Changing An AMU Module On A C2 School Bus

The C2 Freightliner school Bus comes with an Air Management Unit that houses all the necessary valves to run the air system. It's located in front of the drive axle, the valves are accessible once the mounting bar is removed. The brake switch, double check valve and park brake switch are a few of the valves in this unit.

We experienced a problem recently with the park brakes not releasing and found the park brake switch was at fault. When the park brake valve is pushed in, it sends an air signal to the switch which in turn sends a signal back to the bulkhead module indicating that the operator wants to release the spring brakes.

If this switch is faulty you cannot release the spring brakes until the switch is replaced. One way to check the operation of the park brake switch is to check  the park brake indicator on the dash panel, it should be illuminated with brakes on. This will tell you that the electrical circuit has continuity.

 This chassis is a 2006, newer models have the AMU located behind the transmission. It's easier once you remove the mounting bar to flip the whole section of valves over 180 degrees for better access.

The locking clip securing each module has to be inserted properly to seal the mating surfaces and fastens the whole assembly together.It helps to have someone to push the valves together while inserting the locking clip. The rubber gaskets in between each valve have to be in the right slots as well.

Air Brake Theory Revealed

Here is an excellent video on air brakes. This is the best presentation I've ever seen that will give you a detailed overview on exactly how air brakes work. The air brake mechanical and air systems work together. This video is referring to a school bus but is the same brake system found on most transport vehicles.



If The Video Doesn't Show Up in Internet Explorer Get To It HERE

The cutaways and detailed views of the air brake system explained here will help you understand the principles and terminology that you need to know. There are many things that happen in air brake systems like moisture that builds up from the heat that is generated from the compressor.

This is why draining air tanks daily is very important especially in cold weather conditions. This is a lot of information to absorb but the power of video allows you to watch it more than one time.

Haldex Brakes-Self Adjusting Slack Adjusters

Haldex Brakes self adjusting slack adjusters are installed at the factory on all Thomas Buses equipped with air brake systems. This allows much better control over brake adjustment especially for a large school bus fleet operation. At the same time there are operating and maintenance procedures that must be followed to get the best performance out of Haldex Self Adjusting Slack Adjusters.

Here Are Some Important Maintenance Steps To Follow:

• Visual and operational checks must be made at every scheduled PM check
• Check adjusters and anchor brackets for visual damage
• Automatic Adjusters should not be operated as manual adjusters
• Standard chasis grease may be used to lubricate Haldex Slack adjusters
• Lubricate the Standard 409-10 model every 50,000 miles or 3 mths
• Lubricate Reduced Maintenance 409-10 once a year
• No-Lube 429-10 adjusters are sealed and do not require lubrication
• DO NOT use Moly-disulfide grease as this will effect the operation of the slack adjuster
• Replace the slack adjuster if the TORQUE is less than 13 ft lbs when backing off the adjuster. You will hear the ratcheting sound while doing this.
• Haldex Slack Adjusters should be checked for operation between 90-100 p.s.i. in the air brake system reservoir. Anything over 100 p.s.i. starts to measure deflection in the foundation brakes and does not give a true reading
• To self adjust Haldex Brakes the operator has to make several full brake applications [spring brakes released] preferably with brakes hot and over 90 p.s.i in the air brake reservoir.
  
  Here is a very good video explaining proper Brake Adjustment on an air brake system. The self adjusting slack adjuster in this video is not a Haldex but the exact same procedure applies. In my experience with a properly adjusted Haldex slack adjuster the push rod travel averages at 1.5 inches of travel. Well below the Department Of Transport maximum allowed brake adjustment specs.
  
  Self adjusting slack adjusters are now standard equipment on all Thomas Buses and are a real asset for large fleets who must keep up with Department Of Transport Standards especially with air brakes and proper adjustment. I hope this post has informed you on the proper maintenance of Haldex Brakes.

Spring Brakes and What You Should Know!

Here's a great explanation on the Spring Brake that holds all that truck and Bus tonnage from rolling away. When doing a pre-trip the springs must on and to test them for holding power you give the throttle [engine running in gear] a quick hit with your foot. Don't put your pedal to the metal 'cause nothing is going to hold back that kind of torque!
The safety factor is much higher with spring brakes because the highly commpressed spring is now factory clamped so it's left alone and no one can get injured.[years ago they were serviceable and there were fatalities]

Check out this pdf below: Thanks to Mac over at http://skoolie.net

Spring Brakes PDF Instructions

Here's a previous post I made about Spring Brake assembly replacement:

How To Replace A Spring Brake Assembly

Bus & Truck Air Brakes System-Bendix AD-9 Air Dryer

The Bendix AD-9 air dryer is easily serviced by disassembling it from the frame and removing the bottom cover bolts. The unloader located on the bottom housing can be removed and a kit installed. The kit comes with new o-rings, spring, valve and seat. The main filter which is filled with dessicant absorbs the oil and moisture that accumulates in the air brake system.
Since the air compressor creates heat this causes moisture to build up, so this is why air brake systems have an air governor that unloads the compressor at 120 p.s.i. and signals the dryer to dump the build-up of water & oil in the bottom housing to atmosphere. That's the quick air release you hear when building up the air system, the air guage should be reading a minimum of 120 p.s.i.
The dessicant filter has a threaded stud attached so it can be easily removed during a service. The Bendix AD-9 is a very reliable component and is usually trouble free but if you notice a lot of oil coming from the bottom housing exhaust port then that's telling you the compressor is pumping out more oil than it should. Bendix has a splash card that you hold under the exhaust port, that gives an indication of how bad the oil loss from the compressor really is. The next step is to replace the compressor and of course, service the air dryer. Under normal conditions the air dryer can be serviced every 1-2 years depending on the condition of the air compressor and highway miles or hours of operation.

The Bendix Air Dryer also have a built-in heater wired up to sense cold weather and keep the unloader valve from freezing up. Every Truck & Bus air system has an air dryer installed from the factory, it's the most important preventive maintenance item and should not be neglected.