CTIS Overspeed indicator flashes at 30 mph

Alright, I have a M1083 MTV , Stewart & Stevenson built in 11/98
This is a service truck (Active in the military still) and I need some ones help.

Once the vehicle get to 30mph, the overspeed lamp indicator on the drivers display flashes even though im not overspeeding. The vehicle drives fine and seems to shift fine as well. I know that with the CTIS module set at highway, the overspeed lamp will kick on at about 55mph but its kicking on at 30mph. The light will go off of course if I unplug the CTIS module but this is not an option with my job. Ive trouble shot the sensors, wires from the sensors to the controller, wires to the display, and wires to the transducer, and they check out according to the troubleshooting tree of "CTIS overspeed not operating" but that's a different issue.

I recently swapped out the oil CAT motor for another due to a massive oil leak and the reservists killed it in the process. After it was all said and done, the MTV runs fine and the CTIS works as designed. Tires inflate and hold pressure where it is suppose to be in highway, sand, mud, ETC..., the engine speed sensor and transmission sensor are good, the CTIS module is good, and the air transducer in the passenger kick panel is good.
Ive gone through the trouble shooting manual and these isn't anything for a fault like this.

I work at an AMSA shop and this **** thing is kicking my ass. Ive seen one other member on here that mentioned the same fault but did not seem to come to a conclusion.

If there is a sticky or previous fix to this issue I apologize as im an active member of other car forums and understand proper forum SOP's but I just registered today and im actually on the clock right now so im pressed for time at the moment.
If anyone can give me some pointers on this, that would awesome. This thing is so simple, its not lol there are literally almost no computers on this thing other then the CTIS and the TCM. Ive hooked up a prolink to it and the trans is fine. Ive swapped a known good CTIS and it does the same. it is really weird.
Im wondering if there is some kind of manual control on the air side this isn't working properly that and is registering an incorrect PSI to the module, and then it is sending the overspeed indication. trying to find a schematic on this old thing was a pain in itself and there. Ive called Stewart and Stevenson just to find out I need to call BAE because they don't even register those things on their databases anymore. Im still trying to see if there is anyone on that end that is a Subject matter expert but now calls as of yet.

Thanks everyone for your help

The ctis relies on the trans speed sensor (shared with the trans ecu) to know what speed the truck is traveling at. At a lower ctis setting you will get an overspeed at a lower speed. Something is making the ctis think the truck is traveling faster than it actually is. Did you check the ctis and trans computer grounding. I usually upgrade the ground on both. Especially the trans computer it only gets a ground thru the dash mounting.

Good to know which module relies on which. I will have to do a check on this. Didn't really think the ground on the Trans computer could/would cause an issue like this, but at this point, anything is possible. I will check tomorrow and let ya know.

Curious what model prolink you use at the shop? Does it take a card for the Allison trans? Is there a separate cable for it so you can diag an A1 truck? Or is there a different prolink handheld for the A1? It is possible a wire could have gotten nicked or pinched somewhere during the engine swap also.
Will

Well Ive actually hooked up a bunch of diagnostic computers up to this thing. Ive used the STA/ICE, snap on prolink (don't know the model off hand), ONETOOL with Dearborn protol adaptor, Allison program through the databus reader to make sure there wasn't any bugs in the trans module, save for trying out a different TCN, everything has been tested component wise from a known good and verified, still have the flashing overspeed at 30mph. (from what I can read it while driving more like 27-28 mph)
The CTIS module is good. The only thing I can see wrong is the programing in the TCM and I don't have the right program to change the on command over speed at given set pressure/driving condition.
One thing im still trying to figure out is that the engine speed magnetic pickup sensors wiring routing on the schematic goes into something called a frequency divider... WTF is that? it has a bunch of input and outputs with neg and pos writings on it and I cant really make sense of it with the army schematics scattered everywhere on paper. Im decent on schematics, but this is kinda new to me.

Oh and ive checked the grounds on the TCM and the CTIS. They seem to be good to go. No breaks or shorts that I can test.

I was thinking about it I had that low speed warning once before when I had a bad speed sensor. Once I replaced the sensor it cleared on its own though after some driving. The speedo was clearly off with the sensor being bad. I wonder if the speed signal goes into the trans ecm and is divided there. Is your truck wtec 2 or 3?

Im pretty sure its a WTEC 2. This M1083 is just a M1083. Its not even an A1. Its an MTV in its most basic form. The scanners don't even pull up any names on it, like the newer LTAS models we have. I found it weird when I went checking around the wheel hubs to find it doesn't have any wheel speed sensors nor ABS. In order to even see if its receiving a proper tach reading, the STA/ICE has to be connected to it, which we do not have the right one because this one is so old, and our STA/ICE is for new vehicles with the new hook ups.

Yesterday I also noticed while I was going through some testing with the MSD that the TCM was REALY hot when I was running the truck. Strangely hot for something without cooling fins and normally cooped up in the kick panel. Generally computer electronics do not like concentrated heat like that and normally fail after a while (just look at all the older HP laptops that overheated from being in... laps!)

I beginning to think that the combination of the trans computer and the output speed sensor on the transmission may have some disconnect either internally or in combination of each other. but I cant seem to find a way to verify the fault. Id hate to order a trans computer and a new output sensor just to find its doing it again.... or I take the light out lol

And I did think maybe I did pinch a wire or nicked something, but if you have seen how the wiring harness is removed from the engine prior to engine removal, its nearly impossible unless your careless. most of the wiring harness is on top of the engine save for the bundle that runs down to the starter. It neatly folds up and over the engine and you can lay it on the hydraulic reservoir without ever really hitting anything. Its easier to pull a CAT motor out of this thing then pulling a HMMV motor. Only thing easier is a M113 engine pack lol

Wtec2 has the trans computer behind the keypad and wtec3 has it in the passenger kick panel. I would take the speed sensor off, clean it and put it back on. The wtec2 back plugs on the computer can slip out a bit especially if you have been playing in the dash.

Oh ok, so it is a WTECIII. It is in the passenger kick panel.
So far the new engine came with a new magnetic pickup im assuming this is for engine RPM reference since this sensor gets its from the engine bell housing (takes its signal from the flex plate) and goes to a frequency converter under the driver side dash area. This seems to have good continuity. Ive tried a known good speed sensor on the trans and still to no avail. Im trying to see if the sensor in the transfercase might have anything to do with it but I cant be sure yet. That ones alittle harder to pull out and look at since it has wires going into the transfer case. This is the output speed sensor. im still trying to trace the schematic on where this guy sends its information to, but I suspect its the TCM

I think the wiring going to the transfer case is for the diff lock. Can you try swapping in a ctis controller from another truck
and see what happens? The trans computer uses the vehicle speed sensor on the trans, engine speed sensor on the bellhousing and the throttle position sensor, which is above the primer pump with a cable going to the throttle mech. I dont see how it would affect the ctis though.

Under the dash left side by the light breakout box is a small box labeled power supply. I always thought it was for the dash dimmer or something. The manuals dont really describe it and even the mil mechs that I have spoken with dont know what it does. Maybe thats the freq divider.

Suprman said:

I think the wiring going to the transfer case is for the diff lock.....

Click to expand...

??? Diff lock?

Torque is split 30 front 70 rear in normal driving mode. The front and rear driveshaft speed can vary slightly with cornering and such. In mode the center diff is locked, splitting power 50/50 and forcing both driveshafts to turn at the same speed.

Suprman said:

I think the wiring going to the transfer case is for the diff lock. Can you try swapping in a ctis controller from another truck
and see what happens? The trans computer uses the vehicle speed sensor on the trans, engine speed sensor on the bellhousing and the throttle position sensor, which is above the primer pump with a cable going to the throttle mech. I dont see how it would affect the ctis though.

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That wire bundle has the diff lock and the PX71 output speed sensor twisted pairs inside the transfer case. I also tested that wire set just to make sure there wasn't any breaks inside the transfer case to the sensors. All checked out. Mind you, if you ever do this, make sure you have something to catch 3-5 gallons of oil and the same ot fill the trans back up. this is a messy test

There's the PX71(output sensor), PX72 (engine speed sensor), PX73 (Throttle position sensor), and P38 (Engine Speed Magnetic Pickup) that take some kind of signal and talk to the computers.
im at a lose... Im at the point were im gonna have to grab each wire by hand and make sure there isn't physically a break or kink of some kind. Which still doesn't make sense to me, because the CTIS inflates and deflates on command and inflates when set at a lower drive condition such as X/C or sand and switches to Highway automatically and fills the tires to the proper levels and then shuts off, as designed. im not understanding this.
Do you think there is a mechanical issue somewhere im just not considering?

Suprman said:

Under the dash left side by the light breakout box is a small box labeled power supply. I always thought it was for the dash dimmer or something. The manuals dont really describe it and even the mil mechs that I have spoken with dont know what it does. Maybe thats the freq divider.

Click to expand...

Yeah, that is the frequency divider, but Im not sure what it does. Im assuming that it takes the magnetic signal from the engine flexplate and converts it into some kind of usable signal for one of the computers.

Ordering the TCM is my last resort... $2500 of tax payers money on the line there.

If you didnt change anything with the trans I dont see the ecm going bad, The Allison stuff is reliable. The civilian Allison techs I have spoken with have all said the only issues are physical wear on the keypads from use and wiring harness problems. You can try cleaning the connections. Maybe some moisture got in where the trans harness passes thru the cab. You could try swapping the freq divider too. I have seen the pins on the not-mil-spec connector going to the freq divider have corrosion on them. Water seeps in above the windshield around the upper lights and makes its way into the dash and down. Over time it causes little issues.

Yeah, that's what I was thinking in that the trans controller shouldn't go bad especially since its still shifting the vehicle just fine and no other hiccups. I called Dana to see what their take is on it and I might have a solution, but I need to get the software on my Dearborn protocol up and running right. I can change the parameters in the CTIS to accept a different over speed condition by way of changing the frequency per mile. From what it seems like, the CTIS is interpreting double the amount of road speed and there for seeing more pulse from the output sensor then it actually is, either by a new manufactured part that has been replaced in the supply system or just by chance. Either way, if I can get into it and change the Pulse per mile from 30000 to 60000 I might be able to fool the CTIS module into thinking its going the actual road speed its suppose to see, and get rid of this **** light. Time to look up some software

Do you have the Y cable to access the CTIS controller? You can download the software from Dana. If you have the Y cable, whats the part number or NSN I have been trying to get my hands on one of those for a while. I dont believe the CTIS controller can be accessed any other way.