Compressed Air Systems

[dandick66]

I swear my coach is possessed!  This weekend I started it up and intended to take air pressure readings and post them.  
First off, my coach has the Medallion system and I cannot read any pressures unless the engine is running and the low air alarm is off.  So, I installed a dual-reading analog gauge that I can read all the time.  
The pressure built up to 127 PSI and the compressor cut off.  The pressure dropped 10 PSI and then stabilized.  Remember how I said it dropped about 1 PSI every 15 seconds?  Well, it stopped doing that.  The pressure dropped 1 PSI after about 5 minutes.  I ended up shutting the engine off and checking the analog gauges every once in a while.  The pressure dropped very slowly, but still went down 90 PSI in less than 24 hours.  
The good news is the somewhat rapid pressure drop appears to have stopped.  The bad news is I still have a significant leak.  The only thing I can think of that may have stopped the every 15 second drop, is that I changed fuel filters a couple of weeks ago.  I have the Maxxforce 10 engine and the fuel filter is on the engine and I need to remove the floor access plate to get to it.  While I had the access plate off I looked at any and all hoses and lines to check for chafing, etc.  I think I may (must) have moved an air line and maybe it reseated itself on the push to connect fitting.  I will pull the access plate and double check everything.

I can see my rear air tank and it is more or less directly above the differential, so I am going to have to make some ramps and blocks for the suspension so I can fit under there.  The front tank is very accessible and I sprayed it and looked for leaks and didn’t find any.  

[Bob Jones]

Hi folks!

I noticed a while back that the air pressure was dropping quite quickly on the unit. Today I had it running and I noticed that there is air coming out of the bottom of the air dryer at a continuous rate. 

In the past it would pump up to ~120 PSI or so and then vent briefly. Now it leaks continuously out the bottom of the dryer vent. 

It's been some time since I looked at the air dryer, any ideas? 

[96 EVO]

Two choices

1) new air dryer

2) purge valve rebuild kit

I'd go with choice 1

[Tom Cherry]

MODERATOR NOTE 

This post topic is being merged with a more complete one from earlier this year.  The information and comments requested have been covered there.

Thanks

 

1 hour ago, Bob Jones said:

Hi folks!

I noticed a while back that the air pressure was dropping quite quickly on the unit. Today I had it running and I noticed that there is air coming out of the bottom of the air dryer at a continuous rate. 

In the past it would pump up to ~120 PSI or so and then vent briefly. Now it leaks continuously out the bottom of the dryer vent. 

It's been some time since I looked at the air dryer, any ideas? 

Been there....DONE THAT.  If you are mechanically inclined and can remove the total dryer (Probably Wabco), then you can chase down the rebuild kits.  You actually need TWO.  There is a valve on the front and the back.  I don't really know which is which.  I did a leak test of mine several years ago.  It was leaking on both the front and the back.  Your's is worse.  I priced the valve kits.  I could spend 30% more and get a NEW, in the box WABCO....including the cartridge.  I chose NOT to do a rebuild and picked one up and had my OTR shop replace it and also check for leaks on the various fittings....as they do for the Federal FMVSS Brake System checks.

It was not an easy, according to the tech, pull.  BUT, I have good air and they brake pressures stay up.

My recommendation and comments....but, if you do all your own work and servicing and are qualified....then check out the cost of the valve rebuild kits....but do BOTH.

EDIT.  If you do a SEARCH in the TOPICS section and use AIR DRYER, you will find a wealth of info.  I scanned a few....got to the 2nd page of 10 pages.  Therefore, after scanning the one below....I a merged it with the better and more complete one.

Thanks.... 

 

I think that is the one above....but if not, find the TOPIC and scroll about 2/3's of the way down.  There is a link to the info and also two maintenance manual links....assuming you have the 1200 series....

Good Luck.

[Bob Jones]

Thanks guys! 

I'm looking through my docs and mine is a Haldex Pure Air Plus dryer unit. I was in there before some years ago to install a service kit and I pulled out my docs and found that I had printed off a troubleshooting list and parts list 🙂

Apparently you can pull the purge valve and clean it or install a purge valve kit. The kit is about $55 USD from the looks of it. I think I'll pull it in the next few days and see what it looks like inside.

Many thanks for the tips! I'll post up a pic when I get in there if I find something interesting. Does anyone know what the 'turbo-saver' does? I have no idea how the turbo would relate to the air dryer. 

[96 EVO]

Not sure exactly what the turbo saver valve does, but I know if your regeneration valve goes bad, it will release air thru the purge valve. The turbo saver valve may do the same, which is why I recommend you just replace the entire dryer so your not just chasing your tail replacing one part at a time.

[Jdw12345]

Maybe this will help someone,

https://www.wabco-customercentre.com/catalog/docs/mm34_web.pdf

 

   This is a handy document to save.

Edited September 30, 2022 by Jdw12345

[gofastnow]

13 hours ago, Bob Jones said:

Thanks guys! 

I'm looking through my docs and mine is a Haldex Pure Air Plus dryer unit. I was in there before some years ago to install a service kit and I pulled out my docs and found that I had printed off a troubleshooting list and parts list 🙂

Apparently you can pull the purge valve and clean it or install a purge valve kit. The kit is about $55 USD from the looks of it. I think I'll pull it in the next few days and see what it looks like inside.

Many thanks for the tips! I'll post up a pic when I get in there if I find something interesting. Does anyone know what the 'turbo-saver' does? I have no idea how the turbo would relate to the air dryer. 

MIGHT 'turbo saver' be the same as Turbo Cut-off Valve?

Per the referenced Wabco document by Jdw12345 below;

Turbo Cut-off Valve: A valve located in the inlet port of the air dryer.
It closes the path between the air compressor and the air dryer
purge valve during compressor unload. This prevents a loss of
turbocharger boost pressure during a compressor unload cycle,
thereby maintaining boost pressure for maximum engine
horsepower. Figure 1.21 and Figure 1.22 (used exclusively on the
1200 Plus model).
There is no spring in the turbo cut-off valve assemblies used on U
Series air dryers.
The System Saver E Series air dryers use a special turbo cut-off
valve. Refer to the air dryer parts book PB-8857AS for part number
information

[Bob Jones]

I read that in the document Jeff attached, which was very helpful. But I still don't get, at least with respect to the turbo saver. My Haldex unit appears to have all the same features. 

[96 EVO]

Your air compressor is breathing filtered air. The turbo saver valve prevents disruption of airflow at the turbo when the compressor unloads / dryer purges.

[Bob Jones]

I still don't understand what the turbo has to do with the system. My understanding is that the compressor runs off the engine, is lubricated by engine oil, and pumps the air into the dryer assembly where it removes moisture and oil etc to ensure clean dry air to the braking system. But...I know little about it, that's for sure!

I got a chance to go in there today. So...prior to 'repair' the symptom is that that air dryer was continuously venting out the Purge Valve as the air pressure rose and would not expel air at the 120 PSI cut out as it should. 

I took off the lower housing and pulled the Purge Valve from the system. It did not look that bad. The bore that it rides in also did not look bad. Nevertheless, I cleaned all the residual lubricant from the bore and then took the valve and dropped it in the ultrasonic cleaner for 30 minutes. I then dried it off, lubricated it, and the bore, with dielectric grease, and put it back together.

I then fired up the unit and there was zero air escaping until it reached cut out (120 PSI). I did a couple of cycles and what I noticed was that after the valve 'found its place' as it approached cut out it would leak a little and then reach cut out with the normal woosh and expel moisture. Can anyone confirm for me that it should bleed NO pressure until it reaches cut out?

So...it's much better and that confirms that the issue is with the Purge Valve. At this point it's hit or miss with respect to having a perfect seal but it is operational. 

A Purge Valve kit for these unit is pretty cheap so I will order one in rather than install new O-rings and try to 'repair' the existing one further. Here's some pics for those interested.

As always, many thanks to everyone that has responded!

[Ivan K]

2 hours ago, Bob Jones said:

I still don't understand what the turbo has to do with the system. 

The turbo supplies pre-compressed and filtered air to the compressor for further compression. The function of turbo saver valve is to close connection between compressor and now open dryer to not waste engine intake air and pressure from the engine intake side when compressor (and dryer) is in unloaded state, not pumping because it's intake valve is permanently open for the duration of cut-out. 

[Bob Jones]

Now I get it - thank you. I had no idea the turbo fed the compressor but it makes perfect sense and also explains the turbo saver valve function.