| Author | Message | ||
| Simon Ayriss (Design_dog)
Registered Member Username: Design_dog Post Number: 6 Registered: 3-2007 Posted From: 69.227.69.33 Rating: N/A |
How hard is it to isolate, chase down and find slow air leaks in the line or an air bag? And how hard is it to change an air bag on an MC8? If anyone knows someone who is in the Los Angeles area that is good at doing this let me know. My coach seems to take a while to build up air pressure. My Air Low goes off at between 90 and 100. It seems I have to rev up the engine for a while to get compressor to get up to 100. Once I am rolling down the road I am good but if I am too idle for long periods or air pressure is not built up enough then it goes down to 90. I don't think this is normal. Shouldn't I be able to start it up and at almost idle slowly air pressure will naturally build up to 100 and then valves will release excess? Is this normal? I think I have a slow air leak somewhere. It rides fine when I am moving but just seems like it takes too much effort to mark required pressure. How hard is it to replace an air bag? | ||
| john w. roan (Chessie4905)
Registered Member Username: Chessie4905 Post Number: 488 Registered: 10-2003 Posted From: 71.58.117.21 Rating: N/A |
How long does it take for one or more corners or ends of the coach to drop down after shutting off the engine? If it takes quite a while, you need to check the brake system first. If you can hook up a remote air supply, you then can check for air leaks in the system with a mechanics stethoscope and some liquid soap/water mix, without the noise of the engine. | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 112 Registered: 8-2006 Posted From: 207.231.81.22 Rating: N/A |
Good morning Simon, It sounds like your leak rate is excessive, or your air compressor is really worn out. Just as information, the air compressor is turning all the time the engine is running, and the governor loads or unloads it as presure requires. Your bus is equipped with an air protection valve which will not let any air go to the auxiliary systems, suspension, wipers, horn, etc. until there is about 90 lbs in the front and rear brake reservoirs, and the DD3 application reservoir. (The brake pedal is actually two valves in one.) I believe the leak rate allowed by the CHP is only two pounds per minute with the brakes released, and 3/minute with brakes applied, with the engine off. A bus that will not maintain full air pressure idling, (about 120-125) pounds, has a MAJOR problem, which will be easy to find. FIRST OFF, DO NOT GO UNDER AN AIR RIDE BUS WITHOUT BLOCKING IT SECURELY AT THE JACKING POINTS WITH QUALITY BLOCKING MATERIAL!!! Don't go between a tire and the fender when its at ride height either!!! Now, the easy part, get a large spray bottle, and a large bottle of liquid detergent. Make a rich mixture, 1/3-1/2 soap and water, open all the bay doors, and access panels, spare tire compartment will give you access to all sorts of air fittings, start the bus and rev it up in neutral until you get it in the air, BLOCK IT, shut the engine off, and look and listen... Windex doesn't work as well as dish soap because it is too thin, so if you don't hear it, start squirting air bags, tubing, fittings, and valves. Something will bubble. Someone will have to hold the brake pedal down to check for diaphragm leaks, while somebody else is out back. John's comments above are exactly correct, shop air is quieter, but with a leak rate faster than an idling compressor, it should be easy to find. I will be in LA this week with some free time, Tues, Wed, or Thurs, and would be glad to stop by, say hi, and take a quick look. Cell 916 257-0279. George | ||
| Kyle Brandt (Kyle4501)
Registered Member Username: Kyle4501 Post Number: 302 Registered: 9-2004 Posted From: 70.153.26.224 Rating: N/A |
A friends bus acted the same way. Turned out to be the unloader valve in the compressor wasn't working correctly. Changing the air governor didn't change a thing. Replacing the compressor fixed him right up. Get bus up to pressure & turn it off. Do a walk around, do you hear any leaks? It would have to be a big leak for the compressor to be unable to keep up. Once you hear the leak, you will know where to start with George's great leak detection advice. Good Luck! | ||
| Simon Ayriss (Design_dog)
Registered Member Username: Design_dog Post Number: 8 Registered: 3-2007 Posted From: 69.227.69.33 Rating: N/A |
Wow. You guys are the greatest. I appreciate all the help. It sounds like it is possible that it is the compressor. I noticed not so great pressure before but didnt hear any evident major leaks right off. It could just be a slow leak or possibly the brakes but something tells me maybe not the brakes. Could be wrong. And seems like too much of a drop for a slow leak. I will have to investigate more. How can you tell if you have a bad compressor? I'll try to find out more because its shouldnt take this long to build pressure to 110. It has to be a big leakage problem. or bad compressor. This is the one big thing that is bugging me about my bus. I mean, as I said, when I brought it home it ran fine once the pressure is up but takes to long and you have to rev it to get it up to 100 plus. After engine shut off it doesnt take long for it to drop down to 60. So something isn't right. Very frustrating when everything works great but you have an air leak. I am not knowledgable to check the compressor yet or brakes etc. and I dont have a shop compressor. What I do have is a few bugs on my bus and this is a major one. Drives me crazy when my engine runs great but my air pressure is bad. I cant take it out and drive it properly like that. Plus it takes to long to build pressure. But all good advice I will try to take another look. thanks. George - I may take you up on that if its not too much trouble, at any rate I admire just even the offer. Thanks a lot. Would be great to get a second opinion. Major thanks for all the input. I will try to figure more out as to isolating the problem more. (Message edited by design dog on March 25, 2007) | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 114 Registered: 8-2006 Posted From: 207.231.80.150 Rating: N/A |
Evening Simon, I just got my plans changed, and won't be in LA for a few days, but the offer still stands! Lets use a little logic here, and try and figure this out? If the compressor doesn't put out enough air to match the leak rate, the pressure won't build up. The faster the engine is running, the more air the compressor puts out, so... if it goes down the road at hiway speed & full air pressure, its probably leak(s), as mentioned above, an unloader stuck halfway would cause low output, especially at low rpm. Your telling us that it loses pressure quickly with the engine off, sounds like leaks. Leaks through relay valves don't make much noise, neither do small tubing leaks. The quick way to tell where it is is to fire up, rev up and watch the brake reservoir gauge. All the air SHOULD go to both brake reservoirs only, until the pressure reaches about 90 lbs. Then the protection valve should open and allow air to the auxiliaries. 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. Our money is on leaks.. George | ||
| ChuckMC9 (Chucks)
Registered Member Username: Chucks Post Number: 1100 Registered: 10-2003 Posted From: 66.167.95.221 Rating: N/A |
"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." Now THAT's a GREAT troubleshooting tip that I don't think I've seen before. THANKS, George! | ||
| Richard Bowyer (Drivingmisslazy)
Registered Member Username: Drivingmisslazy Post Number: 1871 Registered: 1-2001 Posted From: 75.108.73.45 Rating: N/A |
That George sure does come up with some GREAT information, doesn't he? Thanks, George. Richard | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 115 Registered: 8-2006 Posted From: 207.231.80.150 Rating: N/A |
Thanks for the thanks, guys!! About 90 lbs is where the protection valve opens. So if the bus won't get to about 80-90, the trouble is PROBABLY in the brake system or the first (wet) reservoir? If it gets a little over 90, the protection valve is letting air into the auxiliaries, and thats where to look next? Simon says (sorry) that it rides OK at hiway speeds, so the compressor is able to exceed the leak rate at high rpm, also he says it leaks down fairly quickly, which kind of rules out a bad compressor? Again, thanks for the thanks! George | ||
| Simon Ayriss (Design_dog)
Registered Member Username: Design_dog Post Number: 10 Registered: 3-2007 Posted From: 69.227.69.33 Rating: N/A |
Yeah. You guys are the greatest. George. no problems about reschedule. Just shoot me an email if you get down here sometime. I am 50 miles North of Los Angeles. Off the 101 fwy. Heck with great information like this, it's just what the doctor ordered to figure it out. I have not been able to fire it up and give it onceover again yet but plan to in the next day or so. But this is good. I think this helps. The logic is fantastic. This is all good information I need to help isolate my problem. Just to go back over again. What happens is this. I start the bus. I will show a slight pressure build but certainly not the way it should. Correct me if I am wrong but if I have close to zero leaks then even at almost idle it should build consistantly. Anyway. What it does do is shows slight pressure but by itself it will not go up to 90. Sort of lingers around 60ish and stays there with a bare build. SO. The only way to get it UP to above 90 is to Rev the engine which will slowly build pressure to 90. I have to leave it at rev to build it up to 100ish to get the LOW AIR light off. If I dont drive it or it is below 10mph then it will drop. Once I get it going above 20-25mph then it holds or builds. When I turn off the engine it drops quickly down to 60ish again. I checked for leaks before but figured I would hear something fierce with that drop. I think I have to fire it up and go through your advice again. I guess the easiest is build it up. and break out the soap water. start with the bags and follow the lines. I'll get a scope and listen real good this time. I really not sure what to do check the brake valves yet. But your making sense. It does go up and build pressure when the engine/ compressor is rolling along. Actaully fairly well. But as soon as I am less than 15mph and idle I'm doomed. Your right. Since it does and will ride at highway. That means the compressor could be bad but not completely. Arrgh. Sure would be nice to have a repair manual. Simon says I gotta take a look at that beast, read over George's advice, fire it up and look for a leak somewhere. And once again - Thanks! | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 994 Registered: 12-2000 Posted From: 74.12.78.76 Rating: |
Hold the phone. MCI of the MC8 vintage plumbed the single needle air gauge into what a spring brake man would call an unconventional place. The effects of a pressure protection valve won't be recorded the same way. Simon, you have leaks. You can't determine the condition of your compressor until you get the air system tight. So, get under there, take the safety precautions, find and fix and report back. happy coaching! buswarrior |
