BCM Community
Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: Melbo on March 17, 2019, 04:34:24 PM
-
I seem to be having an air issue. I'm not sure I can quantify it exactly. The bus airs up normally most times. After it is aired up when I shut it off I don't hear any air leaks. Sometimes it seems to take longer than normal to air up so I will turn it off to see if I hear any air leaks and there never are any I can hear. Sometimes when I am driving and stop at a light or have to wait with my foot on the brake the air pressure seems to drop BUT most times it doesn't. I have run the air down with the brakes and watched it rebuild and it seems to do that like it should. When driving on the highway it runs up to 120 then drops to just over 100 and then runs back up in a normal cycle. Just looking for pointers on what I might be missing or where I might check. I has been about 10 years since I replaced the regulator but when I did that it was obvious that it was not working correctly.
Thanks
Melbo
-
Unloader valves are starting to carbon up would be a good guess
-
Thank You Clifford,
Is that a repair or replacement situation. It looks like I have a Bendix air compressor so it should not be hard to source one and just replace it. It is a lot more accessible than the one I did on the 8V71.
Melbo
-
NAPA has them in a kit with two that you will need. Depending on which compressor you have. They are easy to replace. Two bolts 7/16 IIRC on top where the air inlet from horn comes to the top of compressor. They are inside that. Push up on them with needle nose pliers and pull them out reverse to put in the new ones. :)
-
Pull the discharge line if it's getting oily it is only a matter of time before you need a compressor and if you have a separate air cleaner for the compressor check it
-
As Clifford says, disconnect compressor outlet line. Also if it has a ping tank or muffler, drain it to see if you are getting significant oil. Have you been getting oil at the air tanks or drier if you have one? Keep in mine that an air compressor is like an engine. Any oil getting past the rings goes to the air output instead of into exhaust system like a car.
-
Clifford
Yes I do have a separate air filter and that was the first thing I checked. I will pull the discharge air hose and see what kind of air I have there and if it is oily. If I need to replace the compressor it needs to be timed when installed. It is also necessary to remove the fuel pump. I'm hoping that it is a repair per Dave but I will soon know and I will update.
Thanks for the info
Melbo
-
Time the air compressor? Never heard that before. Is that for vibration control?
Jim
-
Mel has a Cummins engine with the PT pump they do need to be timed the pump is driven off the compressor
-
Everything in a row ...
-
Well I pulled the discharge line and it is not oily and seems to have pretty good air pressure. The unloader parts for a TU-FLO 700 bendix seem to be readily available however I am unable to find very specific instructions for the replacement of the parts. Dave's instructions are about what it says in the manual. I guess I will investigate further. Any hints or additional advice is greatly appreciated. The compressor is very accessible so I think it would be a pretty basic procedure. I have one more test I want to do to be sure that the recovery is within the parameters for a bendix 700 series.
Thanks for all the help
Melbo
-
I took the bus out for a cruise yesterday. It aired up like normal and I did an air recovery test per the Bendix spec sheet and it recovered just fine. Took it to the truck wash and fuel stop and filled it up with water getting ready for a trip to Rocky Point. Then I did the air recovery test again after it was all warmed up. The air kept up and operated normally when driving but when parked it was a little slower than the Bendix specs.
The air compressor is supposed to recover from 85psi to 100psi in 40 seconds. When I first started it was close to 30 seconds but after driving about and having it all warmed up it was closer to 50 seconds.
So here is my question. What would cause the recovery rate to change. Is it the unloader valves as Clifford mentioned or is there something internal so the compressor needs to be replaced.
Just looking at my options here.
Thanks
Melbo
-
Unloader valves or D2 air governor is possible also. Might replace it since they are only about 12.00 at NAPA. I keep 2 in the bus in case I need them. :)
-
You are doing the test by airing the coach up all the way, then reducing pressure via the brake pedal down below 85 and start timing as it comes up past the mark, to conduct the test?
Parking brake in the same configuration for both tests?
Governor failure usually is found to make cut-in and cut-out erratic or non-existant.
happy coaching!
buswarrior
-
Dave Thank you. I see now that the unloaders are just behind the supply air connector on the compressor. I did not see that until I got out the exploded view. I can be a little slow sometimes.
Yes BW I get the pressure up to over 100psi when I have the parking brake set. I then press the brake to reduce the pressure to 85psi and turn on my timer and watch for the pressure to reach 100 with the engine at idle speed.
When driving the gauge goes up to 120 then drops and just over 90 psi starts to climb and goes up to 120 and repeats the cycle.
The issue seems to be when I am on and off the brakes a lot. When I'm backing into storage and stopping and checking my mirrors and moving a little and then stopping and moving a little more and stopping etc. I was wondering if I had an air leak at the treadle valve but I don't hear anything unusual. When I shut the bus down I don't hear any air leaks. I'm just trying to stay ahead of maintenance that may need to be done and to stay safe with the brakes.
Thanks
Melbo
-
You may want to check you brake adjustment. If you are consuming a large amount of air by using the brakes while maneuvering, excessive brake stroke will eat up the reserve air pretty quickly while the RPMs are low and the system not building very fast.
-
Alright I took everyone's advice and replaced the unloader valves and adjusted the brakes. I also picked up a regulator that I have not installed ( don't think I need it but I will have it if I do ) Then I tested and it aired up just like before. I have not had it on the highway again. That will be thursday and I will let you know how it does. Thank you for the information and suggestions.
Melbo
-
To be honest it sounds quite normal to me, except for the long compressor recovery time. 40 seconds might be the maximum, but mine recovers in around 15 seconds. Useful tests that I do every day I drive the bus:
One: recovery test is done with the engine at high idle or 1000 rpm if no high idle, it is not done at normal idle.
Two: with all suspension and other air systems fully up to pressure, measure pressure drop with engine off. No more than 3 psi per minute allowed.
Three: As above, engine off, foot brake fully applied, measure pressure drop - again no more than 3 psi per minute allowed. DO NOT MOVE your foot from fully applied - even the slightest relaxation of pressure will release air from the exhaust valve and give an indication of a leak.
When the governor turns the compressor off and opens the purge valve on the air dryer, it does this by sending air pressure to both devices - the unloader valves and the purge valve. Either can leak when the air pressure is slowly declining from 120 psi cut-out pressure to 95 psi cut-in pressure. As soon as the governor gets to cut-in pressure, that leak will stop, since the governor exhausts the air it was using to hold the two devices open.
I really quite enjoy these threads, keeps me current on air systems 101... :)
-
Thank You Bevans
I don't have a fast idle with the L10 so I am sure the idle is much below the 1000 rpm you mention. We are off on a road trip so I will have multiple opportunities to check how the air system is working. I did a bunch of maintenance on it last summer. I will keep you posted.
Melbo
-
40-50 seconds at base idle sounds fine to me.
Let us know if your air consumption while making manoeuvres has changed. Did you measure the brake stroke before you adjusted 'em?
Sometimes, drivers use too much pedal when manoeuvring, be conscious of keeping the pedal travel only as deep as necessary to get a stop, no deeper.
Happy coaching!
Buswarrior
-
Mel has a Cummins engine with the PT pump they do need to be timed the pump is driven off the compressor
Never did like that set up. What were they thinking ?
Changed a few of them at 20 below...on the side of the road.
:(
-
All in a row Cummins accesories...
Gear train for all that also limits the size of compressor you can employ...
We looked into upsizing the compressor in a transit application, trying to defend against constantly being cut-in...
No can do.
In choosing engines, if it ain't one thing, it's another?
Happy coaching!
Buswarrior
-
You've got that! Series 60 were supposed to be the best thing since sliced bread. Then Bull gear goes out in high miles.$$$. All of them have their particular issues.
-
I am not sold on the 60 series, here are some photos of the damage to a low miles fire truck engine. we still have a few 6V92's that are much older and have never been worked on internally. could not post more pics but the pic of the head looks like it lost a valve head
-
"Well I pulled the discharge line and it is not oily and seems to have pretty good air pressure."
Mel, when you pulled the discharge line off the compressor to check it for oil did it continually to leak from the tank side or just what would be in the line (if any)?
-
Jim I removed the air supply to the bus and checked for any oil residue and found nothing so I started the bus up and checked the air supply pressure and it was comparable to an electric air compressor as near as I could tell so I reassembled everything and went about my business. After I got a chance and changed the unloader valves , adjusted the brakes, and took some driving tips from members of the board here everything is operating EXCELLENTLY. I want to thank you all for the tips and information. Here's hoping I don't have to change the air compressor on the side of the road at 20 below. In fact if I am ever someplace that gets to 20 below it has to be a really bad mistake. I'm typing this as I am sitting on the beach in Mexico.
Thanks again
Melbo
-
... everything is operating EXCELLENTLY.
Good news! It's always good to hear the success stories. Good luck with keeping things working right for a long time.
... if I am ever someplace that gets to 20 below it has to be a really bad mistake. I'm typing this as I am sitting on the beach in Mexico. Melbo
Amen, brother!