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Maybe a Problem?
I cranked the bus on Thursday the 27th. It had not been started in 31 days. That is the longest by far since I have onwed the bus. The longest time elapsed previously between start ups was 18 days and that only happened once. We were out of country for two weeks and when we returned it was just too cold outside to mess with it. I kept thinking the weather would improve but it never did. Thursday I was digging out from the snow so I warmed the engine with the Aqua Hot for a couple of hours and the engine cranked right up.
I let it run about two hours and the temps finally came up to about 190. Everything aired up nicely and I put it in reverse and drive just to make sure the brakes released and the transmission shifted.
Friday I left Ohio to try and beat the snow that was coming in Friday evening. I am glad I did because they have 5-6 inches of new stuff on the ground to add to the 5-6 inches we had on Wednesday. I need to be in Neosho, Missouri Sunday so I did not want to get stuck in Ohio.
After driving 4-5 hours we were driving through some construction with a concret wall on the passenger side. My wife was in the back and thought she heard the air exhausting more often than usual. I thought it was just the sound being magnified by the wall but sure enough it was releasing about every 20-30 seconds or so. The wall did help us hear it.
I stopped at a rest area in a very few miles and went outside. By that time is was not exhausting more often than normal but it did sound bigger and longer than normal. That is the only way I know how to describe it. My air gauges held steady at all times between 115 and 120. I tested my brakes and the gauges held properly then too.
I drove another 4 hours and I never did hear it releasing more often than usual.
Is this something I need to be concerned about? When it was doing this it was in the lower 20s outside but the bus had been running 5-6 hours and moving for 4-5 hours. I know the right thing to do is to go as far south as possible as soon as possible. Please do not remind me. 🙂
Thanks for all of your help.
DKO
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18 Replies
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DKO,
This may be semantics, but your post suggests you normally
hear air while driving. You should not. Apart from the occasional hiss as the
dryer purges at the end of a cycle you should not hear escaping air unless you
are intentionally doing something like lowering the bus to level it.Can you be more specific as to the circumstances, the noise,
its location and under what conditions you might hear air escaping? -
You are correct, Jon. Normally we hear nothing while driving. We were driving right next to a concret wall and the sound was amplified.
The sound I am talking about is when the air pressure releases when it is at top pressure. I do not know if it is the air dryer or not. I hear diesel trucks and busses with air systems do it all the time. Maybe I am not explaining it correctly. It is not air leaking or escaping. It is air being released or exhausting when the top pressure is reached, it seems to me.
The sound is near the drive axle. It has been normal for my bus for nearly 5 years. This was just louder and for a few minutes is was doing it more often.
Does that help?
DKO
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DKO,
I have no answers for you, but my bus did the same thing last weekend when I took it at for a short drive. Air purged every 20 – 30 seconds. When I returned home I let it sit running in the driveway for a few minutes and it did it then as well. The air was deinitetly coming from the area of the two rear axles. I have not had time to trouleshoot or reseach the issue yet. I am in the panhandle of Florida and temperatures were in the upper 50s at the time. My air pressure gauge remained at about 105 – normally it is at 115 – 120.
Brian
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My pressure was fine and right at normal but otherwise it is the same as yours.
Let me know if you find a solution. I will do the same.
DKO
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Below freezing temps, the air dryer trying to do it’s job with moisture in the system will freeze up the purge valve everytime. It should be heated and the element may have gone bad.
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Thanks for the heads up. The air dryer was serviced in February. I will be keeping and eye and ear on this.
DKO
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DKO, I suspect your system is just fine
Brian, I suspect you have a problem and it does need to be fixed. Our coaches are supposed to have a 120-125 PSI cut out pressure and a cut in pressure of 20 PSI less.
http://www.bendix.com/media/documents/products_1/compressorsgovernors_1/troubleshootingguide.pdf
That guide above should help you understand how the compressor and dryer are inter-related and give you enough understanding so you can do the repairs or “manage” them.
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Well, it looks like I do have a problem. The air releasing is sounding even “bigger” and longer and on my last trip it was exhausting every 10-11 seconds during one stretch. I believe I may be having a compressor problem. I have driven about 1500 miles since I first posted this and it seems to be getting worse.
I have an appointment to get it looked at when I go through Nashville next week. I will let you know what they find.
DKO
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No way to pinpoint where it’s coming from?
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DKO,
What are your gauges showing? They tell you a lot of information. Are you seeing them stay steady while the air is exhausting from what I presume is the dryer?
Your bus air systems may be working perfectly and what you are hearing as air escaping could be the compressor cycling often because of a problem somewhere in one of the systems. Your repair bill shrinks in inverse proportion to how much information you can relate about your problem. In fact if it is your aux system that is leaking air somewhere causing the cycling of the compressor Prevost is not where the bus needs to go.
What does your aux air gauge show? Do you see the cycling of gauges up and down to match the air escaping from the dryer?
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It sounds to me like the air is purging at or near the air dryer by the drive axle.
I think I mentioned earlier that the pressure on both gauges holds steady through the cycle and that is still true. I drove from Kansas to Ohio on Monday and watched the gauges very closely. They hold between 110 and 120 with the bottom one just slightly lower in pressure than the top one.
DKO
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What is unanswered is what might be happening with your aux system air pressure. That may not be the problem but I wouldn’t rule it out and I wouldn’t go to Prevost until I had.
As far as your gauges, you should learn what each one tells you and which system each one is reading. None of us can help you because your gauge locations or orientation may not be like anyone else’s. I suspect if you only have two on the dash they are your primary and secondary brake systems. If you do not have a dash mounted pressure gauge for the aux system you might have one in the steer compartment. If so see if that holds steady while your bus compressor and dryer are cycling. If it cycles with the level low set to the driving or road position your problem could be related to the suspension such as a ride height valve. Put it in a manual position and see if the cycling stops or continues. If it continues the suspension is not likely the issue.
The problem could be related to the compressor or dryer or anywhere else, but to isolate it use your gauges and some systematic process of elimination to at least know where it might be. Do you hear air leaking when the system is pressurized and you shut the bus off? Can you watch the gauges drop when the bus is turned off and if so, which system is losing air?
If the brake systems are not cycling up and down and the aux system is holding pressure maybe you can hear air escaping around the compressor. By you isolating the problem you save money, but you also start to develop a better understanding of your pneumatic system.
Everybody should understand from this that if the compressor is cycling frequently it is going to destroy itself in a relatively short period of time. Our compressors are designed for a 25% duty cycle. If they are working a greater percentage of time the compressor will not last.
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Still sounds like the dryer purge valve. Any chance it could be caused by the compressor unloader?
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This might be a real long shot but it happened to me several years ago in an old Monoco coach. On top of the Air compressor on the side of the engine sits a governor, the main supply line exits toward the front of the coach, it is held on with two bolts that may have loosened thus not allowing the compressor to shut down with the correct pressire. The only way the system handles the constant pressure is to constantly pop off. The mechinac at the time tightened the bolts and things went back to normal. Worth a try should be able to reach thru rear side door, Good luck.
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When I went through Nashville the first week of February they determined right off that my compressor was going bad. It was blowing oil pretty badly. The compressor itself was not terribly pricey but it was a huge job to change it.
They also serviced the air dryer. Although it had been serviced 11 months earlier it was bad from trying to handle the oil from the compressor.
When I left town and started south, the dryer purging was still loud and long and became worse over the next two months. I was going to be in Nashville a few days this week so I made another appointment. After listening to the the dryer purge they recommended changing the air dryer.
Now it sounds like it is supposed to when it purges. It also purges at more regular intervals. Of course, I am still chasing leaks but I feel better about making progress. I knew the sounds it started making in December were not normal.
Thanks for your help.
DKO
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