M939 Won't build air pressure

[dilligaf13]

I picked up a M932A2, from Camp Shelby, and initially there were no problems. Just recently the air pressure dropped to 30psi on the primary tank. Now when I start it up the air pressure builds to 20-30 psi and just holds there. I checked the pressure with a gauge at the air line just before the air dryer and sure enough it showed between 20-30 psi. I replaced the governor on the passengers side fire wall with a unit from another truck and there was no change. Monday I'm going to pick up a new governor and install that just in case.

My question is, aside from the compressor being shot, is there anything else that could be causing low air pressure before it even reaches the air dryer? Are there any check valves between the air compressor and the air dryer that I missed that may be stuck? Thanks.

 

[doghead]

Did you try the TM, troubleshooting in the -20?

 

[dilligaf13]

-20 TM????

I've read the -10,-23 and -24 TMs. Where is the -20 TM??

 

[bigboy44]

I have a problem with my M936 but it would not build pressure at all, my son crawled under neath and did something to the air dryer and it started working again. I don't know what he did but something was sticking under the truck. That is not much help but it should give you a starting point.

 

[wheelspinner]

The spitter valve probably had crud stuck in it and he cleared it out. If its stuck open you can get up close and feel/hear it.

 

[dilligaf13]

Yeah I think the purge valve at the bottom of the air dryer is acting up, that's why I tested the pressure before it. It's just weird that it builds right up to 30 psi and then just stops.

 

[wheelspinner]

Its not so much that it stops, as much as it has hit the point it can maintain with whatever the leak is. This is still the most likely cause, second option would be the unloader head in the compressor. Also, you can hear the compressor on these engines and can definately tell when they cut out. Have you tried shop air? One other thought, check all your air drains that they are closed tight.

 

[doghead]

-20 TM????

I've read the -10,-23 and -24 TMs. Where is the -20 TM??

I was not specific because I meant whatever the "20" level manual is for that series of truck's TM. In this case it does end with -24.

 

[doghead]

So, there's nothing in the unit level repair manual, that covers "air pressure does not build"?

 

[wheelspinner]

I was not specific because I meant whatever the "20" level manual is for that series of truck's TM. In this case it does end with -24.

Is that like a "900" series truck?

 

[doghead]

Yeah, sure.

 

[dilligaf13]

Its not so much that it stops, as much as it has hit the point it can maintain with whatever the leak is. This is still the most likely cause, second option would be the unloader head in the compressor. Also, you can hear the compressor on these engines and can definately tell when they cut out. Have you tried shop air? One other thought, check all your air drains that they are closed tight.

I charged the system from a secondary compressor and it held steady for well over 10 minutes so there's no leaks there. After tinkering with it some more I'm leaning heavily towards the compressor being the issue. The TM doesn't do a phenominal job in the trouble shooting portion for this issue. It says to loosen the outlet line to the governor and if there is air coming out than the governor is functioning. Next it says to loosen the main outlet line for the compressor and listen for air escaping and feel the line as it should be hot. I don't know what their definition of hot is but that's a pretty crappy way to tell if the compressor is functioning properly. My outlet is warm and air is escaping at the outlet but only 30 psi so..... I just wanted a more definative way to confirm that the problem is the compressor before I spend a couple hundred dollars only to find out that it wasn't the actually the problem.

 

[twinturb089]

I had the same problem, and it was the head gasket on the air compressor was blown out, no one had one local,so i made one out of gasket material to get me off the road and home, but i stock them now. there are 4 bolts on the top of the air compressor. It takes a little wiggle to get the bolts out as they are pretty long and behind the inj pump.

 

[wheelspinner]

I charged the system from a secondary compressor and it held steady for well over 10 minutes so there's no leaks there. After tinkering with it some more I'm leaning heavily towards the compressor being the issue. The TM doesn't do a phenominal job in the trouble shooting portion for this issue. It says to loosen the outlet line to the governor and if there is air coming out than the governor is functioning. Next it says to loosen the main outlet line for the compressor and listen for air escaping and feel the line as it should be hot. I don't know what their definition of hot is but that's a pretty crappy way to tell if the compressor is functioning properly. My outlet is warm and air is escaping at the outlet but only 30 psi so..... I just wanted a more definative way to confirm that the problem is the compressor before I spend a couple hundred dollars only to find out that it wasn't the actually the problem.

Well the only hole with that theory is that the Air Dryer is excluded from the outside air supply system by a one-way check valve at the wet tank. The emergency air supply feed goes directly into the wet tank. This still hasn't tested the air dryer/spitter valve. Although a head gasket, or unloader is entirely possible (unloader if it seems to shut off making air at 30) you should investigate this part further.

 

[M35A2 KAISER]

Well the only hole with that theory is that the Air Dryer is excluded from the outside air supply system by a one-way check valve at the wet tank. The emergency air supply feed goes directly into the wet tank. This still hasn't tested the air dryer/spitter valve. Although a head gasket, or unloader is entirely possible (unloader if it seems to shut off making air at 30) you should investigate this part further.

I second this thought check the spitter valve first. On a recovery of a 929 we had the same problemswith air. When we first started the truck air pressure built fine then after leaving gov liq yard pressure droped to about 25 psi. We ended up loading our gas compressor in the back of the truck idiling with the hose hooked to the rear emergency hookup. This allowed us to get home since the air was after the checkvalve in the wetank. It turned out to be the airdryer when we got home and fixed the isue.

 

[Artisan]

I wonder if you could make an adapter at the compressor so
you can charge the system, less the compressor, w/ shop air.

If all goes well I would say the compressor is the culprit ehh?

 

[dilligaf13]

I just charge the air system, off the emergency air line, using an "air pig" or the shop system and check for leaks. Turns out it was the compressor that was the issue. I sucked it up and bought this remanufactured compressor from NAPA. With the new compressor installed the outlet air line is about 300 degrees F versus the previous compressor's 150 degrees.

http://www.napaonline.com/Catalog/C...mins-Holset-Haldex/_/R-MBI3558006X_0214435721

 

[twinturb089]

Did you remove the 4 bolts that hold the top plate onto the compressor and check the 2 gaskets?you probably only needed the 4 dollar gasket.... i have done a few that did not build air and that was always the problem

 

[ARYankee]

Thanks for letting us know the outcome. I'm glad you got it fixed. It was a really nice truck. I may try and attempt that gasket first since mine has some of the same symptoms. Twin, you don't happen to have some part numbers for those gaskets?

 

[bigboy44]

Thanks for letting us know the outcome. I'm glad you got it fixed. It was a really nice truck. I may try and attempt that gasket first since mine has some of the same symptoms. Twin, you don't happen to have some part numbers for those gaskets?

ARYankee, I bought some gasket material from NAPA and made mine. Was easy and works great!