Foretravel Motorhome Forums => Foretravel Tech Talk => Topic started by: hdff on October 08, 2017, 03:38:02 pm
Title: Air pressure drop
Post by: hdff on October 08, 2017, 03:38:02 pm
Newbie question alert!!!!!! At what rate should/does the air pressure drop while going down the freeway on the gauges on the dash. My newbie unknowing mind says mine is going down to fast. In 1 minute and 25 seconds it will go from the high of 110 to 90 and then go back to 110. This seems like a pretty quick drop to me but what do y'all think? this could be close to normal for all I know, I hope I'm wrong...
thanks Keith
Title: Re: Air pressure drop
Post by: jcus on October 08, 2017, 03:48:37 pm
Newbie question alert!!!!!! At what rate should/does the air pressure drop while going down the freeway on the gauges on the dash. My newbie unknowing mind says mine is going down to fast. In 1 minute and 25 seconds it will go from the high of 110 to 90 and then go back to 110. This seems like a pretty quick drop to me but what do y'all think? this could be close to normal for all I know, I hope I'm wrong...
thanks Keith
It is pretty quick, I would let pressure built up and shut coach off, but leave key on, and leave in travel mode. Then check all height control valves and airbags for leaks. Soap solution works best, but at that rate, should be easy to find.
Title: Re: Air pressure drop
Post by: wolfe10 on October 08, 2017, 03:51:11 pm
No question, something is wrong.
When you shut off the engine, how quickly does pressure drop? Both gauges or just one?
When you shut off the engine, when you walk around the coach can you hear a leak?
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 08, 2017, 04:07:36 pm
One simple thing to check: make sure the water drain valves on all air tanks are completely closed.
Title: Re: Air pressure drop
Post by: hdff on October 08, 2017, 04:24:26 pm
It is pretty quick, I would let pressure built up and shut coach off, but leave key on, and leave in travel mode. Then check all height control valves and airbags for leaks. Soap solution works best, but at that rate, should be easy to find.
I don't know the answers to these 2 questions wolfe and jcus but I will find out.... although my hearing ain't what it use to be I have not heard any significant air loss that I can recall
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 08, 2017, 04:37:11 pm
I know that there is a tank and drain in front of the drivers side rear wheel but where are the others?
Sorry, Keith, I'll have to let some other member answer that for your 2000 model coach. On our old coach, the drains on the 2 front mounted air tanks are valves screwed into the bottoms of the tanks. On your coach, the drains may be remotely mounted somewhere up front.
Title: Re: Air pressure drop
Post by: Mark Duckworth on October 08, 2017, 04:43:12 pm
I know that there is a tank and drain in front of the drivers side rear wheel but where are the others?
The one you know about is the drain for the "wet tank". On our 03, in front of the driver's side steer tire we have 3 drain valves for the auxiliary, rear and front air tanks.
Edit: If your valves are arranged like ours, the valves are:
Outside valve = Aux tank Center valve = Rear tank Inside valve = Front tank
Title: Re: Air pressure drop
Post by: Numbers on October 08, 2017, 06:32:20 pm
I had a similar issue and it was an old pressure regulator under the coach in the front between the two air tanks that failed. The plastic regulator knob was VERY visibly cracked and it basically disintegrated in my hand when I checked it. Motorhomes of Texas had replaced all the air bags on our coach, replaced a stair solenoid, and replaced an air fitting, so I thought the system was good to go. But they left a visibly questionable air pressure regulator in place. Half way through my trip home the pressure on one gauge started fluctuating a lot, and when parked there was a an increasingly noticeable tilt to the coach as my trip home progressed.
Rough road surfaces (Arkansas) made the problem more noticeable. Really really bad roads with lots of bumps, dips, and irregularities can cause the gauges to fluctuate a lot as the airbags try and and compensate, but if the PSI drops as quickly on smooth roads it can be an indication of an issue.
Foretravel's factory leak down specs specify a loss of no more than 6 PSI per hour.
One way to test if the leak down rate is within spec (if you don't hear an obvious leak when the engine is shut off) is to park the coach and keep tabs on the gauges over time.
Before you shut off the engine wait until the front and rear pressure gauges on the dash read the maximum - usually about 100-115 PSI depending on how the air regulator has been set. As you watch the gauges rise while parked and idling, you should hear the pressure relief valve pop off with a "Pssshhhh".
Shut the engine off.
Every couple of hours come back out to the coach, check the gauges, and record the PSI level. This will give you an average leak down.
In a 10 hour period Foretravel's approved leak down rate allows for a loss of 60 PSI. So if you start out at 110 PSI, 10 hours later it is acceptable to be at 50 PSI. Most likely you will see the gauge at 60 PSI for a while.
Chris
Title: Re: Air pressure drop
Post by: Tim Fiedler on October 08, 2017, 06:46:39 pm
One drain for wet tank on. My 2000. In front or driver side rear tire. You have to get on ground and look up at bottom of coach to see it
Title: Re: Air pressure drop
Post by: jcus on October 08, 2017, 06:51:01 pm
"In a 10 hour period Foretravel's approved leak down rate allows for a loss of 60 PSI. So if you start out at 110 PSI, 10 hours later it is acceptable to be at 50 PSI. Most likely you will see the gauge at 60 PSI for a while." If air pressure in your tanks fall below 65 psi in 10 hours, you have another bigger problem, The 65 psi protection valve should stop all air going from tanks to leveling system at 65 psi. This is to maintain enough pressure for your brakes in an emergency.
Title: Re: Air pressure drop
Post by: Numbers on October 08, 2017, 07:12:58 pm
Jim,
Thank you for pointing out that the protection is at 65 PSI and not 60 PSI. I should have been more detailed and specific.
Chris
Title: Re: Air pressure drop
Post by: hdff on October 08, 2017, 09:25:48 pm
I bought the coach at MOT a month ago and they supposedly did a PDI check on all the major components.. I say supposedly cause I found a small air compressor in one of the lower compartments that had the power wire cut... how did they test that system??? Aubrey at MOT told me on the phone that it should have been checked... I put a butt splice spade connection on the wire and the compressor works BUT it bleeds down pretty fast causing the compressor to run often.. I talked to my salesman and he claims that if I bring it back they will check it, we'll see how that works out..... when I get some time I will go do a leak down test and soap the above mentioned places...
So should the gauges ever go below 65 after sitting for a week or more? On October 2 I ran the coach and pressures up the system to ~110, tonite it was 0 on both gauges,
Title: Re: Air pressure drop
Post by: jcus on October 08, 2017, 09:39:10 pm
I bought the coach at MOT a month ago and they supposedly did a PDI check on all the major components.. I say supposedly cause I found a small air compressor in one of the lower compartments that had the power wire cut... how did they test that system??? Aubrey at MOT told me on the phone that it should have been checked... I put a butt splice spade connection on the wire and the compressor works BUT it bleeds down pretty fast causing the compressor to run often.. I talked to my salesman and he claims that if I bring it back they will check it, we'll see how that works out..... when I get some time I will go do a leak down test and soap the above mentioned places...
So should the gauges ever go below 65 after sitting for a week or more? On October 2 I ran the coach and pressures up the system to ~110, tonite it was 0 on both gauges,
Almost all air systems have some leaks. Not surprising it leaked down to 0 in a week. A lot of fittings, components and connections in the air system. Just a matter of finding the big ones and repairing them before they accumulate and cause big problems. If you do not have a slide the small compressor will try to level the coach ever 1/2 hour if you are in auto level. [if you have slide, it will inflate/deflate slide seals] If it runs a lot in auto level mode, start checking all components, connections, and hwh 6 packs. Try shutting leveling off and see what happens. Many put a switch on the line you reconnected, to shut off compressor just in case you develop an big leak and compressor keeps running and overheats. [New one is close to a coach buck]
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 08, 2017, 09:47:35 pm
So should the gauges ever go below 65 after sitting for a week or more? On October 2 I ran the coach and pressures up the system to ~110, tonite it was 0 on both gauges,
Yes, the air system on every coach will eventually go to zero pressure. Some coaches bleed down fast, some hold pressure for a long time. The protection valves are only designed to hold some brake operating pressure in the tanks long enough to bring the coach to a safe, controlled stop. They are not designed to hold pressure indefinitely.
It is up to you to decide how "tight" you want your air system. At a minimum, it should meet DOT standards (refer to your Commercial Driver's CDL Handbook). Most Foretravel coaches will easily exceed the DOT standard. Some owners don't worry too much about a few air leaks - for others chasing air leaks becomes a obsession. A search on this forum for "air leaks" will give you many ideas on how to proceed.
Title: Re: Air pressure drop
Post by: prfleming on October 08, 2017, 10:17:49 pm
When you shut off the engine, when you walk around the coach can you hear a leak?
You need to find a young helper. We had a leak, I asked my 21 year old son to help, he identified the location under the coach in about a minute.
Title: Re: Air pressure drop
Post by: SteveB on October 09, 2017, 11:21:14 am
Back to the OP's post I can comment that my coach does the same thing within the same time frame while going down the road. It has done this for at least the last 20K miles. Static leak down tests indicate to appreciable leaks. I can auto level and both gauges will read about 85 psi for weeks. I' m sure I am missing something so any suggestions that relate to this specific issue will be appreciated.
Title: Re: Air pressure drop
Post by: jcus on October 09, 2017, 11:39:16 am
Back to the OP's post I can comment that my coach does the same thing within the same time frame while going down the road. It has done this for at least the last 20K miles. Static leak down tests indicate to appreciable leaks. I can auto level and both gauges will read about 85 psi for weeks. I' m sure I am missing something so any suggestions that relate to this specific issue will be appreciated.
My present coach did same thing. When driving and in travel mode, height control valve controls frame to axle distance, When stopped in level mode, solenoid to hcv is closed, so after those valves leak out, system will stabilize and no more air will be lost. After replacing leaking hcv my compressor load to unload time went from 80 seconds to 200 seconds. I also held pressure in level mode for a long time.
Title: Re: Air pressure drop
Post by: SteveB on October 09, 2017, 11:44:15 am
My present coach did same thing. When driving and in travel mode, height control valve controls frame to axle distance, When stopped in level mode, solenoid to hcv is closed, so after those valves leak out, system will stabilize and no more air will be lost. After replacing leaking hcv my compressor load to unload time went from 80 seconds to 200 seconds. I also held pressure in level mode for a long time.
So Jim, did you replace all three hcv's? They're pretty inexpensive and worth a shot.
Thanks for the tip!
Title: Re: Air pressure drop
Post by: jcus on October 09, 2017, 11:52:44 am
So Jim, did you replace all three hcv's? They're pretty inexpensive and worth a shot.
Thanks for the tip!
Just replaced drivers side rear, but they are cheap, got a plastic one on ebay for $60. Funny, Foretravel uses the metal ones and Newell likes the plastic ones. I am going to get another one to carry as a spare
Title: Re: Air pressure drop
Post by: hdff on October 09, 2017, 02:04:28 pm
Could you tell your hcv's were leaking by spraying them with soapy water? Do you know the replacement # and is it something that can be done in the driveway?
Title: Re: Air pressure drop
Post by: wolfe10 on October 09, 2017, 02:16:52 pm
Not difficult/complex, but you will want to have safety stands in place, as these control coach height!
Title: Re: Air pressure drop
Post by: jcus on October 09, 2017, 02:24:20 pm
Yes you can tell, mine was leaking around lever shaft. Mine sits in front of tag tire so a little more difficult to get at. But can be changed in your driveway. Just remember position of lever on shaft. Freightliner Kenworth Peterbilt Leveling Valve Height | eBay (http://www.ebay.com/itm/Freightliner-Kenworth-Peterbilt-Leveling-Valve-Height/230534760581?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649)
Title: Re: Air pressure drop
Post by: hdff on October 09, 2017, 02:57:06 pm
Safety stands are on the short list to get. Thanks Wolfe
So my front air tanks are basically under the drivers seat in front of the wheel?
Title: Re: Air pressure drop
Post by: hdff on October 11, 2017, 07:59:29 pm
So I brought the coach home today and had a chance to poke around on it. First I let the air build up to 110 then killed the motor and left it in travel mode. Checked it all day 110# on both gauges @ 9:30 and at 6:30 tonite it was 67# on both gauges, acceptable by fot standards. Then I soaped all the tanks,bags,leveling switches and fittings I could find.. no major leak and I could ... so why do the gauges go down so fast while going down the road? I have to take it to fot tomorrow and I'll monitor the gauges again...
Title: Re: Air pressure drop
Post by: Tim Fiedler on October 11, 2017, 08:07:58 pm
Pop off pressure valve cycling?
Title: Re: Air pressure drop
Post by: jcus on October 11, 2017, 08:15:21 pm
So I brought the coach home today and had a chance to poke around on it. First I let the air build up to 110 then killed the motor and left it in travel mode. Checked it all day 110# on both gauges @ 9:30 and at 6:30 tonite it was 67# on both gauges, acceptable by fot standards. Then I soaped all the tanks,bags,leveling switches and fittings I could find.. no major leak and I could ... so why do the gauges go down so fast while going down the road? I have to take it to fot tomorrow and I'll monitor the gauges again...
When you are moving, hcv is constantly moving and adding and bleeding off air. You may have one position that the valves leaks by. FOT can probably diagnose the problem pretty quickly.
Title: Re: Air pressure drop
Post by: wolfe10 on October 11, 2017, 08:18:13 pm
Clearly, there is a leak (or leaks) somewhere. And if you lost 60 PSI on both gauges/tanks, you might have a leak in the dryer, and a bad check valve(s).
I say that, because of chance of exactly the same size leaks occurring on both independent systems is pretty low.
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 11, 2017, 08:31:23 pm
Situation like this is when a pressure gauge on the wet tank is handy. Comparing wet tank pressure to the other tank pressures can help determine which direction the air is going, and also help narrow down the possible leak locations.
Title: Re: Air pressure drop
Post by: Protech Racing on October 11, 2017, 09:00:19 pm
It would have to be on the charging circuit. Dryer, D2 relief valve . A part/valve, compressor, that runs when the engine runs. IMHO. The static leak down will be the same regardless of compressor action.
Title: Re: Air pressure drop
Post by: wolfe10 on October 11, 2017, 09:13:09 pm
But, for both tanks to leak down, the check valve(s) must be bad.
Title: Re: Air pressure drop
Post by: jcus on October 11, 2017, 09:19:04 pm
Original problem was rapid cycling of compressor when travelling, slow leak down when stopped is another problem all together. In travel mode, only compressor, dryer and hcv's could cause that quick of a pressure loss, if not manifested when stopped. OP says these are not leaking when stopped.
Title: Re: Air pressure drop
Post by: Old phart phred on October 11, 2017, 10:43:22 pm
I am having a similar issue but less severe, as my Ventura vacuum generator is only running non stop whenever the park brake is not "on". My oshkosh air schematics seem to be somewhat lacking info. I am no expert but I believe the park/emergency brake pnuematic logic is it requires air pressure to release the brake. That being said, a leak in that system can only be detected, with the wheels chocked, compressor running and the park/emergency brake in the released position.
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 11, 2017, 11:05:24 pm
The fact that both "front" and "rear" tanks simultaneously leak down to the same pressure (67 psi) might be due to faulty check valves on both tanks. It could also be caused by a leak somewhere in the air system downstream of the protection valves, IF the leak is in a line or component served by both brake tanks. To determine whether this is possible, one must study the air system schematic. I am not familiar with the air system on the 2000 model U320, so I don't know if the two brake tanks are tied together anywhere downstream of the protection valves.
As a example, on our '93 U280, there are 1/2" lines coming off both brake tanks that are joined together, and then supply air pressure to the brake treadle valve (via line #26). After the treadle valve, the air continues to a number of different brake components in both the front and the rear brake circuits. A leak in any one of those components could drain both my "front" and "rear" brake tanks down to the pressure at which the protection valves closed. This would happen around 50-60 psi on our coach, assuming both protection valves closed at the same pressure.
SO, what is more likely - having 2 faulty check valves, or having two protection valves that close at exactly the same pressure?
Title: Re: Air pressure drop
Post by: jcus on October 11, 2017, 11:17:24 pm
The fact that both "front" and "rear" tanks simultaneously leak down to the same pressure (67 psi) might be due to faulty check valves on both tanks. It could also be caused by a leak somewhere in the air system downstream of the protection valves, IF the leak is in a line or component served by both brake tanks. To determine whether this is possible, one must study the air system schematic. I am not familiar with the air system on the 2000 model U320, so I don't know if the two brake tanks are tied together anywhere downstream of the protection valves.
As a example, on our '93 U280, there are 1/2" lines coming off both brake tanks that are joined together, and then supply air pressure to the treadle valve (via line #26). After the treadle valve, the air continues to a number of different brake components in both the front and the rear brake circuits. A leak in any one of those components could drain both my "front" and "rear" brake tanks down to the pressure at which the protection valves closed. This would happen around 50-60 psi on our coach, assuming both protection valves closed at the same pressure.
SO, what is more likely - having 2 faulty check valves, or having two protection valves that close at exactly the same pressure?
Good point Chuck, but problem is rapid cycling of compressor when running. OP is not worried about leak down when braking or stationary, but when driving normally and not using brakes. His problem does not appear to involve brakes or protection valves. He just cycles between 90 and 120 psi too quickly, but only slow leak down when not running.
Title: Re: Air pressure drop
Post by: Chuck & Jeannie on October 11, 2017, 11:29:54 pm
How about a leak in a brake component that leaks a lot of air when driving down the road, and the leaks less air when parked? What is the brake component that is in different positions when parked and when driving? When driving, full system air pressure is applied to the rear brake cans to release the parking brakes. When parked, air pressure is released from those cans so the spring brakes can activate. I don't know much about the rear brake components. Is it possible to have a leak in one of the rear brake assemblies that would produce these symptoms?
Title: Re: Air pressure drop
Post by: jcus on October 11, 2017, 11:39:54 pm
How about a leak in a brake component that leaks a lot of air when driving down the road, and the leaks less air when parked? What is the brake component that is in different positions when parked and when driving? When driving, full system air pressure is applied to the rear brake cans to release the parking brakes. When parked, air pressure is released from those cans so the spring brakes can activate. I don't know much about the rear brake components. Is it possible to have a leak in one of the rear brake assemblies that would produce these symptoms?
A possibility, OP could park coach and set parking brake. If no leaking from release cans, that eliminates that possibility, This is one of those problems you do not figure out, you just eliminate everything it could not be.
Title: Re: Air pressure drop
Post by: Old phart phred on October 11, 2017, 11:47:59 pm
I am having a similar issue but less severe, as my Ventura vacuum generator is only running non stop whenever the park brake is not "on". My oshkosh air schematics seem to be somewhat lacking info. I am no expert but I believe the park/emergency brake pnuematic logic is it requires air pressure to release the brake. That being said, a leak in that system can only be detected, with the wheels chocked, compressor running and the park/emergency brake in the released position.
I believe this applies to all coaches running on the road.
Title: Re: Air pressure drop
Post by: jcus on October 12, 2017, 12:05:30 am
Do not know about your coach but this is the old system. Sold my 72 gmc conversion, rolled down the parking lot and took out a horse trailer, [no horses hurt] I had warned him to chock wheels. http://www.schoolbusfleet.com/forum/topic.asp?TOPIC_ID=5041