LMTV CTIS Question

Stellaevil

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What faults do you suspect would appear??

Overinflate? Based on what? As in not the 50ish-lb the previous trucks were set for??? But then again.... maybe those were under inflated thus decision to change to the 6x6 settings? How much you suspect the pressure will drop in temps below 50 and wouldn't the ctis adjust as the tires got warm while driving?

Thanks.
The system calculates the inflation time based on tire volume, 4 vs 6. I think the 6x6 wrecker setting for Highway is 82psi which could have the cold temp issue. If the system is cycling due to cold you can vent the wheel valve cover screw and re-tighten, this will reset the valve to the ambient temperature. This would not help at high temperatures as the pressure in the cover would be helping the valve to close at higher tire pressures.
 

coachgeo

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It allows me to set .... and set it to memory
There is mention recent of 6x6 CTIS ECU's used on 4x4's maybe having a fault issue due to math (calculating for 6 tires when only 4 exist).... in your diagnostic/edit software do you come across a spot to in there to set how many tires are on the vehicle? (4 or 6 ? )
 

Stellaevil

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CTIS ECUs can have as many as 8 different variants stored in them. Depending on how Pins (Z) and (M) are wired (High, Float, or Ground) The ECU will load the parameters on power up. (Float Float) may be set for a 4x4 and (High, Gnd) may be set for a 6x6 Heavy. There other parameters in these configurations such as Tire pressure tables, inflation time, deflation time, timing for reading the tire pressures, pressure sensor calibrations. If you try and put an FMTV ECU on a 939 it could fault out due to these calibration differences and the ECU CAN circuit will blow because that Pin is grounded on a 939. If you replace an ECU you need to make sure it is compatible with the vehicle. A 2006 ECU can go on a 1998 truck but the truck may not have a connector to hook up the diagnostic software. A 1998 PCU may only deflate to 16 psi, but a 2006 PCU may be able to deflate down to 12 psi. I guess my point is exchanging new and old CTIS parts can be a good thing but can be a bad thing if you don't get it right.
 

spankybear

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CTIS ECUs can have as many as 8 different variants stored in them. Depending on how Pins (Z) and (M) are wired (High, Float, or Ground) The ECU will load the parameters on power up. (Float Float) may be set for a 4x4 and (High, Gnd) may be set for a 6x6 Heavy. There other parameters in these configurations such as Tire pressure tables, inflation time, deflation time, timing for reading the tire pressures, pressure sensor calibrations. If you try and put an FMTV ECU on a 939 it could fault out due to these calibration differences and the ECU CAN circuit will blow because that Pin is grounded on a 939. If you replace an ECU you need to make sure it is compatible with the vehicle. A 2006 ECU can go on a 1998 truck but the truck may not have a connector to hook up the diagnostic software. A 1998 PCU may only deflate to 16 psi, but a 2006 PCU may be able to deflate down to 12 psi. I guess my point is exchanging new and old CTIS parts can be a good thing but can be a bad thing if you don't get it right.
Then the FMTV and 939 series ECU different? It's not how pins Z and M are connected? I would think that if the ECUs or vehicles are wired differently there would be a different connector.

Back to the all lights on error... would configuring Z or M reset the memory?
 

319cssb

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Easley SC
CTIS ECUs can have as many as 8 different variants stored in them. Depending on how Pins (Z) and (M) are wired (High, Float, or Ground) The ECU will load the parameters on power up. (Float Float) may be set for a 4x4 and (High, Gnd) may be set for a 6x6 Heavy. There other parameters in these configurations such as Tire pressure tables, inflation time, deflation time, timing for reading the tire pressures, pressure sensor calibrations. If you try and put an FMTV ECU on a 939 it could fault out due to these calibration differences and the ECU CAN circuit will blow because that Pin is grounded on a 939. If you replace an ECU you need to make sure it is compatible with the vehicle. A 2006 ECU can go on a 1998 truck but the truck may not have a connector to hook up the diagnostic software. A 1998 PCU may only deflate to 16 psi, but a 2006 PCU may be able to deflate down to 12 psi. I guess my point is exchanging new and old CTIS parts can be a good thing but can be a bad thing if you don't get it right.
I can change all those things with the laptop, including the timing for reading the tire pressure.
what I'm saying is that, if I know what the ECU goes in, I can make changes accordingly with my truck and diagnostic laptop. It's easy peasy.
 

DCLund

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MN
Just an FYI
I accidentally replaced relay K52 (ctis overspeed) with a 4 pole relay. The controller and dash overspeed lights did not light but the CTIS functioned normally (including overspeed automatic override).
 

spankybear

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The fmtv ecu is 12 volt and the m939a2 one is 24 volt they are not cross compatible.
The TM must be wrong for the FMTVs if that's the case. The wiring diagram shows pin H on P110 (foldout 14 of 34 E122) coming from CB40 (foldout 17 of 34 C150) and the wiring diagram shows that as 24 volts. Also show 24 volts from that same CB40 setting pin M high (24V) for the configuration on a M1078 and M1079.
 
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Stellaevil

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I can change all those things with the laptop, including the timing for reading the tire pressure.
what I'm saying is that, if I know what the ECU goes in, I can make changes accordingly with my truck and diagnostic laptop. It's easy peasy.
You sure can make the changes; But if the PCU has a 12 psi relief valve, the system will not deflate below 14 psi so setting the emergency pressure to 10 will not work. Setting the tire pressure to 100 will not work. Setting the overs peed's too high could be dangerous.
 

Stellaevil

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I thought this information might be helpful to others
I do not know of any way to fix an ECU that is not responding normally. If the internal power supply is good and the micro is trying to boot but will not checksum there is no fix. The only issue that may be fixable is a stuck keypad button. I am not sure what condition the ECU goes into if this is happening. It is a "tin whisker" thing so massaging the buttons may clear it. DB3 does lock up often and must have the power cycled to reset it. Changing the CTIS config is not a good way to change tire pressures. Use the DDT in the "Settings" tab but remember >80psi tires and colder than 60F, the valves will not hold pressure after inflating or checking. You can put a switch in line the "pressure switch" signal circuit to disable the system and manually fill the tires. This is how they do an "Arctic Kit" system disable for leaking axle air seals in the cold.
 

Awesomeness

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You can put a switch in line the "pressure switch" signal circuit to disable the system and manually fill the tires. This is how they do an "Arctic Kit" system disable for leaking axle air seals in the cold.
Can you elaborate on that some? I'm not sure I followed it completely.
 

Stellaevil

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Can you elaborate on that some? I'm not sure I followed it completely.
When the pressure switch opens, the system is put in a "Hold" state; if it is pressurized, it will vent after 30 seconds and wait for the switch to close. If you brake the signal wire from the switch to the ECU and run it up to a switch on the dash you have created a disable switch. The only fault that can log is a "Pressure switch failed open" after the system sees >5 mph speed for 20 minutes which should auto clear when the switch is again closed. Signal wire is Pin "T" at the ECU.
 

tcarr1083

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I have a similar issue. My 1083 has 223 miles on it. The CTIS lights stay on until the 24V breaker trips. I have swapped relays, breakers, the ECU itself. Unlplugged all of the pressure relays under the truck and it does the same thing. Inspected the wire harness best i could from under the cab and under the fuse box and there is nothing out of order....
Any thoughts?
 

coachgeo

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I have a similar issue. My 1083 has 223 miles on it. The CTIS lights stay on until the 24V breaker trips. I have swapped relays, breakers, the ECU itself. Unlplugged all of the pressure relays under the truck and it does the same thing. Inspected the wire harness best i could from under the cab and under the fuse box and there is nothing out of order....
Any thoughts?
If it tripped the breaker with all the relays unplugged then can only guess you have a short.
 

Stellaevil

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Michigan
I have a similar issue. My 1083 has 223 miles on it. The CTIS lights stay on until the 24V breaker trips. I have swapped relays, breakers, the ECU itself. Unlplugged all of the pressure relays under the truck and it does the same thing. Inspected the wire harness best i could from under the cab and under the fuse box and there is nothing out of order....
Any thoughts?
If all the CTIS lights are on solid the ECU is bad. I am not aware of a bad ECU drawing enough current to trip a breaker but I would think the harness to the ECU would be warm. You may also hear a spark when disconnecting and reconnecting a live ECU if that much current was present.
 

tcarr1083

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Yeah first thought was bad ecu but I swapped it with another truck and got the same thing. Unfortunately the other truck was not running but what are the odds of two of them being bad with the exact same symptoms. Wiring harness looks clean and with only 225 morning les oni it there is not much wear anywhere
 

Stellaevil

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There is something you can do to replace the ECU completely. Build a switch box with power and ground wires to the truck, connect power to one side of the switches and the other side out to the PCU solenoids. Bring the PCU ground wires back to the switch box ground. (Supply) (Control) (Deflate) I like to wire the box so (Control) has to be "On" to provide power to the (Supply) switch, this way Supply can not be energized without control on. High pressure air would blow out the PCU. You could connect the box the ECU connector but those 26 way are more expensive than the 6 way PCU connector. >1.5 amp switches. Some engineers like to see fly-back or clamping diodes in the circuit but larger 10 amp switches will not fail.


1. Tap a pressure gauge 0-125 psi on the PCU channel line so you can read the tire pressures.
2. Control On, then Supply on for 3 seconds the Off = "Tire Pressure Read", Supply back on = "Inflate"
3. Control On, then Supply on for 3 seconds the Off = "Tire Pressure Read" , Supply of, Deflate on = "Deflation"
4. Read deflated tires: Turn off Deflate Switch, back to (step 2.)

I do not remember previously posting this control method so feel free to share with anyone or re-post
 

coachgeo

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There is something you can do to replace the ECU completely. Build a switch box with power and ground wires to the truck, connect power to one side of the switches and the other side out to the PCU solenoids. Bring the PCU ground wires back to the switch box ground. (Supply) (Control) (Deflate) I like to wire the box so (Control) has to be "On" to provide power to the (Supply) switch, this way Supply can not be energized without control on. High pressure air would blow out the PCU. You could connect the box the ECU connector but those 26 way are more expensive than the 6 way PCU connector. >1.5 amp switches. Some engineers like to see fly-back or clamping diodes in the circuit but larger 10 amp switches will not fail.

1. Tap a pressure gauge 0-125 psi on the PCU channel line so you can read the tire pressures.
2. Control On, then Supply on for 3 seconds the Off = "Tire Pressure Read", Supply back on = "Inflate"
3. Control On, then Supply on for 3 seconds the Off = "Tire Pressure Read" , Supply of, Deflate on = "Deflation"
4. Read deflated tires: Turn off Deflate Switch, back to (step 2.)

I do not remember previously posting this control method so feel free to share with anyone or re-post
Interesting..... wish I was smarter and had even a clue about what you just said.
 

Stellaevil

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Michigan
This switch box is for Troubleshooting CTI system issues only it is not intended for vehicle operation.
* * * MY DISCLAIMER * * * Note that inflating and deflating past the normal software values will cause the system to malfunction and could be Dangerous!

Follow the 2018 CTIS Troubleshooting Guide for connections.
f-swbox.jpg
 
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