Truck Parts And Their Functions

Item	Sample Picture	Function
Air Dryer		The air dryer collects and removes air system contaminants in solid, liquid, and vapor form before they enter the brake system.
Air Management Unit (AMU)		The air management unit (AMU) is a collection of pneumatic and electronic valves and pressure switches of modular form that fasten together in a row on a common rail. The AMU is usually located in the rear suspension area of the vehicle.
Air Reservoir		Store compressed air used to apply the brakes and operate other air-powered devices, such as chassis suspensions, engine fan controls, and seats.
Air Reservoir Automatic Drain Valve		Automatically ejects moisture and contaminants from the reservoir in which it is connected.
Automatic Traction Control Valve		The ATC valve controls only the brake chambers for the drive wheels. It is mounted on the service relay valve in the rear modulator valve assembly. The solenoid in the ATC valve controls an on/off air valve, which allows or prevents air flow to the control side of the service relay valve.
Auxiliary Air Valve Assembly (AAVA)		The AAVA is a solenoid-only assembly with a common air supply for the normally closed solenoids. The AAVA solenoids are located on the suspension crossmember, where the AMU was located on earlier vehicles.
Backup Pump		The power steering pump sends pressurized ATF to the power brake booster supply port. ATF returns to the power steering reservoir from the power brake booster return port. If the supply of pressurized ATF from the power steering pump to the power brake booster fails, a flow switch starts the backup pump to keep the power brakes working.
Brake Chamber		As the brake pedal is depressed, compressed air enters the service brake chamber through a port. Air pressure acts upon a diaphragm, which forces the piston rod toward the non-pressure chamber, applying a straight-line force to the slack adjuster, which converts it to a rotational force. This, in turn, rotates the camshaft and applies the brakes. When the brake pedal is released, air is exhausted from the service brake chamber, and the return spring allows the diaphragm, piston rod, and slack adjuster to return to their normal positions, releasing the brakes.
Brake Shoes		Brake shoes are crescent-shaped components with a rough friction material on one side. They sit inside of a brake drum. When the brake pedal is pressed, the brake shoes are forced outward, pushing against the inside of the brake drum and slowing down the wheel.
Brake Pads		Your brake pads apply pressure to the rotor, which is directly connected to each wheel. This pressure creates the friction needed to slow or stop your vehicle. When the rotor slows, so do your wheels.
Caliper		The brake caliper houses your the brake pads and pistons. Its job is to slow the wheels by creating friction with the brake rotors. The brake caliper fits like a clamp on a wheel's rotor to stop the wheel from turning when you step on the brakes.
Double-Check Valve Module		The double check valve module delivers the higher air pressure of two separate supply pressures. There are no internal air supply or delivery ports. There is one external delivery port on the face of the double check valve module.
Electronic Control Unit (ECU)		The ECU constantly monitors the wheel sensors, modulator valves, automatic traction control (ATC) valve (if the vehicle is so-equipped) and the electrical circuitry.
End Block Module		The ported end block module contains ports that are used to deliver air to another AMU on the vehicle or to modules at the other end of the AMU.
Inversion Valve Module		The inversion valve module is a normally open (N.O.) air valve that delivers system air pressure when the control pressure falls below the specified level. There are no internal supply, delivery, or control ports in this module.
Main Cylinder		Converts the pressure on the brake pedal to hydraulic pressure by feeding brake fluid into the brake circuit and controlling this according to the mechanical force.
Modulator Valve		Modulator valves control the air pressure in each affected brake chamber during an ABS operation. Responding to the signal received from the ECU, modulator valves prevent wheel lockup by regulating brake pressure. During normal braking applications, the ABS system is inactive, and compressed air flows freely through the modulator valves to the brake chambers.
Power Brake Booster		The power brake booster multiplies brake pedal effort to increase power to the brake main cylinder. The power increase comes from pressurized automatic transmission fluid (ATF) supplied by the power steering pump.
Pressure Protection Valve		The pressure protection valve module shuts off all delivery air if the supply falls below the determined setting It also shuts off the supply air to the valve if the delivery pressure falls below the determined setting.
Pressure Switch Module "A"		This pressure switch module receives three different air pressure inputs and closes or opens the appropriate switches when the pressures exceed the specified switch settings.
Pressure Switch Module "B"		Pressure Switch Module "B" receives one or two pressure inputs and opens or closes the switch(es) when the pressure exceeds the specified switch setting.
Pressure Switch Module "C"		Pressure switch module "C" contains a low current pressure switch, which receives a single pressure input and closes a switch when the pressure exceeds the specified switch setting. This module also contains a relay for delivering high current.
Relay Valve Module		The relay valve is a normally closed air valve that delivers system air pressure when the control pressure exceeds the specified level. There is one internal control port on the left side of the module that will mate with pressure switch "B" module to receive parking brake air. The control, supply, and delivery ports are on the face of the module.
Rotor		Rotors work hand-in-hand with brake pads to stop your vehicle. When you step on your brake pedal, your vehicle's brake pads are compressed against the rotor creating friction. This friction enables your vehicle to stop while also creating a tremendous amount of heat. Rotors consist of two iron discs connected by ribs.
Slack Adjuster		Slack adjusters are mechanical links between the brake-chamber push rod and the camshaft on cam type brakes. Slack adjusters are used to manually or automatically adjust the clearance between the brake drum and the brake shoe lining.
Service Relay Valve		The relay valve in an air brake system functions as a relay station to speed up the application and release of the brakes. The valve is normally mounted in the proximity to the chamber it serves.
Solenoid Valve Module (AAVA)		The solenoid modules control ON-OFF air options. All solenoid valve modules are normally closed (NC). The solenoid valve module delivers air when electrically activated.
Solenoid Valve Module (AMU)		All solenoid valve modules are normally closed (N.C.). The solenoid valve module delivers air when electrically activated. The inlet to the solenoid valve module mates to the port on the adjacent pressure protection module or another solenoid valve module. Pressure protected air will pass through the module from the internal supply port and out of the internal delivery port to supply the next solenoid valve module.
Tone Ring		Each ABS-controlled wheel has a stationary wheel speed sensor assembly, mounted very close to, or touching, a tone ring mounted on the hub. When the wheel is rolling, the teeth or slots on the tone ring cause interruptions in the magnetic field created by the sensor. The interruptions create electrical pulses that are sent to the ECU which counts them to determine wheel speed.
Wheel Speed Sensor		Each ABS-controlled wheel has a stationary wheel speed sensor assembly, mounted very close to, or touching, a tone ring mounted on the hub. When the wheel is rolling, the teeth or slots on the tone ring cause interruptions in the magnetic field created by the sensor. The interruptions create electrical pulses that are sent to the ECU which counts them to determine wheel speed.