I am familiar with the hiss that the brake system makes on air-brake equipped trucks. Yesterday, while driving, our new to us would discharge about every 10-15 seconds. Brake pressure was around 120 psi for both systems, and everything seemed fine to my newbie senses.
Is this normal? If not, how do I remedy it? Also, do I have tanks I must drain of condensate?
Matt B
1994 U-240
Matt, First maybe you can take a real good listen when the engine is off and nothing running, listen for an air leak, If your governor is spitting off every 10-15 seconds you have a MAJOR air leak and should be able to hear it with the engine running.
Maybe the issue is not air leaking but a terminology issue.
Heck it is hard to tell from your description, and with the air gauges not moving, Most amazing.
Maybe need another description of problem.
I agree with Dave.
First, your air gauges SHOULD move between CUT-IN PSI and CUT-OUT PSI-- generally a range of 12-15 PSI with 120 likely the cut-out PSI of the engine-driven compressor.
You either have a large air leak, or as likely, a problem with the air dryer check valve. Note: the air dryer needs servicing/replacing every couple of years. If an original Bendix AD-2 or even a replacement Bendix AD-9, consider upgrading to a more current Bendix dryer that is easier to service. A new-generation reman dryer from Bendix is a cost-effective upgrade.
Brett
Matt, my 93, U225 does the same thing. It seems to be related to humidity. I spent $400 for to have a new governor installed and it changed nothing. The pssssh you hear is the drying system purging itself. It does it frequently on some days and occasionally on others. Cummins checked mine out after they put the governor on and said everything is working properly. I think it is unique to the 225's and 240's without air bags.
Brett, doesn't your coach do the same thing?
Another description:
While driving at about 65 MPH, when we passed a high divider, I could hear what sounded like an air discharge, duration of a fraction of a second. The sound was like what a compressor makes when it reaches full pressure, though the sound was much shorter, maybe a fourth second or shorter. I have heard trucks make this sound. We could hear this sound about once every ten to fifteen seconds.
If I understand the systems correctly, it would not be an air leak - after six hours with the ignition off, playing with the air hose, and finding and venting the condensate valves, air pressure is 90 and 120. Does this make sense? When I shut the engine off, I hear a hissing for about 10 seconds at the engine front right side, but nothing else. The coach does not make the sound when it is idling - we noticed it only at cruising speed, when there was a sound reflective surface near the left side of the highway. Is the air pump engine driven?
Matt B
The air pump on a Cummins is engine gear driven. I presume the Cat is also. I think what you heard is perfectly normal.
Kent,
Seems to make sense. First hour we drove (to the Walmart to collapse from nervous exhaustion) it vented frequently. Maybe from high humidity or from sitting for a few weeks. Second day, we heard it once or twice in several hours, though we may have missed a few.
Matt B
For anyone with a new to them coach here is a good place to find knowledge: http://www.beamalarm.com/foretravel-links/foretravel-technical-help.htm#Air_Brakes (http://www.beamalarm.com/foretravel-links/foretravel-technical-help.htm#Air_Brakes) Many of us including myself have started off driving an air brake equipped coach with practically no knowledge of air brakes or how to do an air brake test. Anyone that knows how to do the test will be a safer driver. Here is a website that includes the test. http://dmv.ca.gov/pubs/cdl_htm/sec5_a.htm (http://dmv.ca.gov/pubs/cdl_htm/sec5_a.htm) By the way some states require a class B drivers license if you are driving a vehicle with air brakes or above a certain gross weight usually 26K lbs. I drove my coach 2 yrs. with a class C license before finding out that I should have had a class B.
NO, it is not normal for the dryer to purge every 10-15 seconds. On the U240 there is very little air use (air brakes only, no air suspension), so the cut-out PSI is not reached very frequently-- certainly not every 10-15 seconds!
Brett
I just remembered something! For that first drive, a floor mat was stuck underneath the edge of the brake pedal. It didn't seem enough to cause any braking action, but it did keep it from being fully up. Could this be the cause of the discharge?
Matt B
The only time brakes hiss is when they are released, if a floor mat is holding the pedal down you could have dragging brakes. The other hiss comes from the air dryer when the governor cut out pressure is reached. If you get water out of the tank drain after a days drive consistently the air dryer probably needs service. A little bit of water is normal and how humid the air is will have some effect too. Stand outside the vehicle with the engine running, if you get a hiss from the air dryer frequently like every few minutes something is wrong.
I got a chance to go out and tinker some. At idle, the dryer vents when pressure is reached and is then generally quiet. At 15 RPM, it vents about every fifteen seconds. I adjusted the governor slightly (peak pressure per the dash gauge was about 130) down to about 120 pounds. I did this mainly because I was so pleased to have found the governor on my own. The dryer is still venting like before.
I examined the dryer for model number and could find none, though it is likely hidden by stuff. It was every bit as dirty as the oil tank and the other stuff, so it has likely been there a while. The service records I got with the coach do not mention it being replaced, so who knows!
In any case, the plan is to replace the dryer (old) and the governor (cheap). Which should cover the problem I am having, if it really is a problem.
With a driveway full of Toyotas, I have not done much wrenching in a while. It was fun.
Matt B
1994 U240
The air suspension also makes hissing sounds, easily thought of as the air brake system. I hear mine every time I use the retarder full on to slow around town.
Last summer we took a brief tour through Yellowstone National Park in our coach. Weather was beautiful and we were traveling relatively slowly, so we had the windows open. We could hear the air bags venting as the suspension made adjustments on the curving and hilly roads.
This may not apply to a 1994 U240. I don't think these had air suspensions/leveling.
Very true, but everyone reads these posts for guidance.
Matt,
Have you opened the tank drains? If you find ANY water, you must service the air dryer immediately. I service mine every eighteen months, as FT recommends.
My dryer periodically does the same thing Matt has described and it has never had any water in the tanks. I'm not sure he really has a problem.
Based on my understanding of how the air dryer purges itself, it would seem to be excessive cycling. When the governor reaches its set air pressure it will unload the compressor and that triggers the air dryer purge valve. Once the governor unloads it keeps the compressor from adding more air until it drops to the low pressure setting when the cycle starts over. Mine would not fully close after purging the first time and the pressure would take for ever to build up. It was really bad at 8000 feet. After I replaced the purge valve then everything worked properly. I could always tell what was happening by the air gauge which would stay about 120 until I did something like using the brakes. When the air pressure dropped to about 90 then it would rebuild and I would hear the air dryer purge at 120.
It is too bad that it only occurs at speed so that you cannot tell what is causing the issue.
A final word for those who were following the thread:
It was the dryer that was venting frequently, causing the noise. I set the RPMs at about 1200, and walked around and listened. I looked the dryer over for a model number while it was still in the coach, but could find none.
The old dryer was a Bendix, but I could find no model number on it. I had ordered a Bendix AD-9, it arrived today and I replaced it this evening. No big complications. It took a while to get the brackets loosened enough to get the dryer out. I had to pull it down out of the coach to get off one of the lines, which was against the rear of the coach. One fitting was slightly relocated, so I had to think-out which fitting should be tightened first.
Cranked up the FT, the dryer vented when the pump reached 115 psi, and no more vents. Shut off the engine and no hissing from a new leak. Looks like the job is done.
Matt B
1994 U240
Matt, if it was the dryer working improperly, couldn't you have simply cleaned or replaced the purge valve rather than the entire dryer? Also, I thought you had not found there was a problem with holding air pressure. Is that correct? What made you purchase a new dryer?
Kent,
While he could have replaced the check valve, dryers DO require servicing.
And the cost of a reman or even new dryer is not that much more than the cost of an overhaul kit if you count the labor involved. Yes, some have spin on filters that don't take much time to service, but it sounds like he may have had the original Bendix AD-2 dryer. Not worth fooling with.
I do the same-- every 3-4 years I just buy a Bendix factory reman dryer.
Brett
Interesting, what kind of cost are we looking at?
Under $200 for complete replacement.
But there are several different brands on the market with some variation in how they are serviced.
Start by seeing what brand and model you have. Only thing they all have in common is the need to be serviced/replaced every couple of years. More frequently on newer coaches (use a lot more air) and those operated in humid conditions.
Brett
What he said!
I have pretty good service records, but no indication that the dryer had ever been serviced. Even after removal, I can find no identification other than a tiny serial number, and a big "Bendix Air Dryer" sticker on the side. I took great trust in Brett's age advice and just updated the whole unit.
Newbie question:
It sounds like we are talking about two things at one time. I am guessing the check valve is the little screw-in valve that protects from over-pressure, like one a small shop compressor. The purge valve is built into the dryer, and vents liquid, vapor, and impurities at the end of each compression cycle. Do I have that right?
Newbie question two:
Can the AD-9 be serviced without removing it from the coach? It looks really tough.
@Kent There were no significant air leaks, but the purge valve was activating ever ten seconds or so. Best I could tell, I had to remove the dryer and disassemble to access the purge valve. I could not ID the old dryer, so I followed Brett's advice. Upside - same day R&R, pro rebuild, updated and improved equipment. Downside - just some dollars.
Matt B
You have it right.
'Supposedly' can be serviced in place, but its a whole bunch easier to pull it - and not that hard...
I rebuilt mine in the driveway after the purge valve blew apart. I would buy a reman again. The cost and hours for a full rebuild aren't cost effective in my opinion.
I believe it to be the 'least serviced' critical component for a motorhome...
From Matt's email
I removed and rebuilt the dryer on my 1999 U-270 when it was acting up. The rebuild kit was cost effective solution. If I was to do it again, on my 2000 U-320 I would buy a factory remanufactured unit and just do a remove and replace. Not big a cost differential, and greater piece of mind. ALWAYS block the coach anytime you are under it, especially when you are working on the air system.........
FWIW, the truck places around here do not rebuild the units, because by the time the technician rebuilds the units the shop labor charge on a rebuild about equalizes the cost as compared to a re-manufactured unit. If you rebuild your self and your labor is free, the cost side favors using the rebuild kit and doing it yourself.
Three choices - each progressively less expensive
Pay someone to do it (my current approach) and use a new of factory re-manufactured unit Do it your self and replace old unit with factory re-manufactured unit
Do it yourself, buy a rebuild kit, and rebuild your unit on the bench to reinstall
To each their own, good luck, and if you need to go beneath your coach be safe and cautious. They can, will, and have killed owners and technicians.
A really dumb question,....where would I locate this dryer on a 1997 U270? Should I automatically service it every 2 years or is there a way to determine if service is required?
Thank you.
Guess if helps to have a idea of what to look for. but don't ask me where exactly to look. should be near the one of the air tanks maybe.
And no question is dumb!!! Except the un-asked ones!
I still question whether there is a problem if the coach occasionally makes the pssst noise frequently but builds and keeps good air pressure both when running and with the engine off.
It has to frequently do what you describe. But when it starts building up to 150 lbs or more and starts releasing from the tank release instead of the air dryer or governor that the problem becomes extremely serious.
So am I correct that the dryier is functioning OK if the air pressure builds to 115-120 psi and it vents, then does not vent any more??
If yours works like both of mine, some days the dryer releases frequently and sometimes occasionally. It just depends of the humidity and cleanliness of the air. As far as I know if it builds pressure to about 120 and holds pressure unless you use the breaks, it is working ok. My U225 held working air pressure for up to 10 days when not in use and my U300 holds air pressure for several days. Someone correct me if I am wrong.
So let us define frequently and normal. In my case it was about every ten seconds, which just didn't seem normal to me. In an eight-hour drive that would be 2,880 vents. If it had been every ten minutes, I would have suspected nothing and considered it normal.
What say you?
Matt B
1994 Grand-zilla
With an air suspended coach, you have more air usage maintaining the coach level. So the air pressure cycles more often than with a 225/240/ORED
I would call that excessive provided the air system works like they do on a Class 8 rig. Are you losing air pressure when parked overnight. Could be looking at a air leak, bad governor, or re-build dryer.
As Dave pointed out, a air suspension coach will do it more frequently, just because the compressor has to work more.
What would you consider normal for our coaches that have air bag suspensions and the HWH leveling? Thank you.
My 95 U320 had its governor replaced and the dryer rebuilt w/in the last 9 months. I would say on average the purge valve cycles every 10 to 15 minutes.
Prior to the repair, pressure would drop from 120 to 60ish in two days.
Matt B
What is it doing now? And how often is the air releasing?
Here is a good description of how an air dryer works:
http://www.anythingtruck.com/commercial/airb_airdryer-ad9function.mv (http://www.anythingtruck.com/commercial/airb_airdryer-ad9function.mv)
OK, I read it three times. Did I interpret it correctly that the purge valve can activate as often as every 15 to 30 seconds under certain circumstances. That is approximately the frequency at which mine cycle on occasion.
Matt, dropping from 120 to 60 psi in two days seems excessive to me.
For those of you that have the money a rebuilt air dryier is the way to go. I speak from the experience of replacing the purge valve kit in mine and it was a real hassle. It turned out that it was hard to find parts for it even in Houston (it took three days of looking). I did eventually find the Bendix dealer and got his last purge valve kit which includes all the gaskets needed for reassembly. The most challenging problem was trying to torque the bolts with the air dryer sitting on the floor of my garage because I could not mount it in my vise. I also had the issue of trying to get the dryer out of the chassis as it seemed that it had never been done so there was 15 years of dirt and rust making my job harder. It also did not help that I had to do it in the storage lot which has some very rough broken concrete fill in the area of the engine bay. And then there were the fire ants!