M1078 LMTV CTIS issues

[TLG0010]

All, I'm reaching out after scouring the internet, the maintenance manuals with no solution.

Im working on a donated M1078 LMTV to a local fire department and have no experience with these trucks. There are a couple of issues that I am having trouble diagnose:

1. The CTIS module will flash all 5 lights when in highway mode. But the lights will go solid when switched to a different mode once the air depressures to the selected setting. Seems like it goes in to error or emergency mode when it can't air up to the 55psig and bleeds all the air out of the rear exhaust valve above rear axle.

I've manually aired the tires up to 55psig then cranked the truck. Same issue, will go in to error/emergency mode and bleed all the air from the tires?

2. When the truck is shutdown, the air is dumped out of the rear exhaust valve above rear axle?

Any suggestions or opinions where to start will greatly be appreciated!!

Or anybody have experience bypassing the ctis system?

Thanks

 

[GeneralDisorder]

Replace the diaphragms in all the wheel valves: https://www.pgttruckparts.com/599805-Eaton-Diaphragm-Seal-p/2264-599805.htm

The cover botls break VERY EASILY. Heat and a punch to jar them loose and very gently work them back and forth and you might still break some.

Rebuild the PCU: https://www.ebay.com/itm/1535125790..._2U40P_QI2&var=&widget_ver=artemis&media=COPY

More than likely that will solve the issues. If you have further concerns after rebuilding all 5 (don't forget the spare) and the PCU post what it's doing. This is always the first step.

 

[TLG0010]

Replace the diaphragms in all the wheel valves: https://www.pgttruckparts.com/599805-Eaton-Diaphragm-Seal-p/2264-599805.htm

The cover botls break VERY EASILY. Heat and a punch to jar them loose and very gently work them back and forth and you might still break some.

Rebuild the PCU: https://www.ebay.com/itm/1535125790..._2U40P_QI2&var=&widget_ver=artemis&media=COPY

More than likely that will solve the issues. If you have further concerns after rebuilding all 5 (don't forget the spare) and the PCU post what it's doing. This is always the first step.

Forgive my ignorance, but is the M923 PCU Kit the same as the M1078?

 

[GeneralDisorder]

Forgive my ignorance, but is the M923 PCU Kit the same as the M1078?

Close enough, yes.

 

[Ronmar]

Well, if you unplug the controller, that will disable the system. With the tires aired up and the controller unplugged the tires/wheel valves should hold air. You can confirm this 2 ways.
1. The CTIS dump valve over the rear axle and the front CTIS dump valve beside the transmission(passenger side) should have zero air coming out of the exhaust port/horn. The only air that can come from that port is tire air.
2. You can remove the banjo bolt on a wheel and tape a rubber glove over the hose end. If it blows up like a balloon the wheel valve is leaking.


The dump valves are remote air pressure regulators. If they are dumping air it is because there is:
A. More than 5PSI on the truck side of the wheel valve, which holds them open.
B. There is more pressure on the tire side of the dump valve(output ports) than the truck side(input port) and it is doing its job trying equalize output pressure down to input by dumping output pressure(tire air) down to match the input pressure.

if only the rear dump is dumping I suspect you either have:
A. A large air leak back there on the input line to the valve. And it is venting a lot of the applied pressure from the PCU. It is still maintaining > 5PSI to open the wheel valve but less than the tire pressure, so it is lowering the tires to match that input pressure.
B. There is a blockage or kink along that line to the rear dump valve. It passes pressure to open the wheel valves but when the PCU vents to drop the pressure to 0 and close the wheel valves, the restriction is preventing the pressure from dropping cleanly below 5PSI holding the wheel valves open, and the air flowing out of the tires past the restriction is able to keep the pressure above the 5PSI cut off at the wheel valves and the tires continue to dump…

these things are very sensitive to leakage, and perhaps the best way to check for leakage is to disconnect the output line from the PCU down to the dump valves and install a quick connect fitting. Connect a regulated 60PSI air source to that line and then start looking for leakage. A steady 60PSI source should pressurize all the tires to that point and just set there. If the regulator is feeding air, or you start dumping from the dump valves, you are leaking somewhere… Gotta find and fix the leaks first…