Notes about my CTIS repair...

[ppillard]

I finally got my CTIS working. Happy day. It was failed when I bought it, and I *hoped* I could resurrect it.

A bit of reading on this forum lead me to jumper out the tank pressure switch and suddenly she came to life. So all I needed was a pressure switch.

Easier said than done. Replacement pressure switches Ranged from $400-$800 dollars. I was pretty sure I could find an adjustable pressure switch that would work for me, and so I did:

$12, adjustable in the range I needed, and stainless to boot. Not bad. I dialed it in to close at 95 psi and installed her. Fired the unit up and attempted to cycle the CTIS through the full gamut and ensure complete functionality.

The system started bleeding off every other second, and not doing it’s job at all. I disconnected the switch and scratched my head in annoyance. It took quite a bit of contemplation and I finally found an entry in the manual stating that cut-in pressure is 74 PSI. I adjusted my switch accordingly and suddenly the CTIS is happy.

I believe that at 95 psi, when the switch kicked on, the loss of pressure was enough to set the switch back off, and it only took a couple seconds to recover and kick the switch closed again.

Please someone chime in and clear up the mud as needed.

 

[Ronmar]

Well that switch probably does not have the hysteresis range to do the trick. The correct switch is special. It closes its contacts When the pressure goes above 115 PSI which is right before the governor cuts off the air compressor at around 120. This tells CTIS the wet tank is full and tht it can go to work.

The switch contacts stay closed until the pressure drops below 90 PSI where the contacts open. The pressure protection valve stops airflow out of the wet tank to CTIS at around 85 PSI, so when the contacts open, it tells CTIS it is out of available air.

The contacts stay open untill the presure once again exceeds the 115 PSI cut-in point.

Anytime the contacts are closed, CTIS knows it has air available... It is not going to work correctly unless you can find a switch with those cut-in and cut-out pressures...

 

[coachgeo]

Well that switch probably does not have the hysteresis range to do the trick. The correct switch is special. It closes its contacts When the pressure goes above 115 PSI which is right before the governor cuts off the air compressor at around 120. This tells CTIS the wet tank is full and tht it can go to work.

The switch contacts stay closed until the pressure drops below 90 PSI where the contacts open. The pressure protection valve stops airflow out of the wet tank to CTIS at around 85 PSI, so when the contacts open, it tells CTIS it is out of available air.

The contacts stay open untill the presure once again exceeds the 115 PSI cut-in point.

Anytime the contacts are closed, CTIS knows it has air available... It is not going to work correctly unless you can find a switch with those cut-in and cut-out pressures...

hmm.... could he/we plumb two or three switches in a row or on a home brewed manifold set at the different pressures?

 

[Ronmar]

You could do it with two switches and a double pole/2 contactor relay. Basically relay power would come thru the high switch set to close its contacts at 115-120 PSI to initially energize the relay. One set of contacts would provide the contact closure to CTIS. The second set of contacts would provide latching power to its own coil thru the low pressure switch that comes on at 85-90 PSI.

So as wet tank pressure increases from 85PSI, the low pressure switch closes its contacts, but because the relay is de-energized no power can reach the relay coil thru those contacts. As the pressure increases above 115, the hi switch closes its contacts sending power to and energizing the relay. one set of contacts tells CTIS air is available. the other set sends power to its own coil thru the lo switch contacts. As CTIS uses air, the HI switch contacts open as the wet tank pressure drops below 115PSI, but the relay still stays energized, powered thru its own contacts and the low switch contacts. The relay and contact closure to CTIS saying air is available stays energized until the air drops below 90, opening the low switch which de-energizes the relay. The relay open contact tells CTIS it is out of air.

SO 2 switches, T fitting to plumb them, relay and housing and some wiring/electrical skills. About $50? if you have the skillset...

Or you could to find a switch with a span adjustment. They are used on water systems but one of those probably would not do well with vehicle vibrations. Basically two adjustments, one for the cut-in pressure and the other determines how much the pressure has to drop before the contacts cut-out...

 

[coachgeo]

...

Or you could to find a switch with a span adjustment. They are used on water systems but one of those probably would not do well with vehicle vibrations. Basically two adjustments, one for the cut-in pressure and the other determines how much the pressure has to drop before the contacts cut-out...

this place may make what your suggesting. I've set a few parameters..... but should be reviewed left side of page

Pressure Switches

Gems Sensors Pressure Switches cover a wide range of applications from 2 to 6000 psi (40 mbar to 400 bar) in a variety of types including Vacuum, Differential, Specialty and General. With field adjustable or factory set options, these pressure switches are ideal for oil, liquid, air, gas...

www.gemssensors.com

 

[ppillard]

Question: Why do we need the CTIS system to wait until the air pressure reaches 120 psi? Why can't it just go to work a little earlier than previously expected?

 

[coachgeo]

Question: Why do we need the CTIS system to wait until the air pressure reaches 120 psi? Why can't it just go to work a little earlier than previously expected?

to prevent stealing air needed to operate the brakes. It needs to use air for both inflation and deflation; If I understand right., it actually takes pressure to open valves that allow the tires to bleed air for deflation...... and it takes air of course for inflation.

 

[Ronmar]

It only takes about 8PSI To open the wheel valves for deflation.

The reason for the high pressure point I think is to 1. tell CTIS that the air system is functioning normally/is capable. 2. To allow it to regularly top-off the pri/secondary brake tanks. CTIS and these huge tires are a tremendous load on the air system. For example, having timed it, it takes 3 seconds to empty the wet tank Into the tires when they are below 50 PSI. It takes 9-12 seconds to refill the wet tank at high idle-idle. By using this Hi low pressure switch scheme and inflating in pulses, It forces the wet tank to charge to full pressure regularly, which in turn charges the pri/secondary brake tanks regularly...