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Enhancing Vehicle Performance with Brake Pedal Position Sensors

By R& D
Published on March 15h, 2024

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Purpose

The brake pedal position sensor (BPPS) sends a signal used by the body control module (BCM), engine or powertrain control module (ECM/PCM), and transmission control module (TCM) to determine the specific position of the brake pedal within its range of travel. 

Brake Pedal Position Sensor

Depending on vehicle design, the BPPS signal is then used to illuminate the brake lights, illuminate the center high mounted stop light, unlock the automatic transmission torque converter, and activate anti-lock braking and yaw control systems. 

The signal may also be used, in vehicles so equipped, to activate regenerative braking systems that charge the hybrid battery. Some very late model vehicles use the BPPS signal to activate an electronic brake override system reducing engine power if the driver simultaneously presses the both the accelerator and brake pedals.

Location

The BPPS sensor is located on a bracket above the brake pedal or on the pedal arm. It can be identified by locating the electrical pigtail connected to it. 

On some older vehicles, a 'brake pedal position sensor' is located behind the brake master cylinder in the engine compartment. Such sensors are actually simple on/off switches solely used to illuminate the brake lights, and not “true” sensors indicating specific pedal position.

Operation

The typical brake pedal position sensor consists of one or two variable resistors, termed potentiometers, which change resistance to electrical current traveling through the resistor as the brake pedal is moved. 

Each potentiometer has three electrical circuits: a 5 volt reference circuit that supplies voltage to the sensor, a signal voltage circuit, and a ground circuit. As the brake pedal moves through its range of travel, electrical resistance within the potentiometer's signal circuit changes correspondingly, decreasing or increasing the voltage in the signal circuit.

Drivability Symptoms

If the BPPS malfunctions, the brake lights may fail to illuminate when the brake pedal is pressed or may, in some instances, may illuminate when it is not pressed. The brake pedal may need to be pushed further down to illuminate the brake lights or disengage cruise control. 

In some vehicles, "calibration error," occurring when the ECM/PCM cannot determine the brake pedal position, will prevent starting the engine. Because these malfunctions do not affect vehicle emissions, the instrument panel Malfunction Indicator Light (MIL) may not illuminate, though other warning lights may display.

Enhancing Vehicle Performance

Inspection, Test, and Diagnosis

Malfunction of the brake pedal position sensor sets one or more OBD2 "generic" chassis Diagnostic Trouble Codes (codes beginning with "C"). Consequently, if a BPPS malfunction is suspected, retrieving stored DTCs should be the first step. Generic chassis codes are shared by all vehicle manufacturers, but are less well-defined than powertrain codes. 

Not all codes are used by all manufacturers and there may be variations in the diagnostic procedures for particular codes among manufacturers. Vehicle manufacturers may also define "enhanced" diagnostic codes applicable only to that manufacturer's vehicles that further refine diagnostic processes and that can be retrieved with a suitable scan tool.

The most common generic BPPS trouble codes are C0277, indicating a defective sensor or a short or open in BPPS signal or reference circuits, C0278, indicating that the sensor is not calibrated, C0282 through C0284, indicating electrical faults in the BPPS or its circuits, and C0870, indicating a short to battery voltage or ground in the BPPS reference circuit. The code for ABS brake switch malfunction, C0161, may set with C0277 and C0278 and should be diagnosed before C0161.

To test the BPPS itself, first identify the circuit terminals in the pigtail connector. There will be either three terminals (one potentiometer) or six (two potentiometers). If you do not have access to a factory service manual, terminals can be identified by elimination. 

Begin by locating the reference circuit. With the electrical pigtail removed from the BPPS and the ignition key on, but the engine off, connect the negative black lead of a digital multimeter (DMM) set to the voltage scale to a good ground and touch the red lead to each of the pigtail terminals. 

One for each potentiometer should read approximately 5 volts. That is the reference circuit and the 5 volt reading establishes that the sensor is receiving proper voltage. If there is not a 5 volt reading on one terminal for each potentiometer, there is a fault in the circuit to the BPPS or in the pigtail connector.

The remaining two terminals for each potentiometer are the signal and ground circuits. To test these, reconnect the pigtail and "backprobe" the sensor. To backprobe, either use a commercially available "backprobe pin" that fits onto the DMM lead or use a "T" pin or small paperclip bent straight and clip the DMM lead to it. 

Slide the "pin" between the outside sheathing of the wire where it enters the connector and the rubber or neoprene weatherproof seal around the wire that keeps moisture out of the connector. Be careful not to perforate the wire's insulation.

brake light switch under the brake pedal in a car

Insert the pin sufficiently into the connector to touch the electrical contact within the connector. Connect the red positive lead to one of these two terminals and the negative lead to the other. With the key on and the engine off, slowly and smoothly depress the brake pedal. If there is no reading, reverse the leads, and repeat the test.

In one of these two configurations, the red lead will be connected to the signal circuit and the black lead to the ground circuit. The voltage reading should change proportionally as the brake pedal is depressed. If that does not occur, or the voltage reading changes erratically, either the BPPS is defective or there is a bad ground. 

To exclude the ground as the cause, repeat the test with the DMM's black lead connected to an external ground. If the voltage reading now changes smoothly, the fault is in the ground circuit from the BPPS. If the results do not change, the BPPS is defective and should be replaced.

Reprogramming

Replacement of the brake pedal position sensor may require "recalibration," which is reprogramming the BCM or ECM/PCM to learn the resting position of the brake pedal and requires specialized equipment.

Repair/Replacement

The brake pedal position sensor is removed by disconnecting its pigtail and then removing the screws or bolts retaining it to the pedal assembly. Installation is the reverse of removal.

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