M923A2 - Won't build air after 60PSI

[gijoel]

I have been working on and off with an air problem on my M923A2. My truck will not go past 60 PSI on idle and 90 PSI when I have the RPMs at 1800. When the truck is under power (on the road), it will reach 120 PSI. I have replaced the governor, air compressor and air dryer – same issue.

I happened to be at the CP USMC motor pool and was speaking with a mechanic and told him my problem. He suggested I run an outside line to fill the tanks through the gladhand. I did this, and the tanks quickly filled to 60PSI, and at 60PSI I heard an air leak. The air leak was coming from an airline (see attached) in the front of the truck. The line looks like an exhaust port line - I could be wrong. Turning off the air source, the tanks stay at 60PSI, but there is no leak. Only after +60PSI does it blow air from this hose.

 

[98G]

Sounds like you have a pressure protection valve set at 60psi that is supplying air to that open line.

That line is supposed to go somewhere. Not sure where.

Wipers, horn, tcase are all possibilities.

 

[Jbulach]

There should be no exhaust/vent lines up there. Check the size of that tube with a wrench, is it half inch, also it appears to be black? If it’s black and half inch it’s probably goes to the top of the brake QR valve, which is where the other two lines you see in your picture go. The brake QR valve is hanging on a bracket just below the radiator crossmember. Theres also black half inch line that feeds into the top of the CTIS QR valve on top of that crossmember.

 

[Jbulach]

Also are you sure your truck is an A2? What engine, truck year of manufacture, does it have CTIS? Give us an overall picture of the truck taken from the front/drivers corner and we should be able to verify.

 

[gijoel]

It is a M923A21. Had the Cummings 8.3 engine. I had the CTIS removed. I am suspecting we may have damaged this airline when we pulled and had the engine overhauled back in 2022. I use this truck maybe twice a year.

Ill check tonight on the airline placements.

Thanks!

 

[afc2324]

Did you get this issue resolved? Im having a similar issue.

 

[98G]

Did you get this issue resolved? Im having a similar issue.

Pressure protection valve is supposed to open around 70psi, at which point a bunch of stuff downstream gets fed : horn, wipers, tcase, etc.

Hypothesis: something downstream is leaking fast enough to prevent air building past that point.

Suggestion: pressurize the truck with shop air and listen for leaks.

 

[gijoel]

Sorry about the late response.

I use an air compressor to pressurize the trucks air system. At 65/70 PSI, I discovered a leak under the radiator. Probing around, I found a broken hose leaking air. I am in the process of tracing it. A visual inspection of the airline break, it does look like it broke loose. More to come.

 

[gijoel]

Is there a main valve to turn air off to the CTIS system on a M923A2 truck. I stopped the leak and the pressure does go up to 90 psi and just stops. But not leaks. I am thinking the air governer needs to be adjusted as it is new (replaced some 1 year ago).

 

[afc2324]

I finally figured out my M931a2 pressure issue. It wouldn't build past 60 either. I went through the entire air system. I checked everything and most things three times. I removed the top of the air compressor three times to verify and inspect. The last time I took the unloader off I notices what looked like smooth rubbing on the side of the unloader cup looking thing. It was out of round and hanging up. My problem was a stuck unloader. I replaced that one part, polished all the other parts, put it all back to gether and fired it up. It builds pressure like it should and maintains it like it should. I'm so glad that nightmare is over. Thanks for all the assistance. I should have noticed the issue the first time I took it apart but hey.........that was at midnight, in Indianapolis, in a Napa parking lot, as I commuted from Battle Creek, Michigan to Bergheim, Texas.