LMTV CTIS does not deflate

[Suprman]

Check make sure your release valves are clear.

 

[Konichiwa512]

I think you are right to blame the release valves. After going through this discussion, http://www.steelsoldiers.com/showthread.php?152581-Manual-CTIS-Conversion-for-M-1078 I think my CTIS quick release valves might be seized up after a long time without any use when the controller was bad. I am going to Kuwait next week, so I will have to look into it when I get back in three months.

 

[Konichiwa512]

I guess this CTIS does not have a deflation problem after all. After almost a year of not being able to deflate the tires or to figure out why not, I was driving in the rain when the CTIS Overspeed caution came on. The usual three solid lights were still steady on, but the EMER light decided to flash. I soon felt the tires getting squishy. When I stopped, all 4 tires were deflated to what looks like EMER pressure (14 psi). Now the CTIS Overspeed caution is on even after shutting off the engine and cycling the master switch. I let the truck sit all day and the tire pressure looks the same after 8 hours, but now it is stuck there with EMER pressure and the Overspeed light on. It must be a short somewhere. It is unlikely to be in the cabin, since the CTIS computer, wire harness and manifold valve are all new. Plus the cabin stays mostly dry, and I think the rain likely set this thing off in this new direction.
The TMs are great if you have a problem listed in their malfunction matrix, but there is not enough tech data anywhere to troubleshoot problems that are not listed. The most I have learned about CTIS comes from Suprman's posts. If anyone has the slightest idea which component could cause the CTIS to never deflate and then suddenly issue an uncommanded EMER pressure only, I would be glad to hear about it.
The truck is stuck at work because the tires are too saggy to drive without causing much more expensive problems. I will let it dry out for a day or two and hope I can at least get my original problem back so I can drive it home.

 

[Suprman]

You can unplug the controller and manually inflate the tires they will hold pressure. If the controller is stuck in a mode lower than highway pressure then the overspeed alert will come on at a lower speed corresponding to the tire pressure mode. I have not heard of a controller switching modes on its own. It should come on in highway mode to start although I have seen some lmtv ctis controllers that appear to have a memory and will come on in the mode they were last set to. Start the truck press hwy and see if it starts to fill. If it won't at least try to fill like that then it could be a controller issue.

 

[Konichiwa512]

I forgot that I could just disconnect the controller and inflate the tires. Trouble is, since I returned from the deployment, I cannot get any air to my gladhands. The TM has pretty good troubleshooting steps for that. Neither the front nor rear gets air now, so the stuck valve is upstream of the split. I will hobble over to the base auto shop and mess with the gladhand air, or just fill the tires, drive home, and let the LMTV sit until I am in the mood to mess with it again.

 

[Suprman]

Maybe stuck check valves. You can unscrew one of the air tank drain valves and an a standard air fitting should screw in. Done it before to fill a stubborn truck.

 

[mkcoen]

Also on the gladhands make sure parking brake is OFF or you wont't get any air. I found that out when recovering my M1082 trailer and couldn't air up the tire until I called NDT and he told me about that little function.

 

[tennmogger]

Getting air to the trailer gladhands requires the truck controller brakes off (yellow knob) and also the red trailer controller enabled. Be sure to have chocks under the tires because the truck will roll otherwise! This is a terrible way to have to use to get air for tires.

I have seen a situation like yours, where the tires get deflated and will not re-inflate, and the CTIS controller faults out. The CTIS controller gets confused, I think because the front and rear axle/tires get deflated to unequal pressures. Why? Possibly because there is a restriction in either the front or rear quick release valves allowing the faster 'dump' to drop pressures faster on one axle (one QRV on each axle). Then, when the system makes the dump, stops the dump and measures pressure, it sees a changing pressure. That is because the front axle and rear axle are at different pressures, and when the 4 wheel valves get their 15psi-to-open signal, there is flow from the highest pressured axle to the lowest pressured axle, trying to equalize. This test does not last long enough to allow equalization. As the CTIS controller tries to measure the system pressure (and it only looks at one combined system pressure) it sees change and thinks there's a bad leak and shuts down.

You can verify if your axles/tires are at different pressures with a manual gauge. If so, inflate the two tires on the lower pressure axle to match the higher pressure axle. Now the CTIS will not see changing pressure and may take over and work. [edit, it just occurred to me that lowering the two higher pressured tires to match lower pressured tires would achieve the same equalization goal, and does not require hooking up the air hose]

Sounds like you will need to go through your air system looking for bad components. Not a trivial job hour-wise, but not too expensive a repair. Lots of threads on here about CTIS and air system components.

My opinion of the highly-structured trouble-shooting steps in the manual: designed for someone who will not have any idea of the theory of the system, and has unlimited time to blindly follow troubleshooting steps. Hope we are beyond that. We FMTV owners had better understand our trucks or you will undoubtedly get stranded somewhere. A little knowledge of the system, and normal human logic, will allow so much better troubleshooting than the manuals 'brainless' test steps!

Bob

 

[Konichiwa512]

Thanks I got the tires back to 50psi by disconnecting the CTIS computer and manually inflating. I guess when I ops checked the gladhands the first time I happened to have the parking brake off and did not know that was a requirement because it does work now. I hope you are right (tenmogger) about my QRVs being the culprit. That is a big help since I came to the conclusion that the original problem where the CTIS will not let me set a pressure other than HWY may also be caused by the quick relief valves. The problem is, I do not know how to check for proper function of the QRVs, other than the whole CTIS functioning correctly. I guess I can soak them in some gasoline, lube them up and re-install after checking for cracks and bad wiring. All of the other problems I have had with this LMTV (fuel leak, alternator dismounting 4x) can be attributed to the shakedown my truck gets every time I drive on my bumpy road. I will have to go through and check every air and wiring connection to see if anything is loose. Other than that, I do not know what I am doing really.

 

[mkcoen]

Other than that, I do not know what I am doing really.

Welcome to the club! I felt pretty silly when I couldn't get the gladhands to work and had to call NDT. Fortunately I have him and Suprman in my phone so I can email them if I have a emergency issue. I have discovered a lot about the truck in a short amount of time and learned I could do things myself that I doubted I could before actually doing them. It's all an adventure. Now I've had the radiator out, changed the fan clutch and fan, pulled and reinstalled the rear axles shafts and much more. The truck keeps pushing me and I keep bouncing back.

 

[gottaluvit]

If the ctis is the same as the m939 series, you can hit run flat button and it will air up your low tires. As for the deflation, well I am not in the know.

 

[Konichiwa512]

My CTIS works! Finally! Out of desperation, I stopped by the National Guard base to pick the brains of the real LMTV mechanics. They said the likely culprit would be the pressure sensor or the QRVs, which you guys have already mentioned. When I was there they happened to be loading a crate of 'junk' for the scrap yard. They stopped loading just for me and opened it up to offer me some of the goodies. There were a dozen or so CTIS computer controllers, so I grabbed what I could fit in my motorcycle cargo box (two). I didn't see much point, since these were supposedly unserviceable. But the troubleshooting manual instructions for checking the CTIS computer are 1. Replace the computer with a working one. 2. If the new one works, the old one must have been the problem. With that in mind, if figured they might help me with the diagnosis. Sure enough, I removed the 'factory new' computer that I paid a fortune to Eastern Surplus for and put in one of the junked ones from the National Guard and CTIS worked perfect. Tomorrow I'll go off-roading with the Jeepers and show them how things are done. The next project is to ditch the computer and install the manual air pressure regulator and valve setup that someone on SS has already invented. That is definitely the way to go.