M1078A1P2 CTIS Problem

[SailBoat]

The CTIS is constantly pumping air to the front passenger tire, then when it gets to where it needs to be it’ll start exhausting out of the quick release valve. Then goes back to pumping air to the tire and repeats.

Thought it was the quick release valve so I changed it out with a new. Didn’t solve the problem.

Loosen the banjo bolt and can hear air coming out when it starts exhausting. But when I do it, it drops enough to close the wheel valve I believe and stops. So changed the wheel valve out 3 times and still have the same problem.

changed the tire to see if that would do anything to no avail.

undid the hub full plug to see if I could hear air coming out and it’s not. Also plugged the stud where the wheel valve hooks to and my hwy light stays solid and acts fine.

Could really use the help been on this problem of the truck for about two days now

 

[GeneralDisorder]

Rebuild the PCU. It's probably filthy and sticking. Unless the PCU commands the system by sending air to the QRV's it won't dump air. Sounds like it's not closing properly and it's sending air pressure to the QRV's and causing it to dump and then trying to re-inflate and the cycle repeats. The PCU spool valves are usually pretty nasty unless the air system has been meticulously maintained.

Additionally you may have other issues since the system can't just send air or dump air from a single tire. It treats all the tires as one big tire and should be inflating and deflating them all in unison.

 

[SailBoat]

Rebuild the PCU. It's probably filthy and sticking. Unless the PCU commands the system by sending air to the QRV's it won't dump air. Sounds like it's not closing properly and it's sending air pressure to the QRV's and causing it to dump and then trying to re-inflate and the cycle repeats. The PCU spool valves are usually pretty nasty unless the air system has been meticulously maintained.

Additionally you may have other issues since the system can't just send air or dump air from a single tire. It treats all the tires as one big tire and should be inflating and deflating them all in unison.

I will try to see if they’ll let me rebuild it if not I’ll get one off another truck that is working.

And I agree with you on that but the other 3 are sitting at 83-85 and is constantly going up and down which got me attaching my head

 

[Ronmar]

Please clarify what "constantly pumping air to the front passenger tire" means.
CTIS PCU has a single output that feeds all tires simultaneously, via a dump vslve for each axle. The dump valve feeds both tires on its axle equally...

 

[SailBoat]

Please clarify what "constantly pumping air to the front passenger tire" means.
CTIS PCU has a single output that feeds all tires simultaneously, via a dump vslve for each axle. The dump valve feeds both tires on its axle equally...

Sorry about that, it’ll pump the tire up to 83 then it’ll exhaust at the dump valve and the tire will go down to about 60. Then it’ll stop, then pump the tire back up to 83 and repeat.

 

[Ronmar]

Ok. I think i understand. Anytime the truck side of the wheel valve has more than ~5PSI on it, the wheel valve is open. When CTIS finishes a cycle, it vents to atmosphere and the presdure drop below 5 at the wheel valves closes them. This sounds like you have a restriction on that one wheel, and the tire is able to supply air faster than the restriction will pass it, keeping tgecwheel valve open and deflating just that tire.

First confirm the banjo bolt is a banjo bolt and its passages are clear. This has caused issues for a few with a non banjo bolt installed... it will pass enough pressure to open the wheel valve, but not enough to let it close and seal...

Next, while that tire is deflating by itself, remove the banjo bolt and the deflate should stop. If it does, and i suspect it will since you swapped wheel valves, This would indicate a restriction between the banjo and the front axle dump valve up on the right side of the transmission...

Isnt CTIS fun...

 

[SailBoat]

Ok. I think i understand. Anytime the truck side of the wheel valve has more than ~5PSI on it, the wheel valve is open. When CTIS finishes a cycle, it vents to atmosphere and the presdure drop below 5 at the wheel valves closes them. This sounds like you have a restriction on that one wheel, and the tire is able to supply air faster than the restriction will pass it, keeping tgecwheel valve open and deflating just that tire.

First confirm the banjo bolt is a banjo bolt and its passages are clear. This has caused issues for a few with a non banjo bolt installed... it will pass enough pressure to open the wheel valve, but not enough to let it close and seal...

Next, while that tire is deflating by itself, remove the banjo bolt and the deflate should stop. If it does, and i suspect it will since you swapped wheel valves, This would indicate a restriction between the banjo and the front axle dump valve up on the right side of the transmission...

Isnt CTIS fun...

Thank you the banjo is clear and and it does stop as soon as I loosen the banjo bolt. I will check to see if the lines are restricted when I get to work this morning. Will let you know what I find.

 

[SailBoat]

So I took the banjo out and took the line loose from where it goes into the frame and also from where it goes to the dump valve just to try and narrow it down. Blew air through both lines but they don’t sound like there’s any restriction in the lines to me. Could the restriction be in the new dump valve or further back in the lines?

 

[SailBoat]

Also forgot to mention I did take apart pcu and saw that one of the metal circles around the brass colored circle was sticking up a little bit but was able to push it back down. Also the cap that covers the control solenoid was a little loose so I tightened it but it didn’t change anything

 

[Ronmar]

If you disconnect the line to that wheel AT the dump valve beside the transmission, you should be able to blow air into the tire thru the hoses and hub just like CTIS does using a blow gun. when you remove the blowgun the line should vent briefly as the pressure drop closes the wheel valve… if it does not, then something, probably in/thru the hub is restricting the flow…

 

[SailBoat]

If you disconnect the line to that wheel AT the dump valve beside the transmission, you should be able to blow air into the tire thru the hoses and hub just like CTIS does using a blow gun. when you remove the blowgun the line should vent briefly as the pressure drop closes the wheel valve… if it does not, then something, probably in/thru the hub is restricting the flow…

I believe I have found it the fitting that runs the air into the hub before the seals has some sort of brass filter looking thing in it I took it out and ran it and it’s working fine and passing the pressure check hopefully it’ll hold strong for the rest of today before i say that’s the only problem

thank y’all for all the advice on it

 

[Ronmar]

Wow, a syntered bronze filter... thats weird and should not be there. the only filter in ctis should be between the wheel valve and the tire. This system relies on proper flow to allow the pressure changes that control/close the wheel valves promptly... that filter should not be there or in the brake system...

Good find!

 

[Mullaney]

I believe I have found it the fitting that runs the air into the hub before the seals has some sort of brass filter looking thing in it I took it out and ran it and it’s working fine and passing the pressure check hopefully it’ll hold strong for the rest of today before i say that’s the only problem

thank y’all for all the advice on it

.
Those bronze filters can generally be washed out.
Hang on to it - just in case the others have them as well.

.

 

[GeneralDisorder]

Well he is working on an A1P2 truck and they do make running changes to these things. Probably something Oshkosh added to try to save the CTIS hub seals would be my guess. I know my right front CTIS hub seal was leaking gear oil into the CTIS and judging by the rust and particulate in the PCU - that's the type of debris that likely caused the problem.

 

[Ronmar]

Well he is working on an A1P2 truck and they do make running changes to these things. Probably something Oshkosh added to try to save the CTIS hub seals would be my guess. I know my right front CTIS hub seal was leaking gear oil into the CTIS and judging by the rust and particulate in the PCU - that's the type of debris that likely caused the problem.

the dump valves do not feed back very much air toward the PCU. 99+% of the air coming from the tire/hub would exit thru the dump vent... I don't see that little bit of flow carrying much from dump to PCU... I don't think CTIS can work well with anything in the line as it would have to be able to vent the air faster than the tire can supply it to get the wheel valves to close, just like this problem...

 

[ckouba]

I put sintered filters in my wheels at the end of the plumbing delivering air and have noticed no adverse effects. The filter dia is significantly larger than the line dia, but the filter media will reduce the effective area. My experience has shown it to work fine but YMMV.

Edit- found a pic:

Filters are sourced off Amazon. In addition to all the work to break down and reassemble the tires, I had to drill out and tap the OE brass wheel feed line to install them. Lots of work but worth it so far.

 

[Ronmar]

I put sintered filters in my wheels at the end of the plumbing delivering air and have noticed no adverse effects. The filter dia is significantly larger than the line dia, but the filter media will reduce the effective area. My experience has shown it to work fine but YMMV.

Edit- found a pic:

Filters are sourced off Amazon. In addition to all the work to break down and reassemble the tires, I had to drill out and tap the OE brass wheel feed line to install them. Lots of work but worth it so far.

Where/how exactly is this installed? Is that the fitting on the end of the stem feeding air into the wheel? If so that wont be an issue. There should already be a filter at the other end of that line where it attaches to the small port on the wheel valve.

the issue, like the OPs problem is a restriction between the wheel valve, thru the hub to the dump valve. A restriction here, like the filter he found, can keep the air from flowing faster than the tire can replace it, holding the wheel valve above its opening pressure and continuing deflating the tire after the system has finished and vented the PCU.

you can get the same problem with all 4 tires when the PCU vent line thru the floor becomes the nursery fo mason bees or mud daubers. This restriction keeps the system dumping, becuse the little bit of air fed back from the dump valves is still enough to over flow the restriction.

i have helped a surprising number of people with this issue. CTIS runs a cycle and deflates ALL the tires…

 

[ckouba]

Is that the fitting on the end of the stem feeding air into the wheel?

Yes. It's the last thing in line on the way into the tire/first thing to run into when air is venting.

M1088 camper conversion

Making progress on a few fronts. Assembled the two tilting pedestals, recording this fascinating video for you. Do yourself a favor, watch it at 2x speed: Also now trying out tire balance beads, but don't want to see them potentially sucked up the CTIS line. So I installed sintered brass...

www.steelsoldiers.com

 

[Ronmar]

Yes. It's the last thing in line on the way into the tire/first thing to run into when air is venting.

M1088 camper conversion

Making progress on a few fronts. Assembled the two tilting pedestals, recording this fascinating video for you. Do yourself a favor, watch it at 2x speed: Also now trying out tire balance beads, but don't want to see them potentially sucked up the CTIS line. So I installed sintered brass...

www.steelsoldiers.com

Ok, not a problem there, or even on the supply line to CTIS from the wet tank. that might slow the speed it can fill the tires though. You just cant put any restrictions between the wheel valves and where the PCU vents thru the floor, otherwise it might not stop dumping when it is commanded to…