NO CTIS Because Wet Tank Builds to only 70-75 psi

[70deuce]

I have had an inop CTIS system since I got the M936A2 a few months ago. Went through the troubleshooting guide in the TM's and found my problem. The wet tank is not building up to more than 70-75 psi. The wet tank never gets up to the CTIS's pressure switch's 85 or so psi so the CTIS system does not activate. The air dryer is purging at 70-75 psi. I had a pressure gauge on the wet tank in place of the pressure switch and it never got above 75 psi with at least 120 psi shown on both primary and secondary air pressure gauges in the cab. When I jump the "A" and "B" pins on the pressure switch harness the CTIS works perfectly. Is this a governor caused thing or the purge valve?? The air dryer seems to be working good just purging early and I understand when the governor unloads the air dryer purges. Governor need adjusting?? As stated though the system air pressure, primary and secondary, is at least 120 psi.

 

[70deuce]

Filters are in good shape. Here is the Baldwin filter part number for the air dryer if anyone needs it.

 

[70deuce]

TM says

3-207. AIR GOVERNOR MAINTENANCE (Contd)
c. Testing
NOTE
Whenever governor is tested, instrument panel primary air gauge
is used.
1. Start engine (TM 9-2320-272-10) and allow air pressure to build to normal operating range.
NOTE
Engine speed must be adjusted to 1,275 rpm before performing
step 2.
2. Check air pressure gauge (4) for air compressor cut-out pressure. Air governor (1) should stop
pressure buildup at 130 psi (896 kPa). If not, perform adjustment My system cuts out just over 120 psi.

Could 10 more psi make a difference at the air dryer?? Probably wouldn't hurt anyways to bring the cutout up to 130 psi.

 

[70deuce]

Talked myself in the fix. Adjusted the governor up to about 128 psi. Right at about 123 psi the CTIS system came alive!! The system quickly brought up all 6 tires to the the wrecker's 80 psi. I still have to fix a leaking rear rim o-ring but the system held the leaking tire at 80 psi with no problem. I now have all kinds of clicking and venting noises in the cab which is a good thing. This all might help someone who CTIS system is working fine otherwise but is not working because the wet tank pressure is just below the pressure switch threshold.

 

[5tonman1971]

Thanks for the ctis diagnosis write up! Hahahaha

 

[MtnSnow]

Bravo!

 

[corvette9]

What TM # is the trouble shooting in. I would like to fix the CTIS in my 936A2. It too worked when I bought the truck. No idea why it stoped. I have not mastered the TM's yet. If someone can just guide me to the correct one would be great.

BG

 

[70deuce]

This one led me to my solution. Chapter 3

 

[1 Patriot-of-many]

Did you have any red lights at all when yours wasn't functioning? I don't get any red lights or cycling, the ECM box is good as I switched out a known good and tried that box in another truck. I hope mine is as easy fix as that!

 

[corvette9]

70deuce. Thanks. That looks like a great start.

Brian G.

 

[F18hornetM]

on My M931 I had crude in the port where the pressure protection switch/valve is. The valve was full of crude as well. On the end of the wet tank. Not CTIS related since it doesn't have it. But did cause issues with everything that wont work unless air pressure is about 80 psi or so.

 

[70deuce]

Here is what I had. After engine start I had the red highway selection light flashing with no air venting or relay cycling clicking from the pneumatic control unit under the front winch control. It would flash for about 15 minutes and then all five selection lights would flash at the same time. The system was never energized because of the low air at the pressure switch on the wet tank. Once I adjusted the ships air pressure up, that pressure switch closed and there was no doubt the system was working. All kinds of clicking and venting was going on at the pneumatic controller. Every time a new selection is made sand, emerg etc the controller vents, clicks and does its thing which tells me it works!! Even with the slow leaking o-ring in the right rear tire the system quickly fills all 6 tires right up to 80 psi.

 

[70deuce]

1 Patriot-of-many, make sure you have min 23 volts going to the ECM. With no lights coming on I would check that. There may be a fuse in the power circuit to the ECM box. I recall one of the CTIS wiring diagrams show a fuse. Check the trouble shooting procedures in the TM I attached above. It tells you which two pins on the cannon plug that plugs in to the ECM to check for 23 volts. With my pressure switch problem the ECM still energized properly. It just couldn't send a signal to the pneumatic controller.

 

[Hoefler]

Thanks for posting. My wrecker is doing the same thing yours did. It has been to cold to work on her, but with warming, I will tackle it.
pete

 

[Dieseljoe3]

70deuce thanks for posting as we'll. I suspect my problems are the same since my CTIS is operating intermittently. Which section of the TM did you look at to adjust the pressure on the wet tank? I looked at section 149 and it didn't seem to address that specifically on adjusting the governor

 

[70deuce]

I did not adjust the wet tank pressure specifically. All I did is bring the trucks primary air pressure up from just under 120 psi to just under 130 psi at the air governor inside the engine bay on the firewall. The new style governors adjust counterclockwise to increase pressure. They are the ones with the star shaped screw off cover.

 

[MtnSnow]

I did not adjust the wet tank pressure specifically. All I did is bring the trucks primary air pressure up from just under 120 psi to just under 130 psi at the air governor inside the engine bay on the firewall. The new style governors adjust counterclockwise to increase pressure. They are the ones with the star shaped screw off cover.

I was just about to post the same thing as I just got done adjusting mine and mine is the newer style and was opposite of what was in the TM directions I had.

 

[Csm Davis]

Also guys a black ctis box will only light up when the lights are turned on. Not saying that this is your problem but something to watch for.

 

[1 Patriot-of-many]

Also guys a black ctis box will only light up when the lights are turned on. Not saying that this is your problem but something to watch for.

It's not for me. I have the backlights with the lights on, no red dots, no cycling.

 

[70deuce]

On mine the red dot light in the push button area comes on on with mode you select light switch on or off. As soon as power is applied via the master switch, the red dot lights come on. Backlights come on with light switch.