M939 8.3 cta compressor failures, need function info

On a recent trip I suddenly lost air pressure. Checked for leaks and found none. Following the tm I checked the line coming off of the top of the compressor and it does not have any air coming out. The tm says to contact a supervisor. Lol

My questions are, if the compressor is bad wouldn't I have signs of oil in the brake lines?

Is there a clutch mechanism that operates the compressor or is it not always pumping?

Any help on what may cause this condition besides the compressor being shot would be helpfull.

If compressor is shot is there any other possible damage to look for or just replace or rebuild compressor?

I have spent hours with the search function, so it is either useless or I am incapable of properly searching. I have the tm's. I have read the tm's.

Surely I'm not the only one this has happened to.

Thanks,

huntler

Some of the guys here say its usually just the head gasket or cylinder gasket that goes bad. They pump all the time.

Someone posted these cummins numbers for the gaskets 4920062 , 154018

If the gaskets don't fix it and it is grenaded ....

http://www.napaonline.com/Catalog/C...mins-Holset-Haldex/_/R-MBI3558006X_0214435723

$339. exchange

Thanks for the replys guys. So if it pumps all of the time then I would think when it fails it would or could contaminate the brake lines with oil. I say this because it's oiled by the engine oil. I don't seem to have any contamination and engine oil isn't low. I should mention I drove it with air supplied from another M931 being towed. Drove it like this for 400 miles so I would have expected oil to find its way into the air lines if compressor failed. I'll count my self lucky that I don't.

I will take it off and dig into it. Just wasn't sure if it pumped continuesly or had some kind of clutch.

Spray some silicone lube in that fitting and in the intake of the compressor before you try taking it apart the valve on top under the round part is bad about sticking and will cause these symptoms if that doesn't fix it then take the head apart and check it. It can be disassembled without removing the injection pump but it is tight.

The tm says to contact a supervisor. Lol

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Guessing you are referring to a -10 TM.

The higher levels of TM should cover diagnosis and repair.

Many of the M939 series trucks have had sticking expello valves(air drier).

The compressor piston pumps all the time. There is a "unloader valve" built into the pump and there is a governor that regulate the unloader valve. Then there are several sub systems in the air system.

My truck did the same thing a while back, 2012 rebuild truck with super low miles
Lost all pressure on a trip. It was the 'unloader valve' in the compressor head.
It did not contaminate the brakes..

I followed the diagnosis in the field maintenance manual which literally says if this condition exists notify your supervisor.

Great info. Im betting on the unloader being stuck. I'll post what I find.

This is also a 2012 rebuild truck.

Thanks

I would guess it might be a faulty air governor. Try replacing that before tearing apart the compressor. You might find that there could be some dirty junk stuck inside the governor.

Hi I had the same happen to me it was the coil spring on the unloaded valve in the compressor cylinder head broke in half.

FASTNOVA said:

I would guess it might be a faulty air governor. Try replacing that before tearing apart the compressor. You might find that there could be some dirty junk stuck inside the governor.

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Can't be the governor, no air going to it.

I know this thread is six months old, but I just experienced this exact issue. The truck built air fine during the pretrip inspection, but lost air during driving and would not build air again. Thanks to ARYankee's advice in another thread, I checked my air dryer filter and it was loose.

I hope this tip can help save someone else time and headache in the future.

It's not like a bad purge valve where you can hear the air expelling. You can't hear anything with the engine running and it doesn't leak with the truck off.

Mikey

mikey said:

I know this thread is six months old, but I just experienced this exact issue. The truck built air fine during the pretrip inspection, but lost air during driving and would not build air again. Thanks to ARYankee's advice in another thread, I checked my air dryer filter and it was loose.

I hope this tip can help save someone else time and headache in the future.

It's not like a bad purge valve where you can hear the air expelling. You can't hear anything with the engine running and it doesn't leak with the truck off.

Mikey

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So how did you fix it?

I tightened it

Same exact thing happened to me (M931A2) and a friend said it was mostly due to cold fluctuations in winter. It says to tighten by hand and only hand tighten it. Yep that worked for about 100 miles. Tightened again and got 250 miles. Got ticked and used a strap wrench I had in my tool box for taking off oil filters. Gave it a bit more than hand tightened. Problem fixed. Unloader on mine is also sticky. Need to clean/oil it again.

This was a type of failure I experienced, the compressor would not build any pressure over 30 PSI the washer / plate that controls the unloader part of the compressor split in half.

Sent from my SM-G930P using Tapatalk

Csm Davis said:

Can't be the governor, no air going to it.

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The purpose of the governor it tells the compressor when to turn on and off. If it is clogged or it is faulty it will not pump. Ask me how I know.