Community Forums

rpaldrich · 03-14-2021, 04:55 PM

I've been troubleshooting an air problem in coach. The air drier as well as compressor were recently replaced (compressor was replaced as part of an engine overhaul).

It drives fine, but the orange needle seems to drop faster then the green needle while driving.

At Idle -- the orange needle cycles from about 60-100psi continuously. I can hear the air hissing out, then it stops and refills and does it over and over. When the coach is off and the tanks have air in them, there is a buzzing/humming sound coming from the "auto" water drain valves in the forward part of under-carriage which goes away for a couple of seconds if I jiggle the manual drain cords.

My coach has a rear tank, and two forward tanks. Any guidance towards t/s steps would be much appreciated!

Thewellz · 03-14-2021, 05:25 PM

Have you listened under the coach to try and pinpoint where the hissing is coming from? The automatic air dryer relief valve, air bags or ride height valve could be possibilities. The orange needle is likely your Primary Tank( primary and wet tanks likely forward) they are inter- connected and a check valve seperates them and the secondary rear tank so if the priamy/ wet tanks fail the secondary holds air to for redundancy. The primary should build pressure to at least 125psi if operating normally. The humming noise at the valve could just be from the tank being pressurized or small leak, maybe try opening and closing the drain valve a few times to make sure it seats properly and isn't leaking. Spraying Soapy water around the bags and fittings might help you find the issue, but what you describe is a major leak likely more than the drain valve.

Skip426 · 03-15-2021, 09:40 PM

The orange needle will be for your front tank and the green for the rear .
The front tank services the front brakes, air suspension , air horns and air activation of the exhaust brake. The rear tank , the rear brakes .

While driving having the orange needle move more that the green is normal , sitting at idle , both should be same and stable .

Unfortunately crawling around with a spray bottle of soapy water , to check for your leaks is the only solution . Block from the frame to the ground , before going under .

Take your camera an post a picture of any leaking part ,so we can help identify it.

I have air system diagrams for my Feb. 99 built chassis , but they don't show a third tank , only a multi chamber rear ( secondary ) . Big changes on the F/L assembly line Jan. 4th , 1999 . So I guess air system was one of them .

rpaldrich · 03-16-2021, 04:57 AM

Okay thanks! I'll get under there with it running and post a pic of it leaking. Honestly it sounds like a "planned" leak....i.e. it sounds like something is causing it to leak for a certain amount of time, then stop, then once it gets to a certain pressure do it again.

I'll let you know!

Skip426 · 03-16-2021, 10:37 AM

No; it sounds like something that should hold pressure to ( lets say ) 150 psi ( as in the case of the system over pressure protection valve), has weakened to the point where it only holds pressure to 100 psi. Then vents to 60 psi , before closing .

Let us know what you find .
With a new compressor and air dryer , your system should perform way better.

Unfortunately , Freightliner is not supporting the older chassis the way they used to , but maybe a call to the help line with your serial number would get you an air system diagram , linked to you serial number.
1-800-385-4357
1-800-FTL-HELP

Ray,IN · 03-16-2021, 09:41 PM

I suspect the manual drains require replacement as they simply screw into the side of the automatic water drains. This is a common item sold at any truck repair parts store.
OTOH one of my automatic drains was leaking air because it was only hand-tight into the air supply tank.
Air brakes should only operate from the the front wet/dry tank, the tank in the rear of the chassis is an accumulator tank for the air springs.

Skip426 · 03-17-2021, 11:25 AM

Ray,IN ; Important to supply, date and manufacturer info on any posted material , and from what I can see ( blowing your attachment up , all printing is unreadable ) , if your PDF is from Spartan , things change frequently ; as in this Spartan PDF , from July 99. Note that suspension components are not shown only a label at a tank fitting .
Chassis manufacturers are also different as the second PDF from Freightliner shows , two possibilities for tank arrangements and related air system plumbing in the same document , I got the F/L PDF years back when I first purchased my coach . Air system PDF was sent out when I supplied my chassis serial number.

Domo · 03-17-2021, 12:01 PM

You may be hearing the ride height valves adjusting when you stop - they'll re-level the frame, as well as they can, and then stop.

I've never heard of an automatic tank bleed system - new one on me! - - my 2008 Freightliner has three lanyards that must be pulled despite all the other whooshing and wheezing you may hear... Just an FYI

rpaldrich · 03-17-2021, 06:17 PM

Wow! This is great stuff and I really appreciate all the knowledge on this one. It has been pouring the past three days, and not forecast to stop until this storm blows through tomorrow night. I think this weekend will be the first time it'll be dry enough for me to get under the chassis.

The PRV sounds like a place to look at as well. I'll get under there w/ a flashlight and listen hard.

Ryan

Skip426 · 03-17-2021, 06:24 PM

Ryan , a dash photo from another F/L chassis owner . Note both needles are at the same point well above 100 psi.

Ray,IN · 03-17-2021, 10:14 PM

Quote:

Originally Posted by Skip426

Ray,IN ; Important to supply, date and manufacturer info on any posted material , and from what I can see ( blowing your attachment up , all printing is unreadable ) , if your PDF is from Spartan , things change frequently ; as in this Spartan PDF , from July 99. Note that suspension components are not shown only a label at a tank fitting .
Chassis manufacturers are also different as the second PDF from Freightliner shows , two possibilities for tank arrangements and related air system plumbing in the same document , I got the F/L PDF years back when I first purchased my coach . Air system PDF was sent out when I supplied my chassis serial number.

Of course you are fight Skip, your PDF's are illegible too. Mine is for my Spartan chassis and shows the 3rd air tank in front of the rear axle, like the OP has.
This is for DOMO: 12VDC automatic air drain, the manual water drain is shown with cable attached..

Skip426 · 03-17-2021, 10:45 PM

Quote:

Originally Posted by Ray,IN

, your PDF's are illegible too.

Ray ; I don't know what computer or operating system you have to open the PDFs , but Windows 10 opens them and allows 10x magnification , and they read just fine.

rpaldrich · 03-18-2021, 04:15 AM

Quote:

Originally Posted by Ray,IN

Of course you are fight Skip, your PDF's are illegible too. Mine is for my Spartan chassis and shows the 3rd air tank in front of the rear axle, like the OP has.
This is for DOMO: 12VDC automatic air drain, the manual water drain is shown with cable attached..

--Thanks Ray. That 12VDC auto air drain definitely "buzzes" and has a slow leak with the engine off and air in the system. May be a combination of old parts leaking around that area(there is now an air quickcoupler installed where one of the manual drain lines used to terminate). I'll take a picture tomorrow or Saturday when the rain quits.

Quote:

Originally Posted by Skip426

Ryan , a dash photo from another F/L chassis owner . Note both needles are at the same point well above 100 psi.

Thanks Skip -- Now how about driving? What is the range through which my needles should go? I.E. in heavier traffic what should my compressor keep the needle above? I see a similar PDF in my Freightliner RV chassis manual as well.

Thanks for all the help!

Skip426 · 03-18-2021, 10:13 AM

Quote:

Originally Posted by rpaldrich

Now how about driving? What is the range through which my needles should go? I.E. in heavier traffic what should my compressor keep the needle above? I see a similar PDF in my Freightliner RV chassis manual as well.

I'll see if I can come up with the specs on the compressor governor , as to the pressures when it unloads ( high pressure cut out ) the compressor and reloads ( low pressure cut in to recharge the system).
In heavy traffic , I manually select a lower gear to avoid unnecessary up shifts and provide the most hold back from the engine when off the throttle.
3rd in town , and 4th until speed limit 50 and over.
As per Allison .

EDIT: Back in time to add info here.
The pressure governor should , run the compressor to achieve 120 psi then unload , till the tank pressure drops down approx 25 psi. So 90/95 psi should be the low end of pressure in the tanks .
Find out if the compressor replacement included a new governor , if when your leak is fixed you don't get 120 psi the governor may require adjustment or replacing .