No air in the wet tank

[LMTV89]

On my 1998 lmtv I was troubleshooting why the cab air ride doesn't inflate and figured out there's no air in the wet tank. I'm looking at the lines and trying to figure out how it's possible that the two big tanks would have full air pressure but the smaller tank when I open the valve no air comes out so I'm thinking maybe there's no air in it and that's why the cab air ride doesn't work. Any ideas?

 

[Ronmar]

I would say your drain valve is plugged/deffective as what you describe simply is not possible. Set it to the open position and probe the valve port, its a ball valve so you sh-uld be able to push a wire right thru into the wet tank… Is your fan disengaging?

the street T fitting at the to- of your pic(circled in red). Air comes down from the dryer and branches to feed the wet tank and service tanks simultaneously. The other line is to the governor on the compressor.

this is actually plumbed wrong(all the A0 trucks are). The wet tank is supposed to be a stage of filtering. Like pictured, it only ever sees ~20% of the air when it should see 100% of it. Its a simple fix, swap the line to the service tanks with the sense line going to the governor. Then 100% of the air will pass thru the wet tank as it should.

 

[Ronmar]

Here are before and afters of my wet tank plumbing… swap 2 fittings and hoses and 100% of the air now goes thru the wet tank like it is supposed to…

 

[LMTV89]

Looking at the plumbing I didn't think it would be possible either but I just put a wire through the ball valve in it went all the way into the tank.

I'll probably alter mine the same way yours is cuz that makes sense but I'd like to figure out what the issue is first but it's not making sense to me that I have no air in the wet tank at all

 

[Ronmar]

Remove the T fitting on the back end of the fank and confirm the port that feeds the wet tank is not plugged.

air flows from the compressor to the dryer, then from the top center dryer output down to that T on the back of the wet tank… unless someone has altered this plumbing pathway, I do not see how y0u do not have any air in the wet tank. Besides feeding the cab suspension control it also feeds the cab for CTIS and the pressure to release/freewheel the engine cooling fan clutch… When you turn off the ignition switch, do you hear a click and a small woosh under the passenger dash from the fan solenoid releasing the fan air?


can you feed in shop air to the front red glad-hand(on level ground and wheels chocked as this will release the parking brakes).

what pressure are the service brake gauges showing in the dash when the engine is running? Plumbed normally, the governor cycles the compressor and regulates system pressure based on the pressure in the wet tank…

 

[LMTV89]

Update if I hook shop air to the front right glad hand it gives air to the wet tank and cab air ride. What does it mean?

Everything else on the truck that runs on air besides the cab air ride and the front glad hands is operating as normal ctis appears to be working brakes working

If the little elbow thing that was circled is just an elbow and doesn't have a check valve in it I don't understand how that's possible.

 

[Ronmar]

Me either, especially ctis as it uses wet tank air out the back side of the tank thru the protection valve(looks like a regulator)… That second line off the wet tank is the pressure sense line. how is the governor cutting off at 125PSI if the wet tank is not reaching 125 PSI?

 

[LMTV89]

Something must be plumbed weird on my truck. The cab air ride used to work so it's got to be some sort of check valve somewhere. But where...

 

[Ronmar]

The only checks are at the outlet of the dryer and the inlets to the service tanks… you can see them in the photo you shared, or in the photos I shared of the re-plumb… the one on the dryer outlet is built into the top cover of the dryer… Unless the compressor output and sense lines have been plumbed somewhere else, I don’t see the governor regulating to 125PSI, because it does this by monitoring the pressure on that other port on the inboard end of the wet tank.

you never said what your pressures were running at On the dash gauges?

there are 3 lines coming off the compressor. A large one from the middle of the head to the dryer input port. A small one that connects from the governor on top of the compressor to the wet tank via a t that also feeds the front red gladhand. That is the pressure sense line… The 3rd small line runs from the governor to the bottom of the air dryer. When the governor senses 125PSI on the other small line, it unloads the compressor(stops sending air down the large line) and it sends sensed air pressure(125PSI) down the line to the purge port on the bottom of the dryer. This causes the dryer to purge the collected moisture in the dryer out into the atmosphere…

so when you put air in from the front red gladhand, you can get air out the tank drain valve?

 

[LMTV89]

I may have took us down a rabbit hole here... Is it normal for the wet tank to be empty and the service tanks still have air with the engine off?

I was assuming all three tanks should be equal but I found that the wet tank fills with air as long as the truck is running. Maybe I have an air leak somewhere that's draining down the wet tank.

The wet tank did fill when I hooked up shop air to the front red glad hand. Air pressure front and rear brakes are both at about 120. I can hear the compressor kicking off when they hit 120 and something purges air.

 

[Ronmar]

Yes, thats normal. The service tanks have a check valve on their input from the wet tank. This isolates them from the wet tank and each other(fed motor vehicle code)… the accessories fed from the wet tank do so via the air brake protection valve(looks like a regulator) which only opens to allow air to the accessories when the tank is above ~85PSI, saving at least that much for the brakes in case an accessory system springs a leak and tries to drain the wet tank…

chock the wheels, charge the system to 125PSI with shop air via the front red glad-hand and look for leaks(listen first, feel then bubblemix) way easier to hunt down leaks without the caterpillar purring… then use a wood block to push the brake pedal from the,seat(apply brake pressure) and search again on the service lines and brake cannisters…

 

[LMTV89]

Cool thanks for your help at least I learned a bit more about the air system staring at it the past couple hours. ITS CRAZY. I must have a leak somewhere on the accessory side and something else is causing the air ride for the cab not to function.

 

[LMTV89]

The only thing I found so far is the dryer makes a noise about every 3 seconds until it runs out of air. Is that normal behavior?

 

[Ronmar]

The only thing I found so far is the dryer makes a noise about every 3 seconds until it runs out of air. Is that normal behavior?

Where is the noise coming from? There is a pressure safety relief on top, those sometimes leak, but air would have to be leaking back thru the check valve under the top center dryer output port… the only time you should see air out of the safety(pull/twist the knurled nozzle) is when the compressor is feeding air…

another thing that often makes noise is the pressure protection valve on the rear side of the wet tank. As the pressure leaks thru it toward the accessory downstream that is leaking. As the valve hits the 85PSI cutoff point, it sometimes squeals/squeaks as it leaks air past the diaphragm seal and into the accessory plumbing… since it sets down between the dryer and the tanks/battery box, it can be hard to conclude exactly what is making the noise. Get a mechanics stethescope

 

[LMTV89]

The sound the dryer makes sounds like air getting pushed through a tight rubber flap, more of a groan sound. I discovered if I release air from the tank it groans in proportion so I think the groan every few seconds is the sound of my mystery air leak leaking.

New problem- ever since I put shop air to the front now the glad hand is leaking air with the parking brake off- did some research and it sounds like either the anti compounding valve or a check valve is stuck- I see 2 things behind the bumper, which one should I be looking to replace?

 

[Ronmar]

The first pic looks like the front brake quick release valve. The second pic is the check valve that feeds back to the wet tank to charge the brake,system when towed.

from th8e front red gladnand the line splits. One line goes thru that check to the wet tank, the other line goes thru the top 2way checkvalve under the drivers floor. That 2way has 2 inputs, one from the front red gladhand and one from the park control valve on the dash. Air applied to either port will feed thru to the 2way output port and on back to the anti compound to release the park brakes there us a pressure switch on the output of the top 2way, that controls the park/emer dash lights, turning them off when park air is applied/brakes are released… if the front red gladhand is venting air when park air is applied/brakes released, it is the top 2way check valve leaking past between the two inputs…

watch my front glad-hand delete video(youtube: username Rronmar) i explain those valves and what they do in that video…

 

[LMTV89]

watch my front glad-hand delete video(youtube: username Rronmar) i explain those valves and what they do in that video…

I watched that 2x while I was researching- did not realize that was you! Nice job! I was thinking about just deleting but it seems like the delete keeps the check valve that may be weak and you cant cap it or the ebrake stops working. I say "weak" because after sitting its not leaking anymore so I am debating whether I want to mess with it at all if its behaving. Seems like pushing the air in from the front got it stuck and now its moved back to where it should be. Im gonna start it every couple of days and see if it messes up again. If I understand correctly the hands in the front are an input and the hands on the back are output- is that correct?

Re the noise coming from the dryer- I am assuming it shouldn't be groaning- its the older style that looks like a big round heatsink, thinking about this... good idea?

M939 Series MILITARY TRUCK 24V HALDEX ABS AIR DRYER UPGRADE KIT

Upgrade your M939 Series Military Truck with this 24V Haldex ABS Air Dryer Upgrade Kit. Remove moisture and contaminants for cleaner air and better performance. Easy installation with basic tools.

midwestmilitaryequipment.com

 

[Ronmar]

Yes the fronts are inputs and must be vented so they don’t store air in the line and act like an input. IE you step on the brakes and they won’t release as the stored air in the line to the blue gladhand keeps brake pressure applied untill it leaks away, or y0u go to set the park brakes and they don’t set as the trapped air via the two way checkvalve keeps park air applied… the 2way checks leaking are the root cause of these issues and the reason the front GH’s must be vented…

the rears must be sealed as they will output air to a towed vehicle/trailer when park/trailer air is applied or the brake pedal is depressed…

there is nothing wrong with the old style dryer, it has a lot if very good design features, and it can be serviced by you without any special tools or parts, unless y0u need orings(i did a video on this)… the new style uses a cartridge so it is easier to service, which is the only thing going for it… A cartridge that you must purchase and change every year… in the interest of making it smaller, they sacrificed desiccant volume and cooling surface area… i have not seen an old style unit fail, even with a bunch of oil being sent thru it, they still seem to blow out a lot of moisture… i have seen 2 cartridge types fail from neglect and deposit filter media into the air system when their cartridges ruptured… I had someone give me a pair of the old style units. I put them together in parallel on my shop air compressor to dry the air… they work great!