will pull the PPV and see if it's sticking.

$20 is cheap enough even if it's not bad. I'm used to buying Ag tractor parts and truck parts are waaaay cheaper. Something like that from Case/IH or John Deere would probably cost $200.

 

[Bandit02]

Yes, sorry it was the PPV on mine, it opens at 90 psi for accessories, couldn't find one that opens at 65 psi or whatever the stock one was.

 

[Swamp Donkey]

The PPV ensures that the brake side of the air system receives priority over the accessories. It only opens after the brake side has reached 65 psi, allowing air to the accessory side.

 

[98G]

Take it apart, clean it and reassemble... trivial and free.

 

[D6c]

Took PPV valve apart in attempt to clean it....glass beaded the parts, but springs were very rusty. Small spring broken in a couple of places. Couldn't get it to relieve pressure so I'll be replacing it.
With it stuck it might explain why the windshield washer air motor in the reservoir has the bottom plug blown out.....over pressure.

Still doesn't explain why the system pressure was so high and no pressure to trailer valve. I'm thinking I have 2 or 3 problems. Wouldn't surprise me if they're bad too....worst thing for a truck is for it to sit for long periods and military trucks do that a lot.

I plan on removing the truck protection valve and the compressor governor to see how cruddy their guts are.

 

[Swamp Donkey]

Still doesn't explain why the system pressure was so high and no pressure to trailer valve. I'm thinking I have 2 or 3 problems. Wouldn't surprise me if they're bad too....worst thing for a truck is for it to sit for long periods and military trucks do that a lot.

The governor is signaled from the accessory side of the system. If the PPV fails to open then the govenor won't be signaled to cut out the compressor. This results in the compressor continuously running over the cut out pressure.

 

[D6c]

The governor is signaled from the accessory side of the system. If the PPV fails to open then the govenor won't be signaled to cut out the compressor. This results in the compressor continuously running over the cut out pressure.

That might explain the high pressure.....



Removed the tractor protection valve.....didn't look too bad until I noticed the flat seal on the spool was falling apart. It probably isn't the reason I'm not getting pressure to the trailer emergency line as the it's just an open pass through port in the tractor protection valve. At any rate it needs replacing.

 

[D6c]

Success!

Replaced the tractor protection valve (bad spool seal) & the Pressure protection valve (rusted & broken springs) and now everything works like it should.

The main pressure holds around 125/130, the trailer brake button holds in almost instantly, the hand brake operates the trailer brakes. The wipers, windshield washer, horn, front axel lock, etc all work now too.

The only think left to work on are a couple of brake chambers need the diaphragms replaced. The one with the spring brake may get the whole assembly replaced. I'd just use the T-bolt to hold the spring and replace the diaphragm but the aluminum disc that the T-bolt hooks into has one side broken out. Don't think it's worth taking the risk that it will hold. Would hate to have it fly apart about the time I get the clamp unbolted.....could really ruin my day. I can get the entire brake chamber assy. for $179

Got the original surplus replacement valves from Eric's Military & Industrial surplus. Thought the parts were reasonably priced and that way I didn't have to adapt commercial parts to the truck.

Thanks to good advice from this forum it turned out to be a pretty easy job.