[tobflyin]

Hi Folks,

I've searched the forums a bit but I know everyone has different air setups. I also watched the youtube videos which were quite helpful in understanding the basics but I'm trying to understand how my coach works as it has a different setup than the ones mentioned on the video.

From my understanding, I have dual 120v Thomas compressors up front, and then the engine driven air pump (no 12 volt compressor). When letting the coach sit for a day, the front compressor pressure drops to zero, but when I turn key on to check primary and secondary pressure it still reads anywhere between 95-110. I also noticed when we purchased the coach that when the engine was running, we would occasionally hear the front compressor come on. I inquired on this and the seller thought this was normal. 

So, my question is:

1. Should the front compressor run at all when the engine is running (assuming it has built up engine pressure).
2. I would assume there's a shutoff to keep air for the chassis when it falls below a certain point, is that the case? And at what pressure should it isolate?
3. If there isn't a shutoff, how could my front compressor gauge read Zero but my primary tanks read 100?

I am planning on fixing all the leaks and hooking it to shop air, but it would help to understand the systems a bit and fix other issues while I'm there. From what I have read, the older design with 12 volt compressor had different pressure settings for the 120v and 12 volt compressors (far less then engine pump), so they wouldn't kick on until it dropped to different given points. But, I think the 120v dual compressor setup is set to kick on around the same pressure as the engine compressor. This may explain the front compressor running with engine running, but wouldn't explain the front dropping to zero when the coach sits without losing a lot of air from the primary. Anyone with my vintage or perhaps newer coach have my type of setup?  I welcome any/all suggestions!

Thanks all,

[Richard]

Serious Kudo's to you. You obviously have studied the system, done the research, and carefully explained the behavior before you asked the question!

Bravo!!!!

Next, an infomercial. I prefer using the forum over phone calls. If we use the forum, you get the benefit of many contributors. Everyone learns. The information is here for the next guy or gal if they take the time to use the search engine. No one is going to flame you on this forum for any question, we maintain our decorum. If I help via phone call then only you get the benefit. That being said, stuck on the side of the road is another situation altogether.

I do not know the specifics of your coach or how a previous owner may have modified it. However following Newell logic and DOT mandated design, the answer to your primary tanks staying up, and the supply tank going to zero is easy. In the air system fed by the engine air compressor, the first device is a pressure protection valve that will divert ALL air flow to the primary and secondary brake tanks until the pressure reaches around 60 psi. At 60 psi, the PPV opens and all three tanks (sometimes more) fill at the same rate. The primary and secondary brake tanks are equipped with check valves to prevent air from flowing backwards from the tanks to replenish a leaking supply side.

Without knowing the setpoints of the thomas compressors, it is difficult to say if they should run when the engine is running or not. If the engine has topped off the brake tanks at 120, the Thomas compressor should not run. However, IF you have a massive leak in the supply side, it could leak down fast enough to trigger the Thomas based upon it's setpoint, before the engine compressor kicked back in at 90.

I believe, but this is not confirmed by me, that with the dual Thomas setup, that those compressors do supply air to both the supply ( slide seals, potty, doors, leveling) and the brake tanks. On the older coaches with 12V compressors there was a check valve to prevent the tiny 12V compressor from trying to inflate all those systems.

With your supply side going to zero overnight you have some work to do. Bubble solution and ultrasonic listening devices are your friend. Tons of posts on here about chasing air leaks.

[hypoxia]

The Thomas air gauge is just before a check valve, see the overall diagram in THIS thread.  Fairly good chance the water separator plastic bowl is cracked & leaking but it could be any fitting from the compressor to the check valve.  The bowl is available and if you do that you may want a new filter.  It is located just left of the gauge.  The filter/water separator in mine is SMC AFM20-N02C-CZ.   After start the Thomas compressor will come on if the levelling tank needs air.  Filter  Bowl

My chassis tanks lose air overnight but since it doesn't affect anything when I'm not driving I haven't considered chasing that leak.  The only downside I see is the amount of time it takes to build air when leaving a campground.

Water Seperator AFM20-N02C-CZ Electric Compressors Front Run Bay.pdf Size: 460.29 KB  Downloads: 3

SMC AFM20 Manual.pdf Size: 4.66 MB  Downloads: 2

Valid-Auxiliary-Compressors.pdf Size: 2.41 MB  Downloads: 0

Valid Aux Air Compressor Parts Breakdown Valid.docx Size: 622.08 KB  Downloads: 0

Pneumatic Tank Orientation.pdf Size: 82.64 KB  Downloads: 1

[hypoxia]

Just remembering, that check valve may be mounted on the input fitting of the levelling tank.

[RobSC]

On my coach, this is the behavior of my Thomas compressor and engine driven compressor.  

First of all, the pressure gauge at the Thomas compressor always reads 14 psi higher than the Aux digital gauge on the dash.  Not sure if one is just inaccurate or something else explains this.  I think they should read the same, but they never do.

With the engine not running, the Thomas compressor will cycle on when the pressure gauge at the compressor falls to 112 psi, the dash gauge reads 98 psi.  It will run until pressure reaches 125 psi, the dash gauge reads 111.

With the engine running, the engine compressor will cycle on when the pressure(s) falls to 100 psi according to the dash gauge.  The three dash gauges are usually pretty much equal between the Front, Rear and Aux.  I assume that the pressure protection valves are open between the three tanks and therefore they should be equal.  Since the dash gauge reads 100 psi, the gauge at the Thomas compressor reads 114 psi., which is still 2 psi higher than the 112 psi.  This being the case, the Thomas compressor will not cycle on.  The engine compressor will run until the dash gauges reach 120 psi.  The Thomas gauge will now read 134 psi.  So by design, I think the engine compressor is handling all of the air needs.  The Thomas compressor should not run while the engine is running.  On my coach, I think this is the case.

On my coach, the Thomas compressor will not supply any air to the front and rear brake tanks.  It only provides air to the Accessory and Leveling tanks.  If you are parked with the engine off and have leaks in the brake tanks/lines/fittings, they will eventually leak to zero.

Finally, if you have shop air this is the best source for troubleshooting as it provides air to all tanks.  It also allows you to hear major leaks since the engine and Thomas compressor are not running.  For small leaks, an ultrasonic tester is required.  I found the most leaks in the rear passenger side bay that houses all the Valid leveling and tag axle solenoids.

[tobflyin]

Wow, thanks all for this help.

Realizing I have leaks to resolve, I still want to make sure I understand the system. I understand the PPV will shut off to protect air to the critical systems. But, assuming chassis air doesn't drop to 60, then in theory it should stay open and still supply air?

One thing Richard mentioned which I didn't think about was the check valves at the primary and secondary tank, which would then suggest I could have gauges for those tanks reading high but no other air in any other system (unless 120v compressor turns on).

The pneumatic diagram is fantastic, thank you for that! I'm going to try tracing my coach and compare the diagram and then print it for quick reference.

It appears my Thomas compressors are hooked to a coil for cooling and then they go straight to the filter/water separator Jim mentioned. After the separator there is what I assume to be a pressure switch, followed by a manifold which has the gauge installed on the manifold. I'll have to trace where that manifold goes outside the compartment, assuming the leveling tank, and see if there's a check valve there.

While spending a few minutes today looking at it, prior to seeing your responses, I did locate a small leak at the door air release inside the front left compartment, and then found a larger leak on the curb side middle bay manifold that distributes to a bunch of solenoids. The manifold itself has a shutoff valve built-in, and that's what appears to be leaking. I'll take it apart and see if I can rebuild it with some new seals.

I'm sure there's a bunch of other leaks but your insight will help educate me on the systems. Nothing like troubleshooting details without understanding the higher-level concept!

[Richard]

Realizing I have leaks to resolve, I still want to make sure I understand the system. I understand the PPV will shut off to protect air to the critical systems. But, assuming chassis air doesn't drop to 60, then in theory it should stay open and still supply air?

Think of the PPV as a diverter valve, not a check valve. As air enters, all air goes through one of the PPV ports to the brake tanks, after 60 psi is reached, the other port opens to allow air to flow to the supply side. Both exit ports would remain open IF the system pressure is above 60. This is why there are check valves on the brake tanks to prevent the brake tanks from backfeeding a leaking supply side. For some reason, the DOT thought brakes were more critical than potties.

Run this little experiment to watch it work. Turn off the coach, turn off the 120V compressors. Continue to pump the brakes until the brake tanks read close to zero. Operate the door, or bleed the supply side until it is zero. Now crank the engine. You should see the brake tank gauges start to climb, but the supply should remain at zero. The brake gauges should climb until they reach around 60, then you should see them stall and the supply side start to climb. When all three reach 60 they will climb in unison. It's a sanity check to see if the PPV is operating as it should.

[hypoxia]

I added some Valid files to my post #3. Just trying to have everything in one post rather than all over the thread.