Exhaust Brake Description and Operation Without RPO LLY
Exhaust Brake System Operation
The exhaust brake system creates a restriction in the exhaust system that induces
back pressure against the engine. This slows the vehicle using the engine a transmission.
This system is also used for the Manual High Idle System. The exhaust brake system
consists of the following components:
| • | The engine control module (ECM) |
| • | An exhaust brake actuator |
| • | An exhaust brake actuator control solenoid (EBACS) valve |
| • | An exhaust brake switch |
| • | An exhaust brake valve located in the exhaust system |
| • | A system to supply compressed air to the exhaust brake system. The two
air supply systems are as follows: |
| - | Vehicles equipped with air brakes supply the compressed air using the
air brake supply system. |
| - | Vehicles not equipped with air brakes supply the compressed air using
an auxiliary air compressor. |
When the exhaust brake switch is turned ON, the ECM receives the signal and
will operate the exhaust brake automatically when needed.
The exhaust brake is active under the following conditions:
| • | Accelerator Pedal Position (APP) sensor is at 0 percent. |
| • | Vehicle speed is more than 0 km/h (0 mph). |
| • | Cruise control is inactive |
| • | No APP or VSS DTCs are active. |
Exhaust Brake System Components Description
Engine Control Module (ECM)
The engine control module (ECM) controls the exhaust brake through the exhaust
brake relay. If the exhaust brake switch is ON the ECM will engage the exhaust brake
automatically if the proper conditions exist. It will automatically turn off the exhaust
brake if the accelerator is depressed. The ECM will also activate the exhaust brake
to engage the manual high idle system. This System allows the engine to warm up faster.
Exhaust Brake Actuator
The exhaust brake actuator is attached to the exhaust brake valve in the exhaust
system. When air pressure is applied to the actuator, it pushes a linkage attached
to the valve in the exhaust pipe. This closes the exhaust brake, restricting exhaust
flow.
Exhaust Brake Actuator Control Solenoid Valve
The exhaust brake actuator control valve is an electrical solenoid attached
to a valve to control air to the exhaust brake actuator. When the ECM turns ON the
exhaust brake relay, the relay energizes the exhaust brake actuator control solenoid
valve. When voltage is applied to the solenoid of the exhaust brake actuator control
valve, the valve opens, allow air pressure to the exhaust brake actuator.
Air Compressor and Air Lines
The air supply for the exhaust brake comes from an air pump. On vehicles equipped
with air brakes, air is supplied by the air brake air pressure system. On vehicles
not equipped with air brakes, an auxiliary air compressor supplies air pressure to
the exhaust brake actuator control valve. this valve allows air to flow to the exhaust
brake actuator when commanded ON by the controller. The air compressor is energized
by a relay that is controlled by an air tank pressure switch. When the switch is closed
it supplies voltage to the compressor relay. When the air pressure reaches 724-827 kPa
(105-120 psi) the switch opens and the air compressor turns OFF.
Exhaust Brake Relay
The ECM grounds the control circuit of the exhaust brake relay. this allow voltage
to the exhaust brake actuator control solenoid valve.
Exhaust Brake Switch
The exhaust brake switch is located on the instrument panel, to the right of
the steering column. When turned On, the switch completes a ground circuit to the
ECM. This ground is the request signal to the ECM to allow exhaust brake operation.
Exhaust Brake (Valve)
The exhaust brake is the valve inside the exhaust pipe near the transmission.
It is opened and closed by the exhaust brake actuator. There is a small opening in
the valve that protects the engine from damage when the exhaust gas flow is too high,
and allows the engine to idle if the valve sticks closed.
