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M939 Inconsistent air pressure...

quickfarms

Well-known member
3,495
25
48
Location
Orange Junction, CA
If you do not know what a caging bolt looks like or what the COLA test is you do not have the necessary training and knowledge to safely operate this vehicle.

You need to learn about this class of vehicle and how to do a pretrip inspection before you operate it.

These vehicles are not like your pickup truck and need to be driven as trucks

This is for both your safety and more importantly the safety of those around you.
 

The HUlk

Member
469
7
18
Location
Cincy, OH
Removed the compressor unloader top and found sticky goo on the valve mating surfaces(see photo) I believe the goo was intermittently causing the valve to stick in the unload position.

After cleaning & lubing with ATF all seems to be well. Time will tell.

Many thanks to those who offered help.



image.jpg
 
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The HUlk

Member
469
7
18
Location
Cincy, OH
Still having air issues, now of another sort.


I took it for a few drives to get it up to operating temperature and the air performance was excellent. We were thinking unsticking the unloader would be the end of our air problems. Everything was working normal. Blowing off at 120+, maintaining much better pressure. When on the brakes heavy, the secondary would remain higher than the primary.


Then I take it for a quick drive today and the pressure built well at idle. After I leave I'm watching the gauges, and it builds to 150 on the secondary and 130+ on the primary before blowing off. I pump the brakes to get them both back down under 120, and they build back to the same 150 & 130. During the previous drives it blew off at around 120-130 and both tanks would read the same at that time.


Also the wipers stopped working, first time. Before, the right one would always actuate one cycle and park when the air built to around 60. Now it is hanging down and neither will function. When the switches are turned on there is no air sound like there used to be.


I also discovered the round hand adjustment valve on the bottom forward end of the wet tank is seeping air. It's so little that I had to use soapy water to find it, but I could hear it when I shut it down. The air is coming from under the round adjustment handle.


Now I'm wondering...


1. What changed with the air pressure building higher than it did? Is 150 on the secondary too high? I don't want to run over pressure and blow something out in the air system. Would the unloader still sticking/failing cause this? The air dryer and governor are both new. The seller of the governor indicates they are preset to 90-120 which seemed accurate before today.


2. I thought I read about an emergency accessory cut off valve that actuates if the air is low. Could this being stuck in the cut off position be the reason for my wipers having no air?


3. Is there a way to tighten the leaky hand valve to stop the seeping air?


Thank you for any guidance.
 

The HUlk

Member
469
7
18
Location
Cincy, OH
PPV valve is stuck.... and your horn doesn't work :)
Thanks for your response 98G, spot on.

I was shocked to realize all three symptoms were from a single cause but it makes perfect sense when looking at the air diagram in hindsight. Stuck PPV starves the governor for air which keeps the unloader making air.

Removed the PPV and it didn't look bad inside the ports where the fittings were, actually it was clean and so were the insides of the fittings. I poured ATF into the fitting ports on the PPV and used a small diameter steel rod in one of the ports to manually actuate the valve against the spring inside to work the oil in there.

Replaced it, started and idled it for a few minutes, the air pumped up to 130 on both tanks and blew off. Hopefully this is the end of my sticky valve problems. Time will tell.
 

The HUlk

Member
469
7
18
Location
Cincy, OH
Great Thread! I just solved my M929 issues by reading it. Where is the PPV at so when I go looking I'm not lost?
My PPV valve is on the bottom, front end of the wet air tank which is the tank the air dryer feeds into. My valve has a round adjustment handle on it, i'm guessing to set the threshold pressure.
 

The HUlk

Member
469
7
18
Location
Cincy, OH
Took it out for a Sunday drive and it's operating perfectly now. I have been draining the air tanks after each use trying to keep condensation blown out.
 

datadawg

New member
253
4
0
Location
Bucks County, PA
This is a great thread, but for the newbies among us, it would be awesome to see some pictures. It would make the information so much easier to process. Thanks for sharing the troubleshooting.
 
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