M923 Air Pressure not building past 45 PSI...ANy IDEAS?

[Lex_Ordo]

Good afternoon everyone.
I am picking up two M923 trucks on Tuesday from Ft Dix. The Disposition Site Manager called me to tell me that one of the trucks, will not release the Spring Brakes because the air pressure will not build higher than 45 PSI.

I asked him if there was a leak, or if a drain cock was left open or some other valve, and he said no.

Hmm???

He told me that he let the truck run a long time, and nothing changed. Now of course he is not a mechanic, and not too familiar with the 5 Tons, but never the less, the truck was moved into the lot it is sitting in recently, so something doesn't add up here.

Since I am not there to diagnose the problem and determine what if any problem exists, I am curious, if anyone else has had a similar issue, or can give me a checklist of where to begin, and through process of elimination determine if in fact there is an air pressure issue, or governor problem.

I am a deuce guy, and new to the 5 Ton. Love the truck, but like the deuce, need to learn my way around the thing.

 

[goldneagle]

Possible air leak at the air dryer under the truck. There are 4 tank drains on the passenger side next to the step. One of them could be leaking or open.

 

[Csm Davis]

Not just possible but probably a leak on the air dryer, many times you can't hear it if it leaks. Also check all of the gladhands some of the valves are backwards from a civilian valve. If it isn't there start at the pump and work back to the tanks and you can shutdown the primary or secondary at the wet tank to get pressure up.

 

[74M35A2]

There is a "wet" tank on those trucks, and they are not allowed to build air pressure until the system reaches 45psi or so. That tank is basically an accessory tank that runs the wiper, horn, and something else I forgot. It is fed by a pressure priority valve, and you may have a big leak in that wet tank system (wiper hose off, etc....).

 

[jatonka]

That pressure priority valve screwed up on my 923 and I lost all dash air including wipers and front wheel drive engagement. I borrowed one and installed it, everything is fine now. I would like to find one NOS if anyone knows where to get one. Good luck Lex Ordo, my thoughts are with you, JT out

 

[74M35A2]

That pressure priority valve screwed up on my 923 and I lost all dash air including wipers and front wheel drive engagement. I borrowed one and installed it, everything is fine now. I would like to find one NOS if anyone knows where to get one. Good luck Lex Ordo, my thoughts are with you, JT out

The PPV is available at any NAPA. Check the sticky at the top of the forum.

 

[wheelspinner]

There is a "wet" tank on those trucks, and they are not allowed to build air pressure until the system reaches 45psi or so. That tank is basically an accessory tank that runs the wiper, horn, and something else I forgot. It is fed by a pressure priority valve, and you may have a big leak in that wet tank system (wiper hose off, etc....).

A "wet" tank is not the accessory air tank, it is the first tank, which is basically a buffer for the system. The PPV which is mounted on the wet tank feeds the PRIMARY/SECONDARY air system first. It releases air to the accessory system at appx 65-70 PSI. An air leak on a wiper hose etc would never cause a pressure max of 45 PSI because the accessories can't get air at that low of a pressure. The PPV if it goes bad cause OVER pressure and the safety valve to blow off at 150 PSI. Like CSM said, check the glad hand valves, and MOST likely it will be the spitter drain on the bottom of the air dryer. They can be taken off and cleaned to free them up.

 

[74M35A2]

A "wet" tank is not the accessory air tank, it is the first tank, which is basically a buffer for the system. The PPV which is mounted on the wet tank feeds the PRIMARY/SECONDARY air system first. It releases air to the accessory system at appx 65-70 PSI. An air leak on a wiper hose etc would never cause a pressure max of 45 PSI because the accessories can't get air at that low of a pressure. The PPV if it goes bad cause OVER pressure and the safety valve to blow off at 150 PSI. Like CSM said, check the glad hand valves, and MOST likely it will be the spitter drain on the bottom of the air dryer. They can be taken off and cleaned to free them up.

See, through my ignorance, we're all now a little smarter. Thanks for the clarification.

 

[bchauvette]

Not just possible but probably a leak on the air dryer, many times you can't hear it if it leaks. Also check all of the gladhands some of the valves are backwards from a civilian valve. If it isn't there start at the pump and work back to the tanks and you can shutdown the primary or secondary at the wet tank to get pressure up.

The red,emergecy "T" handle glad hand valves are backwards the "L" handle valves are not. On the M944. Wonder why.

You can easily check for proper air governor operation. The air Governor sends air pressure to the unload-er to "Shut off" the compressor. Remove the unloaded signal line from the compressor . If you get air pressure coming out before you reach around 120 lbs you air governor is defective.

 

[Suprman]

I have found the unloader spring in the top of the air compressor can get stuck and produce the same symptoms. It is easily rebuilt if thats the case.

 

[bchauvette]

NHC250 compressor primer.

I recently found out you can sort of rent Microsoft Office. It just so happens I've been playing with power point documenting my NHC250 compressor work. This was part of my excessive air pressure in the coolant system issue. It is in movie form.

http://1drv.ms/1tozGjC

You can see how gunked up the inards are.

File must be too big just to upload. The little circly thing keeps going round and round then nothing after I hit upload.

 

[Lex_Ordo]

Well in the end, I could not get anything to work.
I am not a mechanic but I do like to tinker and most of the time can figure out an issue. Not this time.

I canceled this truck, and drove the other back.

The second was a gem. No problems at all. There might be a small fuel leak developing on the IP, but hat is minor.

I appreciate the help, but nothing seemed to work. Air was moving right to the air dryer, but no pressure built. I think there is a solenoid on one of the tanks on the right side, and someone I was with suspected that was the issue, but it was easier to just walk way and request another.

 

[airborneinfantrygrunt75]

Ok so I have an M923 and I have gone through the tech manual. My gauges won't build air pressure at all and I have not found any leaks. The signal air line from the governor has not air running through it but the compressor is sucking air in from the manifold but possibly may not be pulling in enough air is what my buddy told me. could just the unloader spring being stuck prevent any if not most of the air to charge up???

 

[74M35A2]

Push shop air into the system through a drain valve or glad hand, see if there are any system leaks. If not, then go after the compressor/unloader system.

 

[simp5782]

Ok so I have an M923 and I have gone through the tech manual. My gauges won't build air pressure at all and I have not found any leaks. The signal air line from the governor has not air running through it but the compressor is sucking air in from the manifold but possibly may not be pulling in enough air is what my buddy told me. could just the unloader spring being stuck prevent any if not most of the air to charge up???

Take the line off the drivers side air dryer with it running and see if you have air. If not check the line off the rear or the compressor to see if it's pumping out. If not. Bad unloader or head on the compressor just replace it the compressor unit. If you have air off the compressor rear you have a stopped up line.

Sent from my SM-G935P using Tapatalk

 

[airborneinfantrygrunt75]

Well I found the problem and it happened to be the air dryer. So I bought a major kit for the air dryer took it off the truck, broke it all the way down, cleaned it out with carb cleaner some brushes and the good ole fashioned q-tip, and put her all back together again! I had never noticed the blow off on the truck previously when running but man let me tell you it sounds like a new truck!!! Thank you Everyone for the help on here it is much appreciated!