Air System Mystery

Topic: Air System Mystery (Read 1924 times) previous topic - next topic

Air System Mystery

September 22, 2023, 01:35:21 pm

It all started when I took my coach into NIRVC in Lewisville Tx. For routine fluid and filter service.
Along with engine and transmission service and asked them to change the desiccant filter on the air dryer.
I had it changed on initial purchase 06/2021. Since then I've put a little over 10k miles in the last two years with no problems from the air system. When I picked up the coach it was noted by the tech that the desiccant filter was changed and tested for leaks. None found. OPs normal. When preparing to leave I noticed it seemed to take a little longer than usual to build air pressure. Being pressed for time due to work constraints I drove it to storage. It's mostly all highway so the RPMs were up and everything seemed normal once under way. When I arrived at the storage facility and proceeded to park inside the building I became aware that there was a problem. It takes a couple of approaches to get lined up and after several brake applications with the engine at idle speed the air pressure was low enough to turn on the red low air pressure lights and took an usually long time to turn them off, never really getting above 75psi until idling the engine above 1000rpm. After parking I called the service center and told them I needed to bring the coach back that I think it has an air leak and explained the situation. I took about a week to get it back to them due to a holiday weekend. They called me and said the purge valve was leaking and due to the age of the coach the tech recommended replacing the dryer. It's a Haldex 34100. It wasn't leaking when I dropped it off initially. They said their supplier had them on back order and didn't know when they could get one. I then scoured the internet and could not find one anywhere. So I suggested buying a rebuild kit and rebuilding the one I have. I could get those parts no problem. And since my coach was now being held hostage due to the tech not wanting to sign it off because of the air building to slowly when I mentioned that I would just pick the coach up. 😡
I didn't want to pay $200 an hour shop rate for something that I could do myself.
The service advisor agreed that was the next best option. So I delivered the rebuild kit and was called and told that it didn't fix the problem. And the tech noted that they couldn't find any leaks. I asked if they double checked the filter installation since that is when the problem started. They said yes. They said they were unable to blow shop air through the air dryer so there is a restriction in the air dryer and it has to be replaced. Something that is impossible to find right now. When I asked. If you can't blow shop air through the air dryer how am I able to get air to the brake system? The answer. You need a new dryer. 😡
So $700 labor and a $100 for a rebuild kit I picked up the coach and drove it back to storage.
I asked the tech if they thought that maybe the unloader valve in the compressor could be stuck partially open. Again the answer was until a new dryer was installed no further trouble shooting could be done. I won't be taking my coach back there. EVER!

Here are the symptoms.
At idle the air pressure is extremely slow to build and won't go above about 75psi.
At an idle above 1000rpm it will build air to normal pressure of 125psi.
Then if you let it idle again the pressure will slowly start dropping like there is a leak.
Building the pressure to 125 and shutting the engine off there are no leaks. The air pressure holds.
Crank up the engine and let it idle the air pressure will start to drop.

Ideas?

Thanks,
Robert

Re: Air System Mystery

Reply #1 – September 22, 2023, 02:14:45 pm

Quote from: Res777 – September 22, 2023, 01:35:21 pm

Here are the symptoms.
At idle the air pressure is extremely slow to build and won't go above about 75psi.
At an idle above 1000rpm it will build air to normal pressure of 125psi.
Then if you let it idle again the pressure will slowly start dropping like there is a leak.
Building the pressure to 125 and shutting the engine off there are no leaks. The air pressure holds.
Crank up the engine and let it idle the air pressure will start to drop.

You present us with some interesting symptoms. It certainly sounds like a substantial leak...but the leak is only "noticeable" when the engine is running at idle (above idle the air compressor is able to compensate for the air pressure loss). So the question is: what avenue for leakage is only available when the engine is running? My first assumption would be that the leak must occur upstream of the wet tank (or whatever is the first tank on your coach located down stream from the dryer). If this assumption is correct, the main suspects would be the dryer, the air compressor, and the hose that connects them. Checking the hose is easy enough - you should be able to feel a large air leak with your hand. If the hose is intact, then the dryer would be next on my list. If you feel like rounding up a few parts, you could bypass the dryer and see if that solves the problem. If it does, then you would need to figure out how and why the dryer is leaking pressure. If you still have the leak with the dryer bypassed, then it must be a malfunction in the air compressor.

The link below will explain what you need to bypass your dryer:

Air Dryer Bypass Demo

Re: Air System Mystery

Reply #2 – September 22, 2023, 02:33:39 pm

I agree trying what Chuck has said. At least you will be able to write off if the dryer is bad or not.

Do you have HWH Active Air or do you have regular mechanical height controls?

I would try airing system up to 125 psi. shut engine off and then turn key back on. Walk around and see if you can hear any air leaking.

Re: Air System Mystery

Reply #3 – September 22, 2023, 05:28:36 pm

Well, the quandary that I have is this. I do need to verify that it's not the compressor. Bypassing the dryer would do that. What I don't understand is why the brake tanks hold air with the ignition off. But they will start to lose air with the engine. Idling the number two tank goes down first followed by the number one tank. Which is another weird thing because I would think they would both go down together. I really have no way of checking the wet tank there's no pressure indicator on that tank to my knowledge. On the dash I have pressure number one and pressure number two gauges. On the air schematic It does not show those pressure gauges. So I'm assuming number one is the front brake tank and number two is the rear brake tank with no pressure indication on the wet tank. Since both the wet tank and the air dryer, have one way check valves. It doesn't make sense how air could flow backwards. Or what starting the engine would open to allow it to flow backwards. I'll attach a couple of pictures about what I'm talking about. The first picture is when the engines idled up and I can get it up to pressure. Then I noted the time and just let it sit at idle. The second picture is after it's been sitting at idle for five minutes.
I asked the tech if he ran a test to verify the compressor was putting out rated pressure at idle. They said no that they would have to buy fittings to do that. I found that statement interesting because how would they ever troubleshoot a compressor?

Re: Air System Mystery

Reply #4 – September 22, 2023, 05:34:43 pm

I'm not 100% positive, but I don't think NIRVC generally does any engine work other than possibly fluids and filters. They probably send non-routine-maintenance work out to a Spartan or Freightliner chassis shop.

Mike R. might know of a better chassis service option in the DFW area.

Re: Air System Mystery

Reply #5 – September 22, 2023, 05:38:36 pm

Res777

I am thinking it is possible something in the Hwh is causing the problem. Read my post above and let us know what you find out.

Re: Air System Mystery

Reply #6 – September 22, 2023, 06:16:14 pm

Quote from: turbojack – September 22, 2023, 02:33:39 pm

I agree trying what Chuck has said. At least you will be able to write off if the dryer is bad or not.

Do you have HWH Active Air or do you have regular mechanical height controls?

I would try airing system up to 125 psi. shut engine off and then turn key back on. Walk around and see if you can hear any air leaking.

I think mechanical. I'm not sure how to tell the difference. It's an '08. It has an HWH that controls the airbags to level, raise, lower and set travel mode. I'll take your suggestions and next time I'm at the coach. I'll bring it up to normal operating pressure turn the engine off and then turn the key back on and see what happens. I'm assuming that by turning the key on that activates, the HWH system. Split in the air pressure between the two brake tanks is kind of mind-boggling. They both have check valves that shouldn't allow that to happen. I think. 😂

Re: Air System Mystery

Reply #7 – September 22, 2023, 06:29:06 pm

Check the lanyard/drain valve on the wet tank to make sure they didn't open it and leave it open. Then ditto on the front and rear brake tanks just to be sure.

Re: Air System Mystery

Reply #8 – September 22, 2023, 07:48:24 pm

I have drains valves in the wheel wells on the drivers side. Three in the front and one in the back.

Re: Air System Mystery

Reply #9 – September 22, 2023, 09:06:22 pm

Quote from: Chuck & Jeannie – September 22, 2023, 02:14:45 pm

You present us with some interesting symptoms. It certainly sounds like a substantial leak...but the leak is only "noticeable" when the engine is running at idle (above idle the air compressor is able to compensate for the air pressure loss). So the question is: what avenue for leakage is only available when the engine is running? My first assumption would be that the leak must occur upstream of the wet tank (or whatever is the first tank on your coach located down stream from the dryer). If this assumption is correct, the main suspects would be the dryer, the air compressor, and the hose that connects them. Checking the hose is easy enough - you should be able to feel a large air leak with your hand. If the hose is intact, then the dryer would be next on my list. If you feel like rounding up a few parts, you could bypass the dryer and see if that solves the problem. If it does, then you would need to figure out how and why the dryer is leaking pressure. If you still have the leak with the dryer bypassed, then it must be a malfunction in the air compressor.

The link below will explain what you need to bypass your dryer:

Air Dryer Bypass Demo

I'm going to build a dryer bypass to check the compressor and as a backup. I have a question about this. What do you do with the control line from the governor? Isn't that what tells the governor the cutout pressure has been reached? In other words without this control line sensing pressure will the unloader work properly?

Re: Air System Mystery

Reply #10 – September 22, 2023, 09:56:04 pm

Since you only have 4 air tanks you have mechanical height control . You could have bad mechanical level sensors.

Re: Air System Mystery

Reply #11 – September 22, 2023, 10:37:07 pm

Well. I guess at this point I'll start at the compressor and work forward. 🤷🏻‍♂️
I'm really surprised a so called "tech" couldn't chase this down. My dad who owned an automotive and truck repair business my whole life called them "parts changers".
It's funny when they show up with wife and kids in tow and board a flight, they expect me to get them safely without excuses to there destination. But I'm unable to spend $800 to get an air problem diagnosed.

Re: Air System Mystery

Reply #12 – September 22, 2023, 10:42:55 pm

Quote from: Res777 – September 22, 2023, 09:06:22 pm

What do you do with the control line from the governor? Isn't that what tells the governor the cutout pressure has been reached? In other words without this control line sensing pressure will the unloader work properly?

This is explained in the Air Dryer Bypass Demo link. You plug the control line that goes from the D2 governor to the dryer at the dryer end of the line. Use a flare plug screwed into the end of the hose. Where is your D2 mounted? Is it bolted directly to the air compressor or remote mounted on the side of the engine compartment? There is a line from the wet tank to the D2 that tells the D2 when air system cutout pressure has been reached. The D2 sends then a "unload" signal to the air compressor either directly (if bolted on the compressor) or through a small air line (if mounted remotely). The air compressor should continue to operate normally with the dryer bypassed.

Re: Air System Mystery

Reply #13 – September 22, 2023, 11:02:21 pm

I think it is mounted on the compressor. Honestly I haven't gotten my head that deep into the engine bay. That's coming. I thought I was paying someone to do that. My bad. So if I understand you correctly. I need a three way fitting. Compressor outlet to coach feed and control line. I guess my confusion is. As soon as you bypass the dryer. That control line coming from the governor is reading nothing. Help me understand this.

Re: Air System Mystery

Reply #14 – September 22, 2023, 11:22:23 pm

Quote from: Res777 – September 22, 2023, 11:02:21 pm

I guess my confusion is. As soon as you bypass the dryer. That control line coming from the governor is reading nothing. Help me understand this.

You are looking at the communication flow backwards.

The small line running from the D2 governor to the air dryer transmits a pressure signal from the D2 to the air dryer. This signal tells the the air dryer when to open the purge valve on the bottom of the dryer. There is no communication from the air dryer back to the D2.

The D2 only has one pressure input source. It is marked RES on the D2. The line that connects to the RES input comes from the wet tank. The other ports on the D2 are the signal (control) port that sends signals out to the air compressor and the air dryer, and the exhaust port.

Re: Air System Mystery

Reply #15 – September 23, 2023, 05:12:57 am

Quote from: Chuck & Jeannie – September 22, 2023, 11:22:23 pm

You are looking at the communication flow backwards.

The small line running from the D2 governor to the air dryer transmits a pressure signal from the D2 to the air dryer. This signal tells the the air dryer when to open the purge valve on the bottom of the dryer. There is no communication from the air dryer back to the D2.

The D2 only has one pressure input source. It is marked RES on the D2. The line that connects to the RES input comes from the wet tank. The other ports on the D2 are the signal (control) port that sends signals out to the air compressor and the air dryer, and the exhaust port.

Okay. Now I understand. Thanks for taking the time to explain that.

Re: Air System Mystery

Reply #16 – September 23, 2023, 09:30:28 am

Robert,

You ask some good questions in Reply #3, for which I have no answers.

It is indeed curious that your brake tanks hold pressure with engine off, but the pressure drops when your engine is idling. It is like you actually two problems. First, something in the air system is leaking pressure when the ignition switch is ON. Second, when the leak is "active" the engine air compressor cannot keep up with the pressure loss unless the engine is running at high RPM.

You are also correct saying that the system should have one-way check valves that prevent air pressure from flowing "backwards" from the brake tanks, wet tank, and air dryer. Pressure should NOT be able to flow from the air tanks back to the dryer and engine air compressor. I will say that it is not unusual (at least on older coaches like mine) to find these check valves full of trash and stuck open, so that is one possibility.

Your 2008 Nimbus air system is probably more complicated than that found on our older coaches. If you could upload a copy of your air system diagram, it would help other Forum members visualize what is going on, and perhaps come up with some answers.

Re: Air System Mystery

Reply #17 – September 23, 2023, 12:07:45 pm

It sounds like everything worked until they changed the air dryer filter.
I would start at that filter.
Did they replace with the correct filter?
If they did I think I would change that filter again. Only replace with an original equipment filter, no aftermarket part.
If they put a rebuild kit in your dryer was the complete kit installed?
On the dryer on my coach the turbo boost saver valve was difficult to remove and replace if you were not familiar with it.
If that valve is leaking it will make it hard to build air pressure.
Just something to think about.

Re: Air System Mystery

Reply #18 – September 23, 2023, 12:38:53 pm

Quote from: John Duld – September 23, 2023, 12:07:45 pm

It sounds like everything worked until they changed the air dryer filter.
I would start at that filter.
Did they replace with the correct filter?
If they did I think I would change that filter again. Only replace with an original equipment filter, no aftermarket part.
If they put a rebuild kit in your dryer was the complete kit installed?
On the dryer on my coach the turbo boost saver valve was difficult to remove and replace if you were not familiar with it.
If that valve is leaking it will make it hard to build air pressure.
Just something to think about.

I thought the same thing. I've ordered another filter and was planning to start there. In my world of aviation there is the saying that "if you do something that has unexpected results, undo the last thing you did".
I will start there. If that doesn't work I have ordered the parts to build the EDB mentioned here to rule out a problem with the compressor or unloader valve and/or the governor. I purchased and gave to them the parts to rebuild the dryer I have. I wanted to pick it up and do it myself after watching a video on YouTube it looked very straightforward. Unfortunately at that point the shop was holding my coach hostage by saying the tech would not sign it off vai-se it was building air to slowly. 😡
So I furnished a complete lower unit kit consisting of a new purge valve, check valve and springs/ turbo valve and pressure relief valve. The kit included all necessary o-rings and gaskets for the job. Was it done correctly?
Who knows at this point? They installed the rebuild kit and said it didn't fix the problem. They said they could air the coach up with shop air but could not get shop air through the dryer. That didn't make sense to me because I can get the coach to air up by idling the engine at 1100 to 1200 rpm. It just doesn't hold pressure when you let it idle back to normal 600 rpm. I had no air leaks I was aware of before they changed the filter.

Re: Air System Mystery

Reply #19 – September 23, 2023, 12:41:03 pm

Air governor

Re: Air System Mystery

Reply #20 – September 23, 2023, 12:55:02 pm

Quote from: Chuck & Jeannie – September 23, 2023, 09:30:28 am

Robert,
Your 2008 Nimbus air system is probably more complicated than that found on our older coaches. If you could upload a copy of your air system diagram, it would help other Forum members visualize what is going on, and perhaps come up with some answers.

Since only 4 air tanks the air system is the same as older ones with slides.

Air tanks are Hwh, front, rear, and wet
The hwh tank does the slides and possible other air use items.

Re: Air System Mystery

Reply #21 – October 18, 2023, 07:28:23 pm

Res777,
Any progress?

Re: Air System Mystery

Reply #22 – October 19, 2023, 10:11:12 am

There are check valves into and exiting the dryer
. You should be able to remove the dryer filter with almost zero air loss. Try that first . Next change the d2.
Both are active when running of course.
A small fault in the d2 would do this if the unloader is stuck or has a small flaw.
Not finding these to be faulty , spray some Wd into the unloader line. To lube and clean the valve.

Re: Air System Mystery

Reply #23 – October 21, 2023, 11:08:53 am

Sorry for no updates. I really didn't have much to update on. I did build a dryer bypass. So now I know for sure the dryer is not my problem because when I bypass the dryer, I still have the same problem. Since I did that I really haven't done anything else. I retired from work a week and a half ago and before I could get any more work done on the motorhome, my wife and I left for Brazil and we won't be back until early November. But I did read in a troubleshooting guide that if you can see the air loss on the air gauges on the dash. To look from the wet tank forward. That it's most likely not a supply issue that it could be a brake leak or what I suspect even more is a leveling valve or a six pack leaking. I just haven't had the time to get up under it yet with a bottle of soap and start looking for leaks. When I use the leveling mode of the HWH system, it doesn't seem to be leaking, so that's what makes me suspect the leveling valves because the leak seems to be worse when it's in travel mode, which is when the leveling valves are active. When I get back to the states, I need to take it down to Nacogdoches for some other unrelated work and I may just get them to attack it. Either way I'll post what I find was the cause. I just needed to get it out of the shop where it was at because they couldn't diagnose it. They just wanted to start replacing parts, they were gonna start with a $550 dryer which wouldn't have fixed the problem. I didn't really feel like paying them $200 an hour for changing parts.

Re: Air System Mystery

Reply #24 – March 26, 2024, 02:24:10 pm

What ended up being the problem?

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