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M939A2 COMPRESSOR UNLOADER Remove/Replace

Andrmorr

Member
274
7
18
Location
Burlington, NC
My compressor is pretty loud... (Ron (73m819) you heard it in Atl, what do you think it might be)...
I want to check my compressor unloader for carbon build-up and am seeking guidance. Forum search revealed several mentions of doing it but no pics and or detailed write ups. It doesn't seem that tough but I have several questions that I think may help other 1st timers who are learning their trucks too. I plan on responding back with pics for later reference.

Is the unloader the circle on the top of the compressor held down by two bolts?

To lift up the unloader for a check, do you need a gasket or is it an O-Ring?

If it does need servicing what will it look like (black & crispy, gummy, etc)? what's the key points to check...

Etc...


Thanks in advance for any pointers...
 
Last edited:

Floridianson

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Steel Soldiers Supporter
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To check a unloader you would put pressure on it and see if the pressure holds. If I rember you disconect the line from the governer to compressor at the compressor. Then useing a one inch ID line made up so it will attach to the compressor. Then in the line will be a pressure gage and a valve to cut off the air from an out side source. Then you add pressure to the get up you made. I see it like a holding tank with a small charge. Then you can see if the unloader is leaking down.
That way you are not takeing anything apart you are just checking for seating and unseating if that makes sence. Hope I discribed it well enough.
 

Andrmorr

Member
274
7
18
Location
Burlington, NC
To check a unloader you would put pressure on it and see if the pressure holds. If I rember you disconect the line from the governer to compressor at the compressor. Then useing a one inch ID line made up so it will attach to the compressor. Then in the line will be a pressure gage and a valve to cut off the air from an out side source. Then you add pressure to the get up you made. I see it like a holding tank with a small charge. Then you can see if the unloader is leaking down.
That way you are not takeing anything apart you are just checking for seating and unseating if that makes sence. Hope I discribed it well enough.
You did great!...I've seen that method before and it should work fine to check operation status...I plan on doing that also but really want to know what condition the compressor is in by doing a visual...my concern is it may work fine today but be on it's last leg from previous use...my truck only had 36 miles on it after the 2010 RRAD rebuild at time of pick up so it's been idle for a while...with the compressor being the lifeblood of these trucks, it's right behind the engine oil/tranny oil in importance...loss of any one of those three means a tow truck, trip interruption, and the mega dollars that goes with it...
 
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Floridianson

Well-known member
Steel Soldiers Supporter
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Location
Interlachen Fl.
I don't know but I took apart a Deuce compresser with 40k miles and the rings looked great and still cross hatch marks on the walls. I have not had any trouble out of my Cummins 250 pumps yet and don't know about the 8.3. Seems like they should be pretty tough just unloader problems maybe. Seems like every time I take something apart I break more stuff. But guess the saying was "If it's tight force it, if it breaks Muck it, it needed replacing anyway".


Was just thinking but it might be easer if one was to test the unloader to get a portable air tank. It allready has the the gage and a good size tank to hold a charge plus some have a valve to open the line when needing air. So one would just have to figure out the correct fittings and line to run the the compressor unloader. Then charge the tank with a charge and see if the unloader will hold it or leaks down.
 
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