M923A2 CTIS Problem

Thanks! I'm heading out there with a flashlight. To **** humid and hot in the daytime. Thanks no saying look at the TM's! I have them printed out and in the head (read uninterupted), but that is a long stay on the seat!

My CTIS issue was this:

5 flashing lights and wouldnt inflate front rear tire on passenger side at DRMO. Manually filled to 60psi at the base fuel station, took three complete cycles. Drove 230 miles home to Wyoming and everything held air but no working CTIS. While troubleshooting the system and rebooting (disconnecting barrel connector at ECU), reconnecting then starting I changed to 4 lights. Leak checks all over the place, and I found nothing. My air dryer's bottom wasnt getting hot and outside temps were between -5 to 35F depending. 4 lights should be a leak but I believe it is any pressure error that makes a full loop check. This system is trying to equalize at a certain PSI.

I could get the system to try and work but never deflate, just went to 4 light after a while. I found the small filter/separator that is inline right before the in cab control box to have been Gerry Rigged. The bottom petcock was broken off so a screw + RTV was sealing the petcock hole. $15 later and a C&H unit from Tractor Supply had replaced it. These are generic 150 PSI rated units that mount on home air compressors. The stock one I took out was rated to 230 PIS but my system doesn't get over 130 PSI. Old one did have some dried debris in it so dont skip it!

I read 400+ pages in the TM to include the CTIS portion. While good it's theory of operation is wrong. Here is what I know now:

They system starts and gets to pressure. At that point it sends out a loop test, if that fails 5 lights error. If it passes the loop test but cannot equalize the system to the selected mode, 4 lights error. The TM states many times this is a leak condition, but thats only half of it. The HWY mode wants 55-60PSI my tires were set at 72PSI by DRMO giving noticeable height ( and road bounce!)over the one I filled to spec by hand. The 4 light condition shut the system off since it couldnt equalize to the 55-60PSI it was looking for. I'm sure this is a safety to not keep inflating a tire past safe levels. So in short 4 blinking lights is not always a leak.

What I found was the truck has the CTIS disabled by Maint at some point with valve stems in the T that goes to the tire from the hub mounted valve. I read in the TM that trucks used for heavy loads had to have their CTIS disabled by Maint, and I guess this is how they did it. These valve stems let the system fill but prevented them from deflating via the hub mounted valve.

Solution: Use a 9/16" open end and pull the input line from the T off and unscrew the valve core. Wait for lower PSI from tire and reinstall just past snug. You will need a 3/4 socket and 1 1/16 socket to get the covers off too.

I then had all six tires very low but not on the ground. Rebooted the CTIS and started the truck. The CTIS started and began to fill about 1/2 way and errored out 4 lights. Restarted the truck and hit HWY filled to HWY mode without a hitch. The system failure I believe was since the tires were all that low it got a leak signal and shut off CTIS.

I aired it down and back up 3 times and have let it sit overnight now with no problem. My CTIS is fixed!

Have you guys thought about replacing your air compressor with a 2 stage unit? It will build air much faster than the stock single stage. You can even add another tank to speed up inflation. I also converted to a MRAP dryer.

Hard Head said:

Have you guys thought about replacing your air compressor with a 2 stage unit? It will build air much faster than the stock single stage. You can even add another tank to speed up inflation. I also converted to a MRAP dryer.

Click to expand...

Do you have any info on the two stage compressor?

Wyomer, what part of Wyoming are you at? I'm in western Nebraska and I'm having about the same Ctis problem with my 939 and I thought we might share some ideas on fixing them.

Smckerigan said:

Wyomer, what part of Wyoming are you at? I'm in western Nebraska and I'm having about the same Ctis problem with my 939 and I thought we might share some ideas on fixing them.

Click to expand...

Im in Cody WY, so NW part of the state. I was a 19K DAT before civilian life and when we first got the 5 Tons they were pre CTIS. Germany in the early 90s we still used 2.5T and the 5 Tons were the new rigs. So Im learning CTIS myself, but between the TMs, Dana manuals, and posts here it should be easy to fix.

I'm a Biomedial Engineer by trade these days so this half written, half implied stuff is right up my alley. I'm always up to help out.

Honestly if your system has 4 lights, full tire pressure, tries to work then errors out grab a 9/16" wrench and 3/4" socket, and valve core tool.

Use the 3/4" socket to remove the CTIS wheel valve cover plate and the 9/16" to open the compression fitting from the wheel valve to the wheel. This is the fitting closest to the wheel and going into the wheel valve T. If you pop that hose off and see a valve core just remove the valve core with the valve core tool. Hook it back up minus the valve core and work your CTIS. If the tire fills you have 5 more to do and a fixed CTIS.

This is assuming that the only reason they locked it out was for hauling heavy loads. It may have been locked out for another part failure but this is going to get you close in 1 minutes to the truth.

Hard Head said:

Cummins Bendix BX750

Click to expand...

Is this a direct replacement or are modifications/adapters required?

This troubleshooting manual from Dana/Spicer the OEM for the CTIS is a bit easier to read and understand than the Military TMs.
View attachment AXTS0015 - Spicer CTIS (Central Tire Inflation System).pdf

Thanks MtnSnow. Took me awhile to find that since most of the links in the other CTIS threads were bad.

Good info my unit has begun flashing 4 lights since the weather got cold was working fine before that. Gonna go do some trouble shooting now! Thanks guys

I see a lot of posts saying "not an expert" I am the EXPERT and I may regret joining this site but I have a desire to help people. Lets say I helped develop this product for 25 years, and may have even talked to some of you on the phone. Lets get started.
These Old systems/ECU's are not able to communicate to a diagnostic tool unless you purchase a newer ECU and connect a harness jumper with a CAN break out
Start with service manual AXTS-0015 from Dana website.
Start vehicle ECU comes up right away with 5 lights (Electrical Problem) ECU's hardly ever go bad. No lights check (Blackout)
ECU pressure checks and then faults (Pneumatic Issue) Remember the pressure switch should close at 115psi or nothing will ever happen.
Biggest Issue: Air leaks (can only be checked during inflation or the 1 second pressure check. I like to plumb shop air into the wet tank so I don't have to run the truck.(listen for leaks or snoop) if you don't have >115psi shop air disconnect the pressure switch on the tank and short the harness side with paperclip. Shop air should be higher than tires to properly run system.
Air Leaks in Seals: The air will pressurize the axle housing and exit the axle vent tubes
Second issue: Vehicle inflates to target but "deflates" after shutting off. The system is not deflating. One or more wheel valves did not close at target, when the leak back they sound like deflation. Let them leak back and check the tire pressures, you may find only one or two leaking down. Pull them apart and check for contamination. Other Issue, valves don't like extremely cold temperature, try venting the cover screw (few turns out and back in)
Won't inflate or deflate but tries: Schrader inserts may have been put back in.
Imbalanced tire pressures: Check for pinched hoses, or a clogged wheel valve
I will try and get permission to post service parts information here.

Stellaevil said:

I see a lot of posts saying "not an expert" I am the EXPERT and I may regret joining this site but I have a desire to help people. Lets say I helped develop this product for 25 years, and may have even talked to some of you on the phone. Lets get started.
These Old systems/ECU's are not able to communicate to a diagnostic tool unless you purchase a newer ECU and connect a harness jumper with a CAN break out
Start with service manual AXTS-0015 from Dana website.
Start vehicle ECU comes up right away with 5 lights (Electrical Problem) ECU's hardly ever go bad. No lights check (Blackout)
ECU pressure checks and then faults (Pneumatic Issue) Remember the pressure switch should close at 115psi or nothing will ever happen.
Biggest Issue: Air leaks (can only be checked during inflation or the 1 second pressure check. I like to plumb shop air into the wet tank so I don't have to run the truck.(listen for leaks or snoop) if you don't have >115psi shop air disconnect the pressure switch on the tank and short the harness side with paperclip. Shop air should be higher than tires to properly run system.
Air Leaks in Seals: The air will pressurize the axle housing and exit the axle vent tubes
Second issue: Vehicle inflates to target but "deflates" after shutting off. The system is not deflating. One or more wheel valves did not close at target, when the leak back they sound like deflation. Let them leak back and check the tire pressures, you may find only one or two leaking down. Pull them apart and check for contamination. Other Issue, valves don't like extremely cold temperature, try venting the cover screw (few turns out and back in)
Won't inflate or deflate but tries: Schrader inserts may have been put back in.
Imbalanced tire pressures: Check for pinched hoses, or a clogged wheel valve
I will try and get permission to post service parts information here.

Click to expand...

#1 thank you for joining and giving us an expert. Being new to CTIS its been hard filtering all the information over the internet.

Question: Cold weather issues you say vent the cover screw, can you point that out? I live in Northern Wyoming at 6,200ft so 60% of the year its colder than average. I want to be able to maintain my CTIS better.

Also contamination. I found the cab filter/water seperator had some debris in it. Is there another place to put one that would keep the system cleaner?

And am I right that the 4 blinking lights is a target pressure error not just a leak condition?

WYomer said:

#1 thank you for joining and giving us an expert. Being new to CTIS its been hard filtering all the information over the internet.

Question: Cold weather issues you say vent the cover screw, can you point that out? I live in Northern Wyoming at 6,200ft so 60% of the year its colder than average. I want to be able to maintain my CTIS better.

Also contamination. I found the cab filter/water seperator had some debris in it. Is there another place to put one that would keep the system cleaner?

And am I right that the 4 blinking lights is a target pressure error not just a leak condition?

Click to expand...

Each wheel valve has a screw in the center of the cover. The screw has an 'O ring seal, simply back it our a couple of turns and re-tighten. (*Warning*, don't vent the covers when the system is working)
Water is the biggest issue in cold weather so keep the Wet Tank drained and that should help.
AXTS0015 service manual: Description of 4 light flash code is "Low Tire" could be large leak but that would throw the code all the time.

I've had a CTIS problem pop up on my truck recently. The best I can tell from following the troubleshooting guide in the TM, reading every thread I can find, and using the Spicer pdf file, it's not getting the signal from the pressure switch that tells the system it has 120 psi.

I've found the two low air switches on the tanks on the passenger side of the truck but those two are the only two wire switches I can find. Both function properly, and disconnecting either one will turn on the low air light on the dash. I've found the device in the picture below underneath the truck on the front of the air tank that comes out of the drier, but it has 3 wires in it, not two, which normally tells me it is a sending unit, not a switch. Is this actually the right one to jump to confirm that it's bad? If so, which 2 of the 3 wires do you connect? If not, can someone please take a picture of the correct pressure switch and its location for me so I can find it on my truck?

The pressure switch is the one in your attached image that is on the branch that the PPV is on. It's also the only one with wires going to it in the fore front of the image you posted.

Ok thanks MtnSnow, that is the one I took the connector off of thinking it was the one. I also took it out of the air line and confirmed it wasn't packed full of crud, hence the new PTFE tape on the threads.

Like I said though, mine has 3 wires in it instead of 2 like the parts diagram and wiring diagram both suggest, both on the connector and harness side. Do you know which two I should jumper together to confirm it's the problem? Is there any danger of me frying the computer if I guess and choose poorly?

It should only be a two wire. According to the wiring diagrams it would be wires 535 & 536 and they would just need a jumper between the two.

That is the correct pressure switch, my m925a2 has this switch and it's bad. I jumped the a and b terminal with a paper clip and it's working now.