97 M1078 ctis dumped all air from tires

[Andy Barth]

I'm a member of the volunteer Fire Department and we bought two of these M1078 2.5 ton trucks through the state about 10 years ago.

The ctis has never worked in either truck we just aired the tires up normally in the spring and fall when we use them.

Monday out of blue after a call, the dump valves on the frame rails opened up and dropped all 4 tires down to about 10 pounds then stopped.

The ctis valve under pass kick panel has had the electric connectors unhooked since we have had it and the controller has never had a light on it.

Manual CTIS Conversion for M-1078

After severals request to start a post on this topic, I'm going to give it a try. I purchased my M-1078 in December with the control box missing from the cab. After reading several posts on the CTIS, and learning there is a lot of people having trouble with them, I decided to go the manual...

www.steelsoldiers.com

I made and installed this manual setup, works awesome except the right front tire still vents through the qd on the frame till it gets to about 50 pound then it stops. Other 3 tires stay up. Swaped diaphragm in the regulator on tire with known good part and still does it. Where is the leak? What can hold that regulator open on only that tire? Hub seal?

Can we plug up that wheel some how so I can get through fall fire season and fix it this winter?

Thanks

 

[GeneralDisorder]

You can remove the wheel valve and move the shrader to the valve stem itself and just use it like a normal tire.

What is happening is not a leak - actually it's probably the opposite of a leak. You likely have a partial blockage between the wheel valve and the dump valve on the wheel that will deflate to 50 psi and stop. What's happening is the blockage is holding pressure on the wheel valve because at *more* than 50 psi it is unable to vent the line between the dump valve and the wheel valve to lower than 5 psi to make the wheel valve close. The higher you go in pressure the more flow you need to vent the line down to zero. You are hitting a break-over point above 50 psi where the air can't escape from the line between the wheel valve and the dump valve fast enough to drop the pressure on the wheel valve below it's closing point. Thus you dump to 50 psi and stop where you reach equilibrium between the pressure in the tire and the line's ability to flow air down to 0 psi and shut the wheel valve.

Counter-intuitive isn't it?

 

[Ronmar]

Quick fix, on the back side of the pressure control unit under the passenger dash on the port labeled C I think, disconnect the line to the wheels. The other line is the pressure input from the wet tank. The pressure line typicallt has a T right where it comes thru the floor as that air also feeds the fan control solenoid, also under the dash.

General is right, you have a blockage somewhere, but you also have a leak as it requires pressure in the system to open the individual wheel valves and hold them above ~5-6PSI to keep them dumping. Since all tires deflated the only place they come together is that PCU under the dash, so disconnectinkg that line from PCU to wheels should fix this.

my controller was junk, i replaced it with 3 switches and a relay and control it manually as needed. Pretty easy.

 

[GeneralDisorder]

General is right, you have a blockage somewhere, but you also have a leak as it requires pressure in the system to open the individual wheel valves and hold them above ~5-6PSI to keep them dumping. Since all tires deflated the only place they come together is that PCU under the dash, so disconnectinkg that line from PCU to wheels should fix this.

I disagree. A *restriction* (as opposed to a complete blockage - so a partial blockage) will not allow the wheel valve to close above the pressure at which the line is unable to vent to zero *as fast* as the wheel can refill the line (to 5 psi differential). At 50 psi the flow rate is high enough to get the wheel valve closed again. Above 50 psi the flow rate of the restricted line is exceeded by the flow rate of the valve stem and wheel valve orifices, etc.

I had this problem due to an improperly crimped hose. Mine occurred above 75 psi at just one wheel. Same situation but he has a worse restriction resulting in 50 psi as the equilibrium point.

I just don't think the symptom indicates a leak. There might be one. I am just not convinced by *that symptom* that we are dealing with more than a flow restriction.

 

[Andy Barth]

Best way to try and track the restriction on that line? Hook to shop air and go hunting?

 

[Ronmar]

I disagree. A *restriction* (as opposed to a complete blockage - so a partial blockage) will not allow the wheel valve to close above the pressure at which the line is unable to vent to zero *as fast* as the wheel can refill the line (to 5 psi differential). At 50 psi the flow rate is high enough to get the wheel valve closed again. Above 50 psi the flow rate of the restricted line is exceeded by the flow rate of the valve stem and wheel valve orifices, etc.

I had this problem due to an improperly crimped hose. Mine occurred above 75 psi at just one wheel. Same situation but he has a worse restriction resulting in 50 psi as the equilibrium point.

I just don't think the symptom indicates a leak. There might be one. I am just not convinced by *that symptom* that we are dealing with more than a flow restriction.

he said they are dumping to 10PSI... it must be mostly blocked, as the system must build enough pressure to open the wheel valves in the first place... so air is coming from some place(leaking wheel valve, leaking PCU supply solenoid). a system open to atmosphere would have trouble building this pressure to even open the wheel valves to start the dump that would be sustained by a restriction... Its kind of a chicken and egg thing

 

[Ronmar]

Best way to try and track the restriction on that line? Hook to shop air and go hunting?

Disconnect the line at the PCU i mentioned earlier that feeds down to the dump valves and wheel valves, and using a blowgun on that line, give it a brief shot of 60 PSI air. That shot of air should open the wheel valves and all the tires should equalize at the same pressure. As long as you hold the blowgun sealed on the end of that line the tires should hold air. Have someone get under the truck and check the CTIS dump valves located beside the right side of the transmission for the front axle and over the rear axle(backside of brake component crossmember or the one behind that). any air coming out of the dump vent ports indicates a leak in the system. and as air leaks away, the dumps will release tire air to match the leak pressure loss. they are remote pressure regulators. they match whatever they see on their input to the output by flowing input to output or venting output(wheel side) to atmosphere. IE: the only air that can come out of those vent horns is tire air...

Now remove the blowgun from the line. That line and the dump valves should woosh briefly(half a second) as the pressure drops below the wheel valve cutoff pressure and cutoff the airsupply to the dump valve vents. If the dumps continue to vent air, there is a restriction between the end of that line where you applied air and the dump valves.

Since the axles are 2 separate systems Td together, in order for all four to deflate, it has to be something in common to both axles, and they are Td together right by the right rear of the transmission. so the line where you applied air, the PCU or its vent line are the only things in common with both axles.

a common issue with this type failure is the PCU vent line. When you open the control solenoid to vent the system, the PCU vents into that bottom plastic cover on the PCU. There is a line connected from there down to the floor, and insects sometimes fill that vent port behind the grill with mud/nest, blocking the line. a restriction here causes the situation that general described, basically the tires can feedback enough air thru the dump valves that a bad restriction will still keep enough pressure in the system to cause all the wheels to deflate after the controller cycles because it cannot vent the system cleanly below wheel valve cutoff pressure...

but the system must build enough pressure to open the wheel valves in the first place. With a bad enough clog, a leaking wheel valve or the PCU itself could start this ball rolling.

With the truck just sitting static, if you get under and put a rubber glove over the dump valve vent port, any inflation of the glove indicates a leaking wheel valve on the axle that dump serves. You can repeat this test on the wheel banjo bolt fittings on an individual whel to find the leaking valve.

 

[Andy Barth]

he said they are dumping to 10PSI... it must be mostly blocked, as the system must build enough pressure to open the wheel valves in the first place... so air is coming from some place(leaking wheel valve, leaking PCU supply solenoid). a system open to atmosphere would have trouble building this pressure to even open the wheel valves to start the dump that would be sustained by a restriction... Its kind of a chicken and egg thing

It dumped to 10 psi the first time. After put that manual manifold on it only the front right dumps and it stops between 50 and 60 psi. Held there over nighti just checked it. Other 3 are at 80

 

[Andy Barth]

Disconnect the line at the PCU i mentioned earlier that feeds down to the dump valves and wheel valves, and using a blowgun on that line, give it a brief shot of 60 PSI air. That shot of air should open the wheel valves and all the tires should equalize at the same pressure. As long as you hold the blowgun sealed on the end of that line the tires should hold air. Have someone get under the truck and check the CTIS dump valves located beside the right side of the transmission for the front axle and over the rear axle(backside of brake component crossmember or the one behind that). any air coming out of the dump vent ports indicates a leak in the system. and as air leaks away, the dumps will release tire air to match the leak pressure loss. they are remote pressure regulators. they match whatever they see on their input to the output by flowing input to output or venting output(wheel side) to atmosphere. IE: the only air that can come out of those vent horns is tire air...

Now remove the blowgun from the line. That line and the dump valves should woosh briefly(half a second) as the pressure drops below the wheel valve cutoff pressure and cutoff the airsupply to the dump valve vents. If the dumps continue to vent air, there is a restriction between the end of that line where you applied air and the dump valves.

Since the axles are 2 separate systems Td together, in order for all four to deflate, it has to be something in common to both axles, and they are Td together right by the right rear of the transmission. so the line where you applied air, the PCU or its vent line are the only things in common with both axles.

a common issue with this type failure is the PCU vent line. When you open the control solenoid to vent the system, the PCU vents into that bottom plastic cover on the PCU. There is a line connected from there down to the floor, and insects sometimes fill that vent port behind the grill with mud/nest, blocking the line. a restriction here causes the situation that general described, basically the tires can feedback enough air thru the dump valves that a bad restriction will still keep enough pressure in the system to cause all the wheels to deflate after the controller cycles because it cannot vent the system cleanly below wheel valve cutoff pressure...

but the system must build enough pressure to open the wheel valves in the first place. With a bad enough clog, a leaking wheel valve or the PCU itself could start this ball rolling.

With the truck just sitting static, if you get under and put a rubber glove over the dump valve vent port, any inflation of the glove indicates a leaking wheel valve on the axle that dump serves. You can repeat this test on the wheel banjo bolt fittings on an individual whel to find the leaking valve.

I do finally understand how the system works. Last night after I whipped up that manual setup from the other post to bypass the pcu, the tires aired up normally to 80 psi. I closed the supply line valve, opened the dump valve, valve whoshed, qr valve whistled briefly then it was good I thought. However the front rightis the one that's bothering now, it continued to release through the pass side dump valve until about 50 psi then it quit and held there. So the obstruction must be in the line for the front t right tire some place. I just don't know how to figure out where, I'm assuming there has to be t in it someplace that I should be able to open up and blow air through or just replace the line?

 

[Ronmar]

I do finally understand how the system works. Last night after I whipped up that manual setup from the other post to bypass the pcu, the tires aired up normally to 80 psi. I closed the supply line valve, opened the dump valve, valve whoshed, qr valve whistled briefly then it was good I thought. However the front rightis the one that's bothering now, it continued to release through the pass side dump valve until about 50 psi then it quit and held there. So the obstruction must be in the line for the front t right tire some place. I just don't know how to figure out where, I'm assuming there has to be t in it someplace that I should be able to open up and blow air through or just replace the line?

right front might be a sticking wheel valve refusing to seal. the right line goes from the dump valve thru the hub to the banjo fitting on the wheel stud. then thru a hose to the wheel valve. unscrew the banjo bolt on that wheel and confirm that it is indeed a banjo bolt and not a solid bolt(yes it has happened) and that it is not otherwise clogged. place the banjo end of the hose in a rubber glove and tape it to the hose, then manually inflate the tire to 80PSI using the Schrader on the wheel and confirm the wheel valve holds air to that pressure...

you can do the same blowgun type test at the banjo fitting, pressurize the line toward the wheel valve should open it, and when you release the pressure the banjo should woosh briefly as the wheel valve slams shut. you can also blow air back thru the hollow stud toward the dump valve to test for a restricted path. disconnect a different banjo and blow back thru that hollow stud to compare flow/resistance back thru the dump valve on a different wheel circuit.

 

[GeneralDisorder]

I was referring to the single wheel dumping to 50 psi with his manual setup. That's a partial blockage between the right front wheel valve and the QRV. It's not likely to be the diaphragm in the wheel valve since he already changed that.

You can test this easily. Tell the system to inflate to something like 80 psi. When the system shuts off and the QRV begins venting from the right front wheel - immediately loosen the banjo bolt at that wheel - this should instantly result in the wheel valve closing and the QRV venting stopping and will be accompanied by a release of pressure from the banjo you loosened. That is the pressure that's unable to escape due to the blockage being released and allowing the wheel valve to shut. If the tire continues to lose air from the line to the banjo and it still vents to 50 psi after you have loosened the banjo then the wheel valve is faulty and can't hold pressure above 50 psi.

As to the main issue with the system dumping pressure from all the wheels down to 10 psi prior to his manual conversion - there's pressure in the control line to the QRV's - where that's coming from is the question. When my truck did that after I reinstalled the wheel valves and controller that the previous owner removed it was dirty, sticking spool valves in the PCU that caused it.

95% of problems with the CTIS can be solved by rebuilding the wheel valves, rebuilding the PCU, and making sure the vent from the PCU is not blocked. Assuming you have a working controller. The rare case of a single wheel venting at the QRV down to a specific pressure is a partial blockage from that wheel to it's associated QRV. That will also manifest itself as an imbalance when the system tries to inflate - as you approach and exceed the equilibrium pressure where the tire can vent faster than the line to the QRV it will also not accept air from the inflate cycle as quickly as the other tire and you will get an imbalance where air flows more to the wheel without the partial blockage. Then when the system stops inflating the low tire will vent back to the equilibrium point till the wheel valve can close. When the controller does a pressure verification 60 seconds after the inflate cycle it will find the system too low and go back into inflate again. It will do this 10 times and then throw a fault code that can only be cleared by pressing emergency or cycling the controller power.

One thing that is quite useful with the stock controllers is to wire up the CTIS OFF circuit and lamp in the annunciator panel used by the arctic kit. This way you can cycle power to the controller without shutting down the truck or unplugging the connector. When coupled with switches to change the option pin states you can get several programs with different pressure settings for HWY, etc.