M923a2 air compressor mystery

[dawico]

I have a BMY m923a2 5 ton truck with 8.3l Cummins engine. I have only had it less than two weeks. I have read the TMs and searched here and Google for an answer to my problem. Now I am left to ask for help, or atleast push me where to look next.

I believe my air compressor is bad. From my experience with regular air compressors it seems like a reed is sticking or malfunctioning.

If I remove the feed line from my compressor and place my thumb on it all I get is a back and forth type pump. Not just outward pressure like you would expect. This is at idle and about 1200 RPMs.

The TM I read says if it isn't pumping to notify my Supervisor. My Supervisor (wife) really isn't much help honestly.

So, can I pull the head off it and unstick a stuck reed (or whatever part is in there) or is this going to require parts replacement? I thought I read somewhere someone did something similiar.

Is there anything else I need to check? The line coming from the regulator into the top of the compressor has no pressure in it.

The advice I have been given already is to look for a catastrophic leak (none) and maybe something is iced up (was 32° when it happened and 38° when I took the line off the compressor). I don't think icing is the issue but could be wrong there too.

Any advice or help is much appreciated. Thanks in advance.

 

[porkysplace]

Not building air at all ?
It could be the air dryer needing servicing ( common problem ).

 

[Suprman]

Check to see if air is blowing out the bottom of your air dryer. If so insert finger and push up. Sometimes the unloader section of the air dryer gets hung up. Rebuild kits are available for around 40 bucks. You can get the head off without taking the whole compressor off. 95 percent of the time one or the other is your issue. On rare occasion the air governor on the firewall goes bad.

 

[Coffey1]

The outlet off the compressor should be getting hot.
If not then compressor not working

 

[dawico]

Thanks guys.

I climbed underneath and didn't feel or hear air coming from the bottom of the dryer. I didn't push anything up there though and will try that.

The only place I heard any air was by the driver's front tire with the engine at 1200 RPM. It is just too loud to trace anything though and may not even have been air noise. No noise there at idle speed.

The compressor should pump all the time correct? So the feed line should always be pumping while running, correct? Or does the regulator shut that line off when pressure is up? I am not exactly understanding that part of the system honeslty.

 

[Suprman]

The governor tells the compressor unloader to open when it reaches the proper max pressure. If you have nothing chances are the unloader is stuck somehow or you have a broken feed line. Follow the compressor output.

 

[Coffey1]

The line should be getting hot the head of the compressor should be hot yes it runs all the time as far I know it doesn't have a unloader on it .

 

[dawico]

Not building air at all ?
It could be the air dryer needing servicing ( common problem ).

No pressure at all. Zero on both gauges and opening the drain valves produces nothing.

 

[dawico]

I will have it hooked to a compressor tomorrow to limp it back home and will be able to tell if there is a major leak somewhere. I will report back if there is. I don't think there will be though but that will help answer some questions.

 

[dawico]

The outlet off the compressor should be getting hot.
If not then compressor not working

It isn't even warm to the touch. I heard that it should be very hot so I approached it cautiously but it was cold.

 

[m16ty]

The line should be getting hot the head of the compressor should be hot yes it runs all the time as far I know it doesn't have a unloader on it .

The compressor does have a unloader on it. While it is true the compressor turns all the time, it only pumps air when the governor tells it to.

I've had the unloader apart on one of these compressors (it's in the top of the head) but it's been awhile back. All I really remember is I needed a part for it and I picked it up at the Cummins dealer.

 

[dawico]

Ok, here is the update. I hooked up a gas compressor to the rear glad hand and got it home. It pumped up the system just fine with no air leaks on the truck. The trip was about 30 minutes or so.

Air on both gauges read just fine the entire trip and nothing out of the ordinary.

Now the strange thing. Somewhere along the trip the compressor died but my air stayed up. When I got home I went to shut it down but it was already off. Long story short, apparently my compressor fixed itself. I didn't know that was a feature of MVs.

Anyways, I unhooked the compressor and shut the glad hand valve off. I opened the drain valves to release some pressure and the air came right back up.

After a few hours I started the truck and the air came right back up again.

Ok, now, could this be a regulator issue?

Could the temporary compressor have knocked something loose somewhere (build up or gunk in a line)?

Is icing a possibility at 38°? The drains have not produced any moisture to date.

Should I still pull the top off the compressor (not the whole head but the part that the regulator line comes into)?

I am thinking I am going to blow the line out that connects the compressor to the regulator.

Any other ideas or parts I should look at?

Thanks again.

 

[Suprman]

Sounds like the unloader was stuck and then got unstuck. Probably a good idea to rebuild it anyways. Its not that expensive.

 

[Csm Davis]

I would just run it as is for a while and see if it does it again. It is a pain to remove the valve and head, if it happens again remove the line coming from the top of the unloader valve to see if that helps if not a 1/2" wrench will take off the bolts on the unloader valve and you can check it and the o-ring around it.

 

[dawico]

I would just run it as is for a while and see if it does it again. It is a pain to remove the valve and head, if it happens again remove the line coming from the top of the unloader valve to see if that helps if not a 1/2" wrench will take off the bolts on the unloader valve and you can check it and the o-ring around it.

That is kinda my plan for now. I will keep the gas compressor in the bed just in case.

I think if I start pulling stuff apart I will cause more problems than I will fix. It is hard to diagnose a problem that isn't happening and I don't want to trash any gaskets in the process.

If it happens again then of course I will dig into it.

The crazy thing is now the compressor is pumping faster than it did before. The system just needed a little jump start I guess.

Again, thank you guys for the input.

 

[Big Tex]

TIPS ---
I am trying to service my glad handles so they work to allow me to air up the pressure to the tires or using 1 inch drive air impact gun? Any ideas on how to get the pressure to the glad handles?

 

[dawico]

TIPS ---
I am trying to service my glad handles so they work to allow me to air up the pressure to the tires or using 1 inch drive air impact gun? Any ideas on how to get the pressure to the glad handles?

On mine I use the red glad hands and turn on the shut off valve by the hand. Are yours not getting pressure there?

Follow the piping back under the truck and you may have another shut off valve.

I generally use the front drivers side glad hand and it works well.

 

[shadowdd]

I had the exact same thing happen at Christmas time and it ultimately was the unloader on the top of the compressor. I was not able to pull the whole unloader off since it contacted the bottom of the injector pump. I ended up pulling the line from the top of the unloader and loosening both bolts that hold the unloader in the compressor head. I pulled up the unloader as much as I could and saturated the entire unloader with WD-40. I let that sit and repeated the process a couple of times. I then put everything back together and started the truck up and the compressor was filling the tanks once again. Now it seems to fill the air tanks much quicker. I hope this helps.

Bryan

 

[Big Tex]

2 or 3 as I remember work... but those that do only get a little pressure... I really want them to work that way I can use the air impact for tires and various other work if need be in an emergency. Thanks for the tip Ill trace the lines back and look for another valve cutoff... any other ideas?

 

[WillWagner]

The unloader is probably carboned up. High idle hours does this. ATF can de carbon the unloader, it is high detergent. You can try removing the inlet hose and pour a teaspoon of ATF in the hose, let it sit overnight. the ATF should loosen the carbon and free up the unloader discs/valves. The only other thing besides a stuck/carboned unloader that will cause the air to pulse out the discharge is a broken valve spring.