Foretravel Motorhome Forums => Foretravel Tech Talk => Topic started by: Don & Tys on August 25, 2014, 11:51:04 pm
Title: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 25, 2014, 11:51:04 pm
I replaced the isolator valve and all new associated fittings. I have a question about the normal behavior of the air compressor. My understanding is that once the purge valve releases getting its signal from the governor to do so, the unloader port also causes the compressor to stop building pressure until the air pressure drops to 20psi below its set point. For instance, the governor is set to 120psi, there pressure won't rebuild until the air pressure drops to 100psi, correct? It shouldn't start building again if it only goes down to say 110psi. I understand that if there is a leak internally, the gauge pressure may not track with purge valve cycling. Also correct? Of course a leak anywhere in the air ride system, air brake system, including any check valves and solenoids, this process will be hastened. Does this also hold true if you build air pressure, hear the purge, shut the engine down and wait awhile losing a few psi (say 5 to 7psi), restart the engine will the compressor start building pressure again even if the pressure has not reached the cut in? Right now, after changing the isolation valve, the air system will still cycle approximately every 9 to 10 minutes while idling at 700RPM, though the pressure as indicated by the dash gauges has hardly dropped (maybe 1 psi). Sounds like an internal leak in the (newly rebuilt) air dryer or a defective new (rebuilt) governor, unless the compressor unloader itself is involved. Since the behavior before the air dryer failed was as I described at the beginning of this post (to the best of my recollection), I am doubting that it is the compressor unloader that is the current problem, unless of course it was damaged by driving the coach in the bypassed condition. Since I don't have a lot of confidence in the rebuild of the air dryer unit, I am leaning in that direction. Thoughts, comments,? Thanks, Don
Title: Re: Normal coach air system compressor unloader behavior
Post by: Tom Lang on August 26, 2014, 12:56:43 am
What you describe at the top is how mine works. Start the engine, and air pressure builds to 120psi, at which point I hear an air release and the compressor stops. Nothing happens until the air drops to 100psi, at which time the compressor kicks in again. With everything working properly at idle, it will hold that 120psi for a very long time. I don't recall ever hearing the compressor kick in again except when I am testing the brakes or air horn, or airing up the tires. When airing the tires, I intentionally release air to get more pressure.
Title: Re: Normal coach air system compressor unloader behavior
Post by: Barry & Cindy on August 26, 2014, 01:26:14 am
Don, If your tire air chuck is connected to the wet tank, you could put an air gauge on a quick disconnect and learn the wet tank pressure. This would give you pressure readings of all three tanks. I don't know where you tire filler air hose is connected. On our coach it is hosed to the wet tank. I would think you correct in that if the front & rear brake tanks hold their pressure per your dash gauges, then if the compressor cycles, pressure is being lost in wet tank.
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 26, 2014, 12:57:22 pm
Thanks Tom. that is the what I thought it should be...
Barry. Thanks for the idea. Not sure where the tire fill is feed from. I think it is the wet tank thought, at least I know it heads back through the tunnel. I need to see if it on the air schematic. What I was thinking of doing to hel narrow the possible internal leak sources is putting an air gage on the governor unloader port. The Port right above (as installed on our coach) that one is labeled RES. which means reservoir, which I would assume would be the wet tank. However the isolator valve has a 1/4 inch nylon line which I believe runs back to the wet tank. The wet tank connection on the isolator valve acts on the valve and I believe opposes the unloader Port which goes up to the governor via a T on the other side of the air dryer. The other side of that T is the turbo saver valve, at least I believe that's what it is from viewing the PDF breakdown on the air dryer. I am wondering if it may be advantageous to put a gauge on both the reservoir port and the unloader port to be able to compare the readings... just spit balling here since I don't have a complete understanding of all these functions, though Neal's explanation in my other thread probably lays it all out for me once I have the context to understand it completely. In other words, though I think I am getting the gist of it, I don't have enough depth of understanding to say "if this condition is true, then this result must follow..." and be sure of my reasoning. Don
Title: Re: Normal coach air system compressor unloader behavior
Post by: Barry & Cindy on August 26, 2014, 02:27:00 pm
FYI
Our governor had an empty threaded hole that I assumed was for exhausting air. I did not like the idea that dirt or moisture could get into the hole, so I put a hose fitting in the hole and ran a hose down a few feet below governor.
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 26, 2014, 02:31:07 pm
I felt the same way... It is labeled exhaust and I put a brass "quieter" on it. Basically a fine screen pressed into a ⅛" pipe thread fitting. It seems to me that the purge port ought have a hose with some kind of mesh or something on the end. Mine was full of crud as discovered by the side of the road. Contributory? Not unlikely in my view. Don
Our governor had an empty threaded hole that I assumed was for exhausting air. I did not like the idea that dirt or moisture could get into the hole, so I put a hose fitting in the hole and ran a hose down a few feet below governor.
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 26, 2014, 06:38:53 pm
I installed the gauge on the reservoir port of the governor, started it up and let the pressure builds until cut off. Turn it off and let it sit for about 90 minutes. After that time the front tanks were still at 110 but the wet tank was down to about 60 50. Not sure what it means yet, but I think that the information is significant in the context of finding out why the compressor is cycling more often than it should. Don
Title: Re: Normal coach air system compressor unloader behavior
Post by: red tractor on August 26, 2014, 08:33:43 pm
Are you getting air coming out of the bottom vent on the dryer?
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 26, 2014, 11:40:54 pm
Air comes out of the purge valve for a few seconds when it reaches cut out pressure at 120 psi. Have checked with soapy solution and haven't found any leaks at fittings, though I haven't been able to get underneath to where the six packs are to check there yet. Don
Are you getting air coming out of the bottom vent on the dryer?
Title: Re: Normal coach air system compressor unloader behavior
Post by: John Haygarth on August 27, 2014, 12:10:58 am
Don, if the wet tank pressure is falling that fast maybe the check valve at Dryer output large hose is leaking back thru Dryer. I had that problem and found out that the FT mechanic over a year earlier had installed check in reverse. Once removed and cleaned it and with new check valves on the others tanks have been holding great. Other leaks as I have mentioned have now been fixed and air pressure stays for weeks at a time before falling slowly. Check the checks if not already done. JohnH
Title: Re: Normal coach air system compressor unloader behavior
Post by: Neal Pillsbury on August 27, 2014, 12:11:13 am
Don, Just shooting from the hip here as I don't have the air system schematic for your coach. But, in general, most of the FT 's are pretty much the same. Since your engine running air pressure cycling is 9 to 10 minutes, you already know that you can drive for months without excess cycling. Since with the engine off, the front and rear tanks stay up for 90 minutes and more, but the wet tank decays to 50 psig, you know that the check valves from the wet tank to the front and rear brake tanks are good. Since the HWH leveling systems and braking systems come off the front and rear tanks, then all of the HWH and braking components seem to be good too. There aren't many things left to be the source of the problem:
Governor, fittings and associated control air signal tubing
Compressor, fittings and associated control air signal tubing
Wet tank to air dryer, fittings and piping
Wet tank to front and rear brake tank check valves (valves might be on wet tank, might be on front and rear brake tanks)
Air dryer itself and associated fittings
Wet tank pressure relief valve
Wet Tank blowdown valve and fittings
Wet tank low pressure sender and fittings (dash low pressure alarm)
I don't recall anything else. If the Air Dryer (Tower) has no escaping air (fittings and relief/blowdown port), I'd start the engine and build pressure until the Air tower blows down. Then shut off the engine and soap bubble every line and fitting from the wet tank outward (except the fittings and lines AFTER the front and rear tank check valves).
It seems like it has to be outwardly detectable.
The only two "non-outwardly-detectable" possibilities would be:
The internal unloader of the compressor itself. That leak possibility would be hidden, and at this leak rate (90 minutes to decay 50 psig or thereabouts), it would be difficult to hear. Because it is internal, I don't know of a way to outwardly prove that leak possibility short of taking the wet tank control line off of the compressor unloader fitting and capping it (or inserting a valve and valving it shut) to see if that stops the bleed down of the wet tank.
A leak inside the air dryer tower that is feeding back to the compressor, so that once again, the leak path is internal to the engine air supply. I"d have to think about that one but I think that capping the small wet tank line to the isolator valve (or, once again, inserting a valve and valving it shut) would prove/disprove that possibility.
Wish I were there. Good luck, Neal
Title: Re: Normal coach air system compressor unloader behavior
Post by: John Haygarth on August 27, 2014, 10:55:47 am
Wet tank was dropping pressure because of the check valve between Dryer and this tank. Air was feeding back thru dryer and was not detectable.The valve in backwards did not help but reason for mentioning it is that many do not realise there is one there on that line. JohnH
Title: Re: Normal coach air system compressor unloader behavior
Post by: John Duld on August 27, 2014, 11:35:52 am
Don, Don't forget the wet tank drain valve behind the driver side rear wheels.
JD
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 29, 2014, 10:15:17 pm
Thanks John, I did soap that up and didn't see any bubbles or hear any air escaping from the wet tank drain valve. At this point I think it is most likely a check valve or internal in the Air Dryer (turbo saver, etc.). At least the air is getting clean and dried without too much extra wear on the compressor, so even if I can't resolve this over the "pit", it won't hobble our travels before I can address it in depth at our home base. Don
Don, Don't forget the wet tank drain valve behind the driver side rear wheels.
JD
Title: Re: Normal coach air system compressor unloader behavior
Post by: fkjohns6083 on August 29, 2014, 10:25:14 pm
Don ---- Dont know if this is any help or not, but on our 91GV, temperature makes a big difference on air sys leakage. It appears that when the temperature is higher, the boundary valve leakage is lower. This may also be a factor in the operation of other components such as the unloader. Have a great day ---- Fritz
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 29, 2014, 10:36:11 pm
Thanks for that comprehensive list! Our air system schematic is probably the same as yours. It was drawn by AAL (initials, I suppose) on 3/22/95 and the latest date on it is 7/28/95. It is a very useful document, but there are references to components and circuits which have letters and numbers not in the "Port Legend" These are presumably on another page or pages that wasn't included in the owner documentation. Some of them I can guess at (i.e.; Check valves on the air tank inlets are letter "I"). I wish there was an air line list that showed the numbers of the air lines like there is for the wiring. Anyway, internet access has been very sporadic up here in Canada and mostly limited to my iPhone, so posts with any detail are a pain to write (and read!). I have been studying the Air Schematic over the last week in my spare time and it is starting to become more familiar as I notice things I overlooked before. As I said earlier, it very useful but by no means comprehensive. I will take it to some Kinko's like place and make several copies of it including an enlargement (a la Bill Chaplin ^.^d ) to mount on a poster board for my "war room". This is one war I intend win, even if winning is only temporary :o I already know a lot more than when I started this process, and I am nothing if not persistent! Tahnks also for the troubleshooting suggestions... more to come! Don
Don, Just shooting from the hip here as I don't have the air system schematic for your coach. But, in general, most of the FT 's are pretty much the same. Since your engine running air pressure cycling is 9 to 10 minutes, you already know that you can drive for months without excess cycling. Since with the engine off, the front and rear tanks stay up for 90 minutes and more, but the wet tank decays to 50 psig, you know that the check valves from the wet tank to the front and rear brake tanks are good. Since the HWH leveling systems and braking systems come off the front and rear tanks, then all of the HWH and braking components seem to be good too. There aren't many things left to be the source of the problem:
Governor, fittings and associated control air signal tubing
Compressor, fittings and associated control air signal tubing
Wet tank to air dryer, fittings and piping
Wet tank to front and rear brake tank check valves (valves might be on wet tank, might be on front and rear brake tanks)
Air dryer itself and associated fittings
Wet tank pressure relief valve
Wet Tank blowdown valve and fittings
Wet tank low pressure sender and fittings (dash low pressure alarm)
I don't recall anything else. If the Air Dryer (Tower) has no escaping air (fittings and relief/blowdown port), I'd start the engine and build pressure until the Air tower blows down. Then shut off the engine and soap bubble every line and fitting from the wet tank outward (except the fittings and lines AFTER the front and rear tank check valves).
It seems like it has to be outwardly detectable.
The only two "non-outwardly-detectable" possibilities would be:
The internal unloader of the compressor itself. That leak possibility would be hidden, and at this leak rate (90 minutes to decay 50 psig or thereabouts), it would be difficult to hear. Because it is internal, I don't know of a way to outwardly prove that leak possibility short of taking the wet tank control line off of the compressor unloader fitting and capping it (or inserting a valve and valving it shut) to see if that stops the bleed down of the wet tank.
A leak inside the air dryer tower that is feeding back to the compressor, so that once again, the leak path is internal to the engine air supply. I"d have to think about that one but I think that capping the small wet tank line to the isolator valve (or, once again, inserting a valve and valving it shut) would prove/disprove that possibility.
Wish I were there. Good luck, Neal
Title: Re: Normal coach air system compressor unloader behavior
Post by: Don & Tys on August 29, 2014, 10:56:22 pm
Thanks for the idea Fritz, though as far as I can tell, the behavior of the compressor and motor is pretty consistent across a wide range of temperatures and conditions. Don
Don ---- Dont know if this is any help or not, but on our 91GV, temperature makes a big difference on air sys leakage. It appears that when the temperature is higher, the boundary valve leakage is lower. This may also be a factor in the operation of other components such as the unloader. Have a great day ---- Fritz