Exhaust Brake Description and Operation (LGH or LML)

Exhaust Brake Description and Operation (LGH or LML)

Exhaust Brake Description and 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 and transmission. The exhaust brake system consists of the following components:

* The exhaust brake switch
* The Engine Control Module (ECM)
* The Transmission Control Module (TCM)
* The turbocharger assembly

Under certain conditions, the ECM will automatically close the turbocharger vanes to build back pressure in the exhaust. This increase in back pressure causes the engine speed to be reduced. The following conditions are required for the exhaust brake to be enabled :

* The exhaust brake switch is ON.
* The antilock brake system (ABS) is not ACTIVE.
* The automatic transmission fluid temperature is greater than 5°C (41°F).
* The coolant temperature is greater than 30°C (86°F)
* Transmission torque convertor is LOCKED.
* The fuel system is in deceleration mode.
* The vehicle speed is greater than 20 km/h (12 mph).
* No Accelerator Pedal Position (APP) or Vehicle Speed Sensor (VSS) DTCs are active.

The ECM may also enable engine braking while the cruise control is active, to maintain the set target speed. This is called cruise grade braking. If cruise control is active and the exhaust brake switch is ON, then the exhaust brake will activate when the vehicle speed is greater than 3 km/h (1.9 mph) over the set speed. The TCM makes this request along with a modified shift schedule, (more downshift availability), to hold the cruise set point on downhill grades.