XCS chassis, air system purge/relief every 3 minutes or so

[ualdriver]

Hello-

Not having much luck with my camping year this year. Refrigerator failure, water tank leak, panel air conditioning failure, stair motor failure, all within two months : ( First world problems, surely. Now this......

I was driving down the highway today and noticed the "Chhhhhh" sound coming from the engine area every 3 minutes or so, and also saw slightly abnormally high air pressure showing on my 2 brake system gauges. The "Chhhh" sound is the same sound you get when you initially start the RV up after the air system airs up completely. Also the same sound you get after you apply the brakes while driving and I assume the air system is recharging. Whatever that sound is (relief valve? purge valve?), that's the same sound I'm getting every three minutes driving down the highway.

I attached a picture of the two air pressure gauges when they were reading their highest. Normally, both gauges sit exactly half way between the 75psi and 150psi indications, but now they're running closer to 150psi as seen in the picture. Sometimes the gauges with both drop down to that "half way" point between 75psi and 150psi, but then both gauges would drift back to the pressure shown in the picture. When the relief valve/purge valve sounds, there is no change in indicated pressure on the gauges.

I called a couple of Freightliner shops, and of course they either won't work on RVs or in the case of the Oasis shop, they're booked until mid-August.

Any idea what's going on? Can I continue with my camping trip where I will be driving about 1000+ miles over the next two weeks, or does this need to be addressed now? Although if it can be addressed now, I doubt any shop will see me anyway anytime soon, as usual with these sorts of problems.

[twinboat]

You need to address it now.

You have an air leak somewhere in the air pressure governor system. The governor controls the tank pressures.

Having the compressor cycle over and over, like it is, is damaging to it. They are not designed to run fulltime like that.

Find a shop or a mobile repair service.

[ualdriver]

Originally Posted by twinboat

You need to address it now.

You have an air leak somewhere in the air pressure governor system. The governor controls the tank pressures.

Having the compressor cycle over and over, like it is, is damaging to it. They are not designed to run fulltime like that.

Find a shop or a mobile repair service.

OK, thanks. Local shops can't or won't fit me in because I'm a RV. A mobile repair service can handle a repair like this?

[twinboat]

Call some and ask. Your looking for a mobil truck mechanic who knows air brakes. Most do.

There are also Mobil RV mechanics who know air brake systems.

[ualdriver]

Originally Posted by twinboat

Call some and ask. Your looking for a mobil truck mechanic who knows air brakes. Most do.

There are also Mobil RV mechanics who know air brake systems.

Thanks twinboat. I finally found a shop that would take me first thing in the morning after many phonecalls. Couldn't find a mobile mechanic that wasn't backed up for a week or two.

I appreciate your help.

[jelag]

The air compressor is designed to run 15-20% of the time other wise it will overheat and shorten the life of the compressor... each time it shuts off, that is the D2 governor sees a high set point, it also sends a signal to the air dryer.. the dryer goes through a purge process when the compressor shuts down and that noise you hear is that purge process...


Remember in a shop, the mechanic will do the same tests I've started to outline above... it might take him 20 min or 3 hours to trace the leak and finally repair it... OFTEN it can be in a location where the air line runs in the frame or against the frame where you can't see it but hear it from the air line rubbing against the frame....



If you do this yourself find an industrial air supplier and only purchase DOT approved air line... its manufactured to handle some of the abrasion... if you see a portion of an air line that rubs the frame either nylon tie to away or get some heater hose.... split the hose and let it sit around your air line and absorb the abrasion...
Hope this helps..

Take a spray bottle(water and soap) and run the air lines... I don't hear well but often if its a bad leak you can hear the leak and follow it.. there is a one way check valve after the compressor so your problem is between that check valve, down through the air tanks, air lines, valves, spring brakes... etc...



If you have an older coach and HAVE NOT annually changed the dryer CARTRIDGE you can expect excessive moisture in the air tanks.. and if you've not drained those tanks regularly you might find pin holes in the bottom of the air tanks, so make sure you spray the bottom of the tanks as well..


This is a simple job and I would rather add air with a connection to the main tank than let the engine run... this allows me to listen and follow the noise when I use the water/soap spray..


Each tank is isolated from the other tanks with one way valves so that air can't follow backwards in the system...

[ualdriver]

Originally Posted by twinboat

You need to address it now.

You have an air leak somewhere in the air pressure governor system. The governor controls the tank pressures.

Having the compressor cycle over and over, like it is, is damaging to it. They are not designed to run fulltime like that.

Find a shop or a mobile repair service.

To tie a ribbon around this in case someone searches on the forum and has the same problem, a diesel mechanic looked at my rig this morning and talked to Freightliner via the 1-800 customer service line. He then found that there was excessive water coming out of the air purge valve. He replaced the air dryer assembly and the governor. $260 for the air dryer, $92 for a new governor, $300 in labor, plus tax, so I'm out about $700. No mention of an air leak? Not bad assuming this all fixes the problem. It drove home without excessive purging but the air pressure was still slightly higher than I thought(?) I have seen in the past. Usually the needles in the brake system are around 110psi. The pressure seemed to sit between 110psi-130psi.

I don't understand how the water got in there. I changed the decessant (sp?) cartridge last year in the annual service ( I think the Freightliner maintenance schedule says change every 3 years off the top of my head), and I regularly purge the air lines via the 3 lanyards in the chassis.

Originally Posted by jelag

The air compressor is designed to run 15-20% of the time other wise it will overheat and shorten the life of the compressor... each time it shuts off, that is the D2 governor sees a high set point, it also sends a signal to the air dryer.. the dryer goes through a purge process when the compressor shuts down and that noise you hear is that purge process...

I'm still not understanding how this compressor interacts with the rest of the air system. So if you and/or twinboat have the patience perhaps you can fill in the blanks......

On my rig, the purge valve was excessively cycling, but I don't understand the relationship between water in the air system and the purge valve going off every 3 minutes. And if it was going off so often, why wasn't it purging the water as well?

Further, I'm not understanding how the compressor works to control air pressure in the system. After talking to the Freightliner rep and the diesel mechanic, my understanding is that the air compressor is mechanically connected to the engine so it is ALWAYS running and pumping air. I don't understand where the pumped air goes when the system is up to pressure, and I'm not sure what the governor does in all of this and how it works? And if the compressor is pumping air anyway, what does it matter how often the purge valve purges?

Originally Posted by jelag

Remember in a shop, the mechanic will do the same tests I've started to outline above... it might take him 20 min or 3 hours to trace the leak and finally repair it... OFTEN it can be in a location where the air line runs in the frame or against the frame where you can't see it but hear it from the air line rubbing against the frame....

I appreciate the explanation as it was interesting, but the search for an air leak would be beyond my skill set. I wouldn't even know how to pressurize the air system externally.

[Milford 47]

Originally Posted by ualdriver

To tie a ribbon around this in case someone searches on the forum and has the same problem, a diesel mechanic looked at my rig this morning and talked to Freightliner via the 1-800 customer service line. He then found that there was excessive water coming out of the air purge valve. He replaced the air dryer assembly and the governor. $260 for the air dryer, $92 for a new governor, $300 in labor, plus tax, so I'm out about $700. No mention of an air leak? Not bad assuming this all fixes the problem. It drove home without excessive purging but the air pressure was still slightly higher than I thought(?) I have seen in the past. Usually the needles in the brake system are around 110psi. The pressure seemed to sit between 110psi-130psi.

I don't understand how the water got in there. I changed the decessant (sp?) cartridge last year in the annual service ( I think the Freightliner maintenance schedule says change every 3 years off the top of my head), and I regularly purge the air lines via the 3 lanyards in the chassis.



I'm still not understanding how this compressor interacts with the rest of the air system. So if you and/or twinboat have the patience perhaps you can fill in the blanks......

On my rig, the purge valve was excessively cycling, but I don't understand the relationship between water in the air system and the purge valve going off every 3 minutes. And if it was going off so often, why wasn't it purging the water as well?

Further, I'm not understanding how the compressor works to control air pressure in the system. After talking to the Freightliner rep and the diesel mechanic, my understanding is that the air compressor is mechanically connected to the engine so it is ALWAYS running and pumping air. I don't understand where the pumped air goes when the system is up to pressure, and I'm not sure what the governor does in all of this and how it works? And if the compressor is pumping air anyway, what does it matter how often the purge valve purges?



I appreciate the explanation as it was interesting, but the search for an air leak would be beyond my skill set. I wouldn't even know how to pressurize the air system externally.

The governor is what engages the compressor to make it pump and then releases so it can idle (not pump) until the air pressure drops to the governor kick in pressure, same over and over with the kick out pressure stopping the air from being compressed

Are you or have you been in a very wet humid area for a long time? Perhaps for some reason you were only getting a partial purge, air only instead of the water. If you see it purge you should see a slight mist or haze if any moisture is being purged

Will be interesting to hear other thoughts

[ualdriver]

Originally Posted by Milford 47

The governor is what engages the compressor to make it pump and then releases so it can idle (not pump) until the air pressure drops to the governor kick in pressure, same over and over with the kick out pressure stopping the air from being compressed

Are you or have you been in a very wet humid area for a long time? Perhaps for some reason you were only getting a partial purge, air only instead of the water. If you see it purge you should see a slight mist or haze if any moisture is being purged

Will be interesting to hear other thoughts

I read a little bit about air systems this evening so I think I get it now. That compressor is always pumping air, but when it isn't pumping up the air system it's pumping to atmosphere. I guess pumping to atmosphere is less strain than pumping up to 120 psi (or whatever system pressure is), so the compressor will last longer unloaded most of the time rather than constantly pumping the air system up to 120psi. The purge valve opens when the governor directs compressor output to atmosphere so that the air dryer system can rid itself of moisture and contaminants.

I live in IL so I wouldn't say that it is particularly humid, but we do get humid summer days like anyone else I guess. Also, I haven't been to any particularly humid climates, either. I've never seen any mist when I pull the lanyards. If I had, I would have kept pulling until there was no more mist.

[twinboat]

When the governor reaches a set point, the new one may be a bit higher and that's ok, it sends an air signal out to the compressor and air dryer purge valve.

That signal lifts the intake valves of the compressor causing it to simply push the air back and forth between it and the engine intake manifold. The compressor is moving but not really working.
The signal also open the air dryer purge valve, purging all of the air in it and the discharge line between the compressor and dryer.

Inside the air dryer is material that absorbs moisture during the compressing cycle and discharges it during the purge cycle. If the purge cycle is to short, all of the moisture doesn't drain, causing it to pass thru into the tanks.

The air leak, that caused the exesive cycling, may have been a bad air dryer purge valve.
An air leak there would cause the wet tank ( first tank the compressor supplies and does not have a gauge ) to drop pressure. That tank supplies the governor and dropping pressure would cause the governor to cycle.

[ualdriver]

Originally Posted by twinboat

When the governor reaches a set point, the new one may be a bit higher and that's ok, it sends an air signal out to the compressor and air dryer purge valve.

That signal lifts the intake valves of the compressor causing it to simply push the air back and forth between it and the engine intake manifold. The compressor is moving but not really working.
The signal also open the air dryer purge valve, purging all of the air in it and the discharge line between the compressor and dryer.

Inside the air dryer is material that absorbs moisture during the compressing cycle and discharges it during the purge cycle. If the purge cycle is to short, all of the moisture doesn't drain, causing it to pass thru into the tanks.

The air leak, that caused the exesive cycling, may have been a bad air dryer purge valve.
An air leak there would cause the wet tank ( first tank the compressor supplies and does not have a gauge ) to drop pressure. That tank supplies the governor and dropping pressure would cause the governor to cycle.

Thanks twinboat. That makes sense. Very much. appreciate your help.

[jelag]

The air compressor runs all the time.... when the D2 sees the preset pressure it supplies air from the D2, to the compressor... this air pressure pops OPEN the intake VALVES... thus the pistons go up and down... sucking air in when the piston goes down... and pushing the air out when the piston goes up... if the valve closed as designed... the air would be pushed out the exhaust valve into the tanks...

When the air pressure drops... the D2 stops the signal to the compressor... allowing the intake valves to function and the compressor will start to pump air...
When the signal to the compressor opens the exhaust valve and the compressor stops pumping.... a one way check valve stops any air in the tanks from backing up in the system and exiting through the open valves... At the same time the intake valves open the same signal signals the air dryer and it will purge..

I've personally not seen an air dryer that had a problem with the purge valve and it would cycle as you describe... and will have to sit here a while and consider the operation of the dryer for an answer...
The object of the purge valve is to push out any excessive moisture that's accumulated in the dryer...
I don't understand the replacement of the D2.... its function is to start and stop the compressor... I assume that it has been starting and stopping normally which means to me it was working.... IF the D2 was sending signals in error... it would start the compressor early and shut it down at the high limit setting.... the D2 is nothing more than a spring setting... the tighter the spring the higher the cut out pressure and the lower the spring pressure the lower the cut out pressure...
If you see a higher pressure now its because the new D2 is set to a higher level or set point....
So I wrote this before I read what twinboat had mentioned... he is correct in what he has shared just said it in a different way....

Hope this has helped...
Any other questions....

[ualdriver]

Originally Posted by jelag

The air compressor runs all the time.... when the D2 sees the preset pressure it supplies air from the D2, to the compressor... this air pressure pops OPEN the intake VALVES... thus the pistons go up and down... sucking air in when the piston goes down... and pushing the air out when the piston goes up... if the valve closed as designed... the air would be pushed out the exhaust valve into the tanks...

When the air pressure drops... the D2 stops the signal to the compressor... allowing the intake valves to function and the compressor will start to pump air...
When the signal to the compressor opens the exhaust valve and the compressor stops pumping.... a one way check valve stops any air in the tanks from backing up in the system and exiting through the open valves... At the same time the intake valves open the same signal signals the air dryer and it will purge..

I've personally not seen an air dryer that had a problem with the purge valve and it would cycle as you describe... and will have to sit here a while and consider the operation of the dryer for an answer...
The object of the purge valve is to push out any excessive moisture that's accumulated in the dryer...
I don't understand the replacement of the D2.... its function is to start and stop the compressor... I assume that it has been starting and stopping normally which means to me it was working.... IF the D2 was sending signals in error... it would start the compressor early and shut it down at the high limit setting.... the D2 is nothing more than a spring setting... the tighter the spring the higher the cut out pressure and the lower the spring pressure the lower the cut out pressure...
If you see a higher pressure now its because the new D2 is set to a higher level or set point....
So I wrote this before I read what twinboat had mentioned... he is correct in what he has shared just said it in a different way....

Hope this has helped...
Any other questions....

jelag-

Yes, it helps very much. I really appreciate the technical information you and twinboat are providing about the operation of the system.

[twinboat]

In 35 years of heavy equipment repairs, I may have replaced 1 or 2 governors. That was because oil and water had contamanated them.
Some of my co-workers randomly changed them but that didn't fix the problem.

It seems that some shops just replace them because they are cheap and they don't understand the function.

Most governor issues are due to leaks in the 1/4" lines that run to and from them. That or an undetected wet tank leak.