Air Pressure Loss

[Jim Wasson]

Before traveling last summer we had Freightliner replace the air compressor dryer cartridge on our 2004 Santiam.  I am pretty sure Freightliner created a leak, but proving it is another thing.  Attached is a photo of the dash air pressure gauge. What do the red and green dial hands tell me? Is one of the hands for front and the other for back or is red for left and green for right like navigation lights.  Any help is appreciated.
BTW, this leak if very slight and normal pressure is maintained while driving. Regardless I still want to  fix the leak.
Thanks,
Jim

[Joel Ashley]

Jim-

My take is that the green pointer reflects the air charge of the front tank, and the red the rear tank;  since the front is first in-line from the compressor and dryer, its green one should slightly lead ahead of the red pointer during charge-up.  They approximately equal out at 120-130 psi, where the relief valve operates to protect the system, and the air governor signals the dryer's purge port to expel moisture - the short blast of air you regularly hear.

Others may have more expert, complete, or alternative input than my answer provides.

-Joel

[Gil_Johnson]

I have exprience with that gauge.  However, many of those gauges allow the user to set the low point for quick visual association.  If yours is like these, you can set the red pointer by moving a knob on the center front of the gauge.

Gil

[Jim Wasson]

Gil;
Is the red pointer then used only as a "set point" indicator and the green pointer as the actual PSI indicator?  When I start the coach both pointers move to 110-120 psig in tandem.
Thanks,
Jim

[Marty and Suzie Schenck]

There should be 2 air systems on vehicles with air brakes, a service system and an emergency system. The green arrow  should be for the service or A system and the red arrow for the emergency or B system. If the the air in the emergency system falls below about 60psi it trips your parking brake to go on and or prevents you from releasing it if it is below that. The tanks are not necessarily left or right and or front or rear, they are generally put where there is room but may be marked A or B, service or emergency. Marty

[Paul Schwalen]

Jim-

Back in 2005 I cut and saved an article from FMC September 2005 issue.  I just dug it out because I was curious about the issue of front/rear air lines and the whole air system in general.

Here is what the article says, but be careful because it is based on a Freightliner XC chassis and may or may not exactly apply to our Beavers.  I intend to go under my Contessa and try to verify some of the colors and tank locations but I don't have the ambition today.

The author, Martin Sherlock, refers to the supply/primary and secondary air tanks. After the compressor the air passes to the Air Dryer and then to the supply/primary air tank that he says is between the rails over the front axle. This tank has two compartments and is protected by a relief valve set at 150 PSI. The air from the dryer enters this tank on the supply side where it cools and then is routed to the primary side where the air is used for the systems.  This side of the tank supplies the air for the rear air brakes.  This tank also supplies air to the secondary tank.  This secondary tank is divided into three compartments and the compressed air is supplied to the secondary compartment.  The other two compartments supply air to the front air brakes, the entire air suspension system, the exhaust retarder and the customer air manifold. On chassis equipped with air bags there may be another separate air tank that supplies air to the rear air bags and is usually located between the rails over the rear axle.

The air pressure is monitored through a gauge on the dashboard.  This gauge has two pointers; the green one indicates the pressure in the primary tank, and the red one indicates the pressure in the secondary tank. Sherlock says the air lines under the coach are color coded.  Green is for the rear service brakes , red is for the front service brakes, black is for the park/emergency brake system, yellow is for the park/emergency control, and blue if for the air suspension system.

I know that I have green, red and black air lines but I don't recall seeing yellow or blue.  I intend to have a look soon and determine how my system is layed out.

I know this is a lengthly reply to your question but I have only entered a small portion of the article.  Perhaps some member can figure out how to acces the article on line so all can peruse it.

Paul

[Joel Ashley]

Thanks for the more complete description, Paul.

Here's the link to the article you mention;  members will have to sign-in at FMCA to see it:

http://www.fmcmagazine.com/back-issues/2005/september/6102-the-chassis-compressed-air-system

We got our '06 coach a year after the article was printed, and weren't FMCA members then, so didn't get to read it.  As Paul points out, not every detail in the article is directly applicable to any one Beaver Coach, so keep that in mind as you read it;  your rig may vary from certain system feature descriptions, as ours does.  

I'm printing out a copy, trimming it to fit, and placing it in my Owners Manual under the air systems section for future reference.  

-Joel

[Jim Wasson]

I appreciate the help from our BAC family. This is good information.
Thanks,
Jim