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post #1 of 40 (permalink) Old 10-05-2011, 06:27 AM Thread Starter
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CTIS in use questions

Hi Folks

I read a while back that tyres on the same axle are linked with the CTIS system, so one puncture means two deflations.

Assuming I've remembered that correctly,

If you got a big puncture on the highway and both deflate does the system (analogue on SBUs and digital on new stuff?) "know" to not dump the air tanks out of that hole? Or will you lose the brake pedal and the parking brake will apply as the tanks drain?

How do you change one tyre without two deflating?

If one debeads at low pressure they both will?

I can't for the life of me reseat my 395s using a bicycle inner tube to seal the gap during inflation. Can I use ether etc off the truck with the other mounted tyre still inflated, then mount it at my leisure?

Just wondering for future planned for (dreaming probably) purchase

Cheers chaps

Jason
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post #2 of 40 (permalink) Old 10-05-2011, 07:11 AM
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You are correct that the air pressure is based on the axle, not the tire.

My U500 had a pressure display, but to change pressure was still manual. You can't just say "stay at this pressure"

There is a separate air tank for brakes versus the CTIS. You will never lose your brakes due to a CTIS failure.

It is hard to lose a bead on CTIS trucks. They all come with beadlocks.

I never did change a tire. I assume if you disconnect the air hose, you would lose all pressure.
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post #3 of 40 (permalink) Old 10-05-2011, 07:19 AM
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From what I remember hearing from Greg S. (ex NA Unimog salesman) If a newer Mog (400-500) comes with CTIS it also comes with internal bead locks.

He was showing a demo of a snow cutter on a U500 and some one at MB messed up and did not install the bead locks with the CTIS option. He had the system at low pressure and de-beaded one tire and then they both went flat and de-beaded. They had to spend over a day digging to find the tires (cutting deep snow) and have them put back on the rims.

Long story short if you put CTIS on use bead locks.
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post #4 of 40 (permalink) Old 10-05-2011, 07:34 AM
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From the UGN International Sales material:

"4.5.3.1 Tyre pressure control system (code A30)
The tyre pressure control system is an electropneumatic system installed on the vehicle for tyre inflation and deflation on the front and rear axle.
The inflation pressures can also be changed while driving.
The tyre pressure control system is pneumatically actuated via auxiliary consumer connection 24 on the 4-circuit protection valve. The compressed
air for tyre inflation is supplied via a branch upstream of the 4-circuit protection valve (high pressure 18.3 bar)."

From the UGN U500NA Options List:

"Technical description:
Tire inflation/deflation at front and rear axle can be controlled separately or together,
LCD display at instrument panel
Tire air-supply line connected to tire via axle body; can be controlled while vehicle is in
motion
Notes/Product advantages:
Off-road operation: improved off-road capabilities, better traction and minimized surface
damage due to low tire pressure
On-road operation: improved handling, lower tire temperature and reduced tire wear at
high speed
Optimized tire pressure can be guaranteed in every situation: lower tire wear and higher
efficiency
Specifically applicable for fire-fighting, utility, and agricultural operations"

The attached pictures are from the U500NA options list and are not at all very good. I would have to assume that there must be a check valve somewhere in the mess to allow you to change a wheel without horror.
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post #5 of 40 (permalink) Old 10-05-2011, 07:42 AM
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Here's some SBU (U1550) stuff.

I've always wanted CTIS
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-Trev
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post #6 of 40 (permalink) Old 10-05-2011, 08:25 AM
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Quote:
Originally Posted by bigmellon View Post
...........

My U500 had a pressure display, but to change pressure was still manual. You can't just say "stay at this pressure"

............
"Stay at this pressure" is exactly the purpose of CTIS. But I think your saying on the U500 you can't dial in a new pressure setting and the system automatically fills to that setting. That wouldn't work well with tire heating at speed.

I find CTIS very useful. I change tire pressure frequently. For example going 35mph on washboard the perfect tire pressure makes a big difference in ride. That change just isn't going to happen without CTIS.

Have'nt changed a tire yet. I assume there's a valve on the hose end. If not I have hose clamps.

Apparently reinstalling the rubber bead lock is a special skill. I seem to recall that Charlie A's shop couldn't do it. I don't think it's doable in the field by mortals. I've never heard of the 395's with the bead lock debeading, however. I believe Charlie lost a bead when his truck was new and without beadlocks.
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post #7 of 40 (permalink) Old 10-05-2011, 09:08 AM
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A theoretical system can maintain any given air pressure as long as you have a temperature reference, ideal gas law and all that. If the system knows the volume and the temperature it can deduce pressure based on math and display the corrected value.

I would think that with this system you would have to put faith in the display and it's corrected value when airing up or down and not try to second guess it by trying to work from actual measures.
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post #8 of 40 (permalink) Old 10-05-2011, 09:20 AM
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There should be no math involved, the system monitors the pressure and let air out or in to maintain its set point.

As the tire gets hot with use the pressure rises and the system dump the air to atmosphere, as the tire cools and pressure drops it add air from the tanks.
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post #9 of 40 (permalink) Old 10-05-2011, 09:25 AM
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Originally Posted by sdo View Post
There should be no math involved, the system monitors the pressure and let air out or in to maintain its set point.

As the tire gets hot with use the pressure rises and the system dump the air to atmosphere, as the tire cools and pressure drops it add air from the tanks.
Not on a U500
The system doesn't want the tires bled as they heat up to highway speed. I don't believe the system ever dumps pressure automatically. It will add air.

Last edited by ddmiller; 10-05-2011 at 09:29 AM.
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post #10 of 40 (permalink) Old 10-05-2011, 09:57 AM
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The system in the U500 doesn't have a temperature sensor and thus is only accurate with a "room temperature" tire which makes sense if you follow the logic that you only air down for adverse traction conditions like ice, snow, slush, sand, loose loam etc which are all conditions in which you would not be traveling at highway speeds so tire heating would be minimal.

DDMiller is correct in that the system is designed to not automatically bleed off overpressure from heating. Airing down is simply a matter of creating a leak and letting physics do the rest. To maintain a tire with a puncture the system just runs continuously because the pressure high cut off limit is never reached.
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