Tuesday, July 06, 2010

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.


Anonymous said...

Thomas C-2 park brake light stays on all the time ,park brakes do release but light stays on I suspect the valve in the amu that works the park brake . What do you think?

John Whelan said...

The park brake valve in the amu also sends a signal to the dash indicating that the park brake is on or off. If the contacts are sticking it could be a faulty valve assy. Test the output on the amu park brake valve.

Anonymous said...

Would the amu affect high idle? Idles normal when one of the connectors is unplugged. Has a pink hose going into it.

John Whelan said...

Yes AMU runs accessory systems like exhaust brake for example. The high idle that runs on air would be controlled by this system.