Airing up an MC8

[Melbo]

When I start my bus the air builds up to 60 lbs in a couple of minutes but takes what seems forever to go from 60 to 90 so I can get moving. When I bought the bus I took it in and had stewart and stevenson mechanics mark all the air leaks they could find and they also said I need to replace the air compressor which I did. When I took it in this spring and had them service it and check the brakes I asked if there was anything I need to be aware of or have done and they ( after adjusting the brakes and replacing some parts ) said it was doing fine but need new start batteries which I have since replaced. When we go somewhere to eat it holds air so we can start right up moving however it does leak down some. So here is the question what is happening that the last thirty pounds takes so long to fill up when the air is all the way down after being parked for a week or so???  Any ideas or places to check would be appreciated.

Melbo

[JackConrad]

The first 60 lbs. is filling the main tanks.  At about 60-65 lbs., the pressure protection valve opens allowing the accessory tank to fill. Sounds like the leak is probably in the accessory tank or associated  equipment (wipers, air horns, etc.)  Jack

[gumpy]

Jack,

It sounds like his suspension is kicking in, and filling up the big air beams.

I'm still unclear on the sequence of tanks and the order in which they fill, but the scenario he describs is pretty much what I was seeing when my bus had been sitting for a month or two. I've got most of my suspension leaks fixed now, so I don't see it that often, but my accessories leak off quickly and my gauge goes to zero, but it only takes 5 minutes or so to air that up.

Maybe someone has a link to, or will post a detailed description of the order of events that occur in airing up the MCI systems.

craig

[JackConrad]

Craig,
    You are probably right. I am pretty sure the suspension system feeds off the accessory tank. We have our bus suspension changed to rolling lobe (eliminating those big air beams), I forgot about the air beams.  Jack

[Busted Knuckle]

Jack,
It sounds like his suspension is kicking in, and filling up the big air beams.

I'm still unclear on the sequence of tanks and the order in which they fill, but the scenario he describs is pretty much what I was seeing when my bus had been sitting for a month or two. I've got most of my suspension leaks fixed now, so I don't see it that often, but my accessories leak off quickly and my gauge goes to zero, but it only takes 5 minutes or so to air that up.

Maybe someone has a link to, or will post a detailed description of the order of events that occur in airing up the MCI systems.

craig

Craig,
I believe you have hit it on the head! All the MCI's I've ever worked on, on a regular basis (8's, 9's, 96A's and our 102A3) all have done this exact same thing! I have been told that after the main tanks hit 60 they open up to "fill the rest of the system" (that being aux tank, air bags/beams, and etc.). I've been told that since the bags do not deflate quickly that it is why restarts are so much quicker! This is what an "MCI Tech Support Employee" told me,and he went on to say on older coaches it's best (read cheaper) to live with it! I didn't make the rules, I just try to follow them! LOL!
Now on the other hand on my Setra's I can hit the lower switch and start it and I'm up over 120 PSI almost instantly, then as I put it in gear I return the suspension switch to normal ! (My dad hates it when I do it that way, as he prefers it to be all the way up before starting off down the driveway/road! ) FWIW BK 

[Melbo]

Thanks for the info that makes sense -- what is the cost of going to rolling lobe and having it done at a shop

Melbo

[ChuckMC9]

"If the air doesn't want to go above 90, probably the leak is in the suspension. If it won't reach over about 80, the leak is probably in the parking portion of the DD3 valve, or a brake valve or relay valve, maybe just a reservoir drain valve."

More related at:
http://www.busnut.com/bbs/messages/11/17225.html?1174886590

[jjrbus]

On my MCI5C I have changed the air leveling system to a manual setup. So have effectivly eliminated that as reason for the air coming up slowly. Past the first 60 lbs the air does seem  to take an excessive amount of time to reach over 100lbs.
It would seem that we need to use actual times in minutes and seconds to determine if there really is a problem. I will be starting my bus tomorrow and post the times (if I remember )

[Melbo]

I will be starting up from 0 as well tomorrow and I will note times in minutes and seconds if I can pay attention while it airs up. I am usually moving about checking stuff and securing stuff as it airs up. Once my bus hits over about 93 lbs or so it will release the brakes and I can drive. As I drive it builds pressure to 120 and does the purge as I'm driving. When I'm on the road it stays about 110-120. My only question was about the slow air up from between 60 and 70 up to the 90 plus mark so I can get on the road.

Thanks

Melbo

[gumpy]

Suggest you step outside and observe the height of your wheel wells above your tires as you start airing up, and again as it reaches the 60 lb area. I think you will see that it remains constant, and then starts coming up as it reaches that critical area. Use a tape measure, or a piece of wood that will just fit between the tire and wheel well, and watch the relative movement.

[jjrbus]

I had started bus earlier this week and it still held air. I started with 45lbs and went to 75 lbs where fast idle came on in 2 min. 85 lbs where low air goes off in 4 min. Will try again after bus sits and bleeds down to zero. I realize this is not much help but I did say I would post the times.

[Melbo]

Ok I aired my bus up and here is the time and pressure.

2 minutes it was at 30 lbs -- 3 minutes it was at 50 lbs -- 4 minutes 68 lbs with no fast idle

5 minutes 72 lbs no fast idle -- 6 minutes it was 72 lbs and the suspension started to go up

8 minutes 72 lbs and fast idle kicked in and suspension was going up visibly on a tape measure

9 minutes 75 lbs -- 10 minutes 77 lbs -- 11 minutes 79 lbs and the suspension was still going up

12 minutes 83 lbs -- 13 minutes 91 lbs -- 14 minutes 103 lbs and the suspension had gone up more than four inches

at 15 minutes it did it's first purge and was at about 120 lbs

So the suggestions posted were most probably correct that the air beams just have to get pumped up for the cycle to finish.

Thanks for all the info and I hope this helps someone else as well

Melbo

[jjrbus]

One way you could speed up your air time is to change to a manual leveling system. it is not a dificult or expensive job, but does take 1 to 2 days.

[gumpy]

Ok, so you've pretty much shown what we thought, but I'm concerned about the overall time of 15 minutes to air up completely. That seems like it's way too much time, especially since you had high idle on. 

I'm wondering if you have a large leak in your airbag or beams which would not only cause the suspension to leak down, but would also contribute to the overall long time for airing up.

If you leave your bus idling after it's fully aired up, how often does the purge valve cycle (how long does it take to leak down to cuting pressure)?

craig

[Hank]

Melbo,
I had the exact same symptoms as you with my 96A3 (which has air beams). After a lot of fooling around, I isolated the problem to the accessory circuit and sure enough, it was leaky windshield wiper control knobs. I noticed they would hiss while the bus was airing up but never thought it was enough of a leak to quadruple the time it took to air up. With that fixed, it airs up from zero to cutout in less than 5 minutes. It will be impossible to pinpoint leaks in the engine compartment with the engine running so air it up with shop air and get out the soapy water! For me it just a process of elimination.
Good luck,
Hank