Theory Of Operation

WARNING: This page is about a different car, the 2017 Jeep Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

The brake switch has two internal switches. The Powertrain Control Module (PCM) runs a diagnostic to monitor the operation and rationality of both switch inputs. The switches do not apply simultaneously and are configured differently.

Brake Switch 1 is in the normally open state at rest.
Brake Switch 2 is in the normally closed state at rest.

Brake Switch 1 closes at initial pedal application and is solely responsible for disabling cruise control. Since Brake Switch 1 is solely responsible for disabling cruise control, it is also checked for a stuck on and stuck off condition. Brake Switch 1 Signal is hardwired to the PCM. Brake Switch 2 opens after additional travel is applied. On a properly operating switch, Brake Switch 1 will always change states before Brake Switch 2. This is important to understand for the Brake Switch rationality diagnostic. Brake Switch 2 Signal is hardwired to the BCM and is transmitted to the PCM over CAN C.

The Brake Switch 1 rationality diagnostic will fail immediately if the PCM receives a brake switch 2 pressed signal while brake switch 1 is not pressed. In some cases Brake Switch 1 can be applied without Brake Switch 2 due to an improperly adjusted switch or during two foot driving conditions. It is important to confirm that these conditions do not exist.

Brake Switch 2 rationality diagnostic will fail after the PCM sees approximately 50 consecutive state changes of Brake Switch 1 with no changes in Brake Switch 2.

The brake switch stuck on test will set a fault if the brake switch is determined to be in the on state (brake pedal depressed) when the vehicle is moving. Once the enable conditions are met, the diagnostic will be enabled. If, while enabled, the brake switch is seen to be in an on state (depressed), a fail timer will start to count in one second increments. This timer can be decremented if the brake switch state returns to the off position. If the fail timer reaches a calibrated fail time, a malfunction will be declared and the timer will be reset.
The brake switch stuck off test looks for no sign of the brake switch being on (brake pedal depressed) during repeated stopping maneuvers. If the vehicle comes to a complete stop and the brake switch is off, a fail timer will start to count in one second increments. If the brake switch is on, the fail timer will decrement back down to zero. If the fail timer reaches a calibrated fail time, a malfunction will be declared and the timer will be reset.

NOTE:

Since Adaptive Cruise Control (ACC) is capable of bringing the vehicle to a complete stop, this diagnostic could potentially false fail because the vehicle reaches a vehicle speed of zero without the brake pedal being dressed. If ACC is actively applying the brakes this diagnostic will be disabled to avoid a false failure.