My M923A2 wont build air pressure past 30-40 PSI in Primary/Secondary air tanks??

[Oldworldtenacity]

Ok, So I probably put about a 100 miles on my truck since yesterday and finally my luck ran out and I encountered another issue. Luckily I was close to a good place to park the truck not far from home.

Truck was building air fine for about an hour driving around today and all of a sudden my low air pressure warning buzzer comes on and I only have about 60PSI in both tanks. I had just enough Air to get it parked basically.

To make a long story short, fellow local member Sharkbait was nice enough to come and help troubleshoot the problem. He even had a new Governor and put one on in case that was the issue. It wasn't unfortunately.
Finally we just had to charge the system with air from his truck which was enough to get me back to my place driving slow.

Just put a new Air dryer put on about a month ago and couldn't find anything wrong with it or Expello valve. The Compressor is also working...although cant tell how good though.

Also, there are no hissing or obvious leaks anywhere. The truck just won't build past 30-40 PSI after draining tanks empty...it is also verrrrry slow building to 30-40 PSI as well...then it stops....sometimes makes it to 40..other times just 30. Strange.

Totally puzzled here.

Has anyone else had this problem or know the best way to continue troubleshooting?

 

[rosco]

Its not too hard for a valve to be leaking - a relay valve, quick release, hose fitting, spring pot, etc., and you not hear it, especially if the truck has to be running to build what air you have. Sometimes you have to plumb in another air compressor (with the truck off), and crawl around under it, listening for the leak. Don't forget to look/listen behind the dash too. Good Luck.

 

[Oldworldtenacity]

Its not too hard for a valve to be leaking - a relay valve, quick release, hose fitting, spring pot, etc., and you not hear it, especially if the truck has to be running to build what air you have. Sometimes you have to plumb in another air compressor (with the truck off), and crawl around under it, listening for the leak. Don't forget to look/listen behind the dash too. Good Luck.

Well, we actually did that(hook a compressor to it) with the truck off and can't find a leak. The tanks never lost all air pressure to begin with...as I was driving around and using the brakes the air pressure kept dropping all of a sudden and the truck just refused to build air past 30-40 PSI driving around or parked after that. The tanks will hold the 30-40psi or however much you charge it with so far...just won't build air on its own past those numbers.

 

[Rmtaunton]

There is. Bypass valve on the compressor that bypasses around 120 or so since the compressor has no clutch and runs continuously , that's where I would start

 

[Floridianson]

Yea there was a good thread on the unloader valve on the compressor and an easy way to check it.
http://www.steelsoldiers.com/showthread.php?137127-Unloader-Issues&highlight=unloader+valve

 

[Coffey1]

OK don't know if its the same. Andy's truck on recovery did same thing.
Slow air build up would not build high.
Then all of sudden it started to build air.
Then a few miles down the road the gasket on the compressor let totally go.
We never heard a leak any where.
Check it out really good then look again.

 

[dawico]

Yea there was a good thread on the unloader valve on the compressor and an easy way to check it.
http://www.steelsoldiers.com/showthread.php?137127-Unloader-Issues&highlight=unloader+valve

This. It sounds like an unloader issue to me. I would start there as rebuilding and cleaning it is relatively simple and cheap.

It may need a new spring if it is constantly stopping at the same pressure.

 

[dilligaf13]

Throw a gauge on the outlet line for the compressor and let it run. Also the outlet line should be hot to the touch, if producing sufficient PSI. If the pressure is good there, at a constant rate, then begin troubleshooting down the line. Generally if there's no leaks, and no valves are stuck, then it's an air compressor or governor issue. Good luck.

 

[sed6]

My unloader was giving me fits (wouldn't build air) and I didn't have the time or gumption to tear it apart. I removed the governor line and gave it a nice shot of WD-40 using the straw. I was shocked by the bunch of rusty water was shot back out the fitting! I followed that up with a shot of compressed air. Field expedient, and not a permanent fix but it got it working again (and I haven't had trouble since).

 

[EO2NMCB]

When trouble shooting a air leak, I close the valve to both the primary and secondary from the wet tank. This should allow you to determine which side of the system the leak is on.

 

[Ddmk18]

When mine did that it was the top gasket on the compressor. Not a hard fix just a little time consuming.

 

[Truckoholic]

Not that this has anything to do with your problem, but on my truck, someone ran the air line for the left front, red glad-hand, in a position that allowed it to get smashed between the left rear engine mount and the engine mount support bracket. I found it pinched pretty much completely shut in there. Could have easily pinched it in half, and that would be a massive leak! Or even with it pinched closed, had I ever went to try getting air from the red glad hand on the left front, I would have been wondering why the heck there was no air getting to it. But yeah, I agree with others. If you can't find any leaks, then sounds like an issue with the compressor.

 

[Oldworldtenacity]

Ok, will check the unloader and compressor Gasket seals and report back. Hopefully this will get to the bottom of it. Thanks for all of the suggestions everyone.

 

[Oldworldtenacity]

One thing I should mention is that the main braided Air supply line coming from the compressor on the inside of the frame rail (that runs to the air dryer I believe) was shaking violently in repetition with a loud air clicking type sound when compressor is running. When we disconnected the line it was putting out air, but not as much as what we thought it would in order to build air in the tanks.

 

[Oldworldtenacity]

Ok, so we determined that the air issue is coming from the compressor/unload er valve as I unhooked the main Airline from the unload er and the air was coming out in spurts that you could easily hold your finger over just like back at the Air dryer....so we proceeded to take the unload er valve off.
Got the valve off and the spring was working great as it was pushing the top of the Unload er up with good force during dis assembly...although the cap with the two bolts on top(pictured) did not want to come out as it took about 5 mins prying it out.

The gasket was in good shape too....so not sure where to look next as nothing looked wrong with the unload er valve including anything partially stuck or dirty etc.

Is it possible the compressor is just dysfunctional/weak and needs to be replaced?

 

[dawico]

You can pop the head off of it and look at the piston. You have to drain the coolant though. It may just need a new gasket. On mine the unloader body (the four bolts on top hold that and the head on) came right off but the head was stuck tight. I had to drive a chisel in the seam to get it loose. My cylinder still had the hatch marks in it.

You may also want to blow out the line that connects the governor to the unloader.

My whole compressor leaked like a sieve through the gaskets before I rebuilt it. Soapy water in a spray bottle is your friend.

 

[Oldworldtenacity]

You can pop the head off of it and look at the piston. You have to drain the coolant though. It may just need a new gasket. On mine the unloader body (the four bolts on top hold that and the head on) came right off but the head was stuck tight. I had to drive a chisel in the seam to get it loose. My cylinder still had the hatch marks in it.

You may also want to blow out the line that connects the governor to the unloader.

My whole compressor leaked like a sieve through the gaskets before I rebuilt it. Soapy water in a spray bottle is your friend.

Yeah, same thing here with the unloader head being stuck on tight as can be. Also, we blew out the line connecting to the Governor as well. I'm still trying to decide what to do at this point as I am waiting on a mechanic that worked on these trucks in the Army to possibly come and take a look at it.

 

[R Racing]

One thing I should mention is that the main braided Air supply line coming from the compressor on the inside of the frame rail (that runs to the air dryer I believe) was shaking violently in repetition with a loud air clicking type sound when compressor is running. When we disconnected the line it was putting out air, but not as much as what we thought it would in order to build air in the tanks.

My braded line between the engine and frame had a pin hole in it and wouldn't build up past 60 psi. installed a new line and all is good. You couldn't hear mine leaking. But I sprayed some wd40 in the intake to the air compressor it showed me where it was leaking with a big mist .

 

[Coffey1]

Take the head and put some sand paper on flat surface and sand the head to see if it's true.
Could not tell by strait edge on it but sanding sure showed it.

 

[Oldworldtenacity]

So, I ended up getting a new compressor and am going to have another mechanic with me when I go to take it off. For any of you that have changed a compressor before....is there anything in particular I need to look out for or do you need to tighten the bolts to a certain spec when putting the new compressor on.
Never did this before and kind of hesitant about doing it myself.