Dash Air Pressure Gauges

[Michael Kauffman]

[size=14][face=Arial]This has been on my mind since I've owed this coach.  The air pressure gauge on the dash has 2 needles, one orange, one green.  They kind of move independent from one another except when fully charged.  Then they are both read about the same.  Inquiring minds need to know, why 2 needles?  Thanks, Mike[/face][/size]

[Joel Ashley]

Mike-

I'm sure others may give a more complete answer, but very basically, each needle represents one of your two air tanks.  One fills before the other because it is first in line from the engine compressor.  Often they are referred to as "dry tank" and "wet tank".  

In actuality, they are front tank and rear tank;  the front one, first in line from the compressor and air dryer, has a wet side and a dry side in it.  As air cools, any moisture in it tends to condense out, and it cools fastest as it enters that first tank, and that, therefore is that tank's "wet" side.  Having cooled substantially, the air moves to the tank's "dry" side where it enters with less moisture in it.  Then it goes into the second (rear) tank.

I am open to correction, but I believe the green needle on your gauge represents the front tank, and the orange one the rear tank.

To protect the tanks from corrosion, there are purge valves (our coach has 4 next to the accessory airline port) that should be used regularly to drain all the air out of both tanks, so any accumulated moisture is discharged.

Joel

[Jeremy Parrett]

 Mike, here is the correct procedure to test and monitor your air brake system:

AIR GOVERNOR TEST
This test identifies the upper and lower limits of the air pressures. Start the engine and allow the air pressure to build up until it reaches its maximum and the compressor turns off (this should be no more than 130 pounds of pressure). You may hear the spitter valve pop when the maximum pressure is reached. Then depress the service (foot) brake several times. Each time you depress the service brake pedal the air pressure will go down and you can see the pressure go down on the air gauge. Wait about 2 seconds between each time you depress the brake pedal in order to see if the compressor restarts and the air gauge needles start going up again. Watch the air gauge and keep doing this until the compressor restarts. The compressor should restart at or above 85 pounds of air pressure. Now you know the maximum air pressure in your system and the air pressure where the compressor restarts. NOTE: There are two needles in the air gauge. One is for the front air tank and one is for the rear air tank. They should be close together, no more than 2-4 pounds apart. If they diverge too much it may indicate a problem with one of the air tanks.

STATIC BRAKE TEST
This test checks for leaks in the air tanks. Continuing from the AIR GOVERNOR TEST above, the air pressure should be at its maximum for this test. You should be stopped in a level position (block the wheels if it is not perfectly level). Place the automatic transmission in neutral (manual transmissions can be put in reverse gear), turn the engine off, keep your foot off the service brake, and release the parking brake. NOTE: Be sure the vehicle does not roll. Looking at the air pressure gauge, the air loss should not exceed 2 pounds in one minute.

APPLIED BRAKE TEST
This test checks for leaks in the air lines. Continuing from the STATIC BRAKE TEST above, depress the service brake pedal and keep it fully depressed for one minute. Watch the air gauge needles. After the air pressure has stabilized (the needles stop moving), the air loss should not exceed 3 pounds in one minute (tap the gauge occasionally). One caution, do not apply too much pressure on the service brake pedal.

LOW AIR WARNING DEVICE TEST
This test checks to see that the low air warning system is working. Continuing from the APPLIED BRAKE TEST above, turn ignition switch fully on but do NOT start the engine. Look at the air pressure gauge and continuously pump the service brake until you can see the warning light come on and/or hear the warning buzzer. If your buzzer sounds because of the low oil pressure switch, just look for the low air warning light to come on. The low air light should come on at pressures below 60 pounds. NOTE: If the low air warning light or buzzer comes on while you are driving, immediately move off the highway because soon after the low air warning comes on the emergency brake will be applied automatically and you will stop where ever you are at that time.

EMERGENCY BRAKE SYSTEM TEST
This test checks to see whether the parking brake applies automatically if the air pressure gets too low. Continuing from the LOW AIR WARNING DEVICE TEST above, turn off the ignition and continue to pump the service brake pedal until the parking brake handle pops up of its own accord. This should happen at pressures below 45 pounds. After it pops up, start the engine but do NOT touch the parking brake (it is on at this point). Try to SLOWLY drive the vehicle with the low air pressure warning on. The emergency brake is still on so it should hold you back.

EMERGENCY BRAKE TEST
This test checks whether the emergency brake works manually. Continuing from the EMERGENCY BRAKE SYSTEM TEST, keep the engine running and allow the air pressure to build up until it is at full pressure (the spitter valve will pop). Put the coach in Drive and release the parking brake. Drive the coach at about 5 miles per hour and apply the parking brake (not the service brake pedal). It should stop you fairly quickly.

CHECK SERVICE BRAKES
This test checks whether the service brakes are working and also whether they may need adjusting. Continuing from the EMERGENCY BRAKE TEST, drive the coach at about 5 miles per hour, loosely hold the steering wheel and apply the service brake pedal firmly to see whether the steering wheel pulls to the left or the right. If it pulls in either direction you may need to have your brakes adjusted. Furthermore, if you apply the service brake and the two needles on the air gauge move farther apart than their normal (small) divergence, this is an indicator that the brakes may need adjusting.

This taken from the RV Forum.    Here is the link:   http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=173:motorhome-air-brake-system-testing&catid=24:tech-topics

When you start up each time ,wait for the air pressure to reach 120 psi with both red and green needle . Then with engine in neutral and spring brake (parking) applied ,pump the brake pedal until the pressure drops to below 80 psi (or whatever is set) so that the emergency brake will actuate. Then let the compressor pump the system back up to 120psi .When it reaches full pressure you will hear the releif valve lift briefly at the rear of the coach. Then watch your needles to see they maintain the correct pressure.
This should be done EVERY time you operate your coach.
 
Hope this helps.  
Jeremy

[Michael Kauffman]

[size=14]Great Information, Thanks Guy's!!  When parked for a while, my rig losses air pressure over several days and takes about 30 seconds to fill once the engine is started.  Is that common?  Mike[/size]

[Marty and Suzie Schenck]

Google Bendix Brake schematics. You will find diagrams for truck/trailer and straight truck. The one for straight truck is the one that our motorhomes will mimic. There are 2 systems: primary and secondary and they each have there own reservoirs. The primary is the green arrow system and usually is the rear brakes. The secondary is the orange arrow system and operates  the front brakes. The accessories that the manufacturer adds (air suspension) to the chassis are most likely tied into the system with the largest air reservoir or may have a reservoir of it's own.
Marty

[Dick Simonis]

Ahhhh, that explains why only the orange arrow drops to 0 when we dump air.  The green sits at about 30 psi for days.

This is a good read.

http://www.wsafc.org/WSFMA/Shared%20Documents1/Bendix%20Air%20Brake%20Handbook.pdf

[Larry and Heidi Lee]

How often does one drain moisture from the four purge valves? (Coach used and stored in So Cal)

[Joel Ashley]

If your coach is stored, and you are level without using the air suspension, you probably should dump air and purge the tanks when you put it in storage.  I'd have to look to see what my manual recommends, but I'd think at least every 3 months would be a reasonable time frame if you're on the road a lot.

Be sure to leave the valves open for 5 minutes or so after you no longer hear air discharging;  this lets residual moisture escape.

Joel

[Michael Kauffman]

If your coach is stored, and you are level without using the air suspension, you probably should dump air and purge the tanks when you put it in storage.  I'd have to look to see what my manual recommends, but I'd think at least every 3 months would be a reasonable time frame if you're on the road a lot.

Be sure to leave the valves open for 5 minutes or so after you no longer hear air discharging;  this lets residual moisture escape.

Joel

[size=14]I've never purged mine yet (1-1/2yr).  Are those the little petcocks in the rear oil filter bay?   Dumber in Seattle, Mike [/size]

[Larry Fisk]

Don't feel alone, I have never purged mine ether. I thought the release of air when it pressured up was the purge to remove moisture. Glad I read this thread!

[Tom and Pam Brown]

All air brake systems are required to have an air dryer I'd some kind.  Most require servicing should recommendatios in your chassis manual.

Even with  a dryer moisture will still find it's way to the tanks due to changen in temps etc.

Follow the guidlines for your specific chassis and dryer and all will be good.

[Joel Weiss]

Ahhhh, that explains why only the orange arrow drops to 0 when we dump air.  The green sits at about 30 psi for days.

This is a good read.

http://www.wsafc.org/WSFMA/Shared%20Documents1/Bendix%20Air%20Brake%20Handbook.pdf

Ours works exactly the opposite to this; I was convinced that the green was the airbag pressure and the orange the brakes.   Now I have no idea.

[Gerald Farris]

All coaches do not have an air dryer. Several of the coaches that have air over hydraulic brakes are not equipped with an air dryer. On these coaches the tanks need to be drained more often than the tanks on coaches that are equipped with an air dryer. If your coach is not equipped with an air dryer, the tanks should be drained at least every month or less depending on how much the coach is driven and how much humidity is in the air that is being compressed. In some cases like a 10 hour drive along the Gulf Coast, you will need to drain the tanks daily. On coaches that are equipped with an air dryer you should check the tanks for water every month or so of use, however if the dryer is functioning properly you will get no water at the drain.  

Gerald

[Joel Ashley]

Larry, the air release you hear is not through the tanks, but from the drier releasing screened out moisture following full pressurization.  As Gerald said, there probably won't be much tank moisture with a working dryer, but the tanks still should be regularly purged to be sure.

Mike, tomorrow I will try to post a photo here of my valve set for those who don't know what to look for;  however different coaches have different configurations, and some have to crawl under the rig to drain tank water.

Joel

[Keith Moffett]

Gerald, regarding the air over hydraulic system.  You say to drain the tanks monthly, do you mean to use those pet valves in the filter bay or a direct drain on the tank?  We have the Haldex drier, what about a little oil in the air system as indicated by one mechanic and how is that done?  I notice that the air drier over pressure valve doesnt make as much noise as it once did.  Is this an indication of service or just keep an eye on it?
Keith