The Bosch 5.3 ABS debuted in 1997 and was first used on the 1998-1/2 Corvette. The 5.3 system (a more compact version of the Bosch 5) combines the Brake Pressure Modulator Valve (BPMV) and Electronic Brake Traction Control Module (EBTCM) into one assembly. The pump motor is a separate item and can be serviced (some sources say not serviceable), unlike the pump on the earlier Bosch 5 system.
The Bosch 5.3 (like the Bosch 5) is a nonintegral ABS system with a conventional master cylinder and vacuum booster. The 5.3 is smaller (by almost half), lighter (by 2-1/2 lbs.) and costs about 60 percent less than the Bosch 5. The module and hydraulic modulator unit are combined as an assembly, held together by six screws.
General Motors refers to the 5.3 controller (which actually contains two microprocessors to check on one another) as the Electronic Brake Control Module (EBCM), or the Electronic Brake Traction Control Module (EBTCM) on models equipped with traction control. The hydraulic modulator is called the Brake Pressure Modulator Valve (BPMV).
The Brake Pressure Modulator Valve contains the ABS solenoids and valves that modulate brake fluid pressure within each individual brake circuit as needed during ABS braking, acceleration (traction control) or maneuvering (Corvetteâ€™s Active Handling system). During the anti-lock mode, the BPMV can maintain or reduce brake fluid pressure independently of the pressure generated in the master cylinder. However, the BPMV does not provide more pressure than is applied by the master cylinder in this mode. If the unit is called on to provide traction control or handling assistance, the BPMV will apply pressure independently of the master cylinder to the individual wheel circuits.
Pressure is generated by the pump that is part of the BPMV. The pump also recirculates fluid back to the master cylinder when brake pressure is being reduced during an ABS stop.
On applications that have Magnasteer variable assist power steering, the Bosch 5.3 control module also helps control steering assist. It does this by monitoring vehicle speed via its four wheel speed sensors.
The brain of the system is the Electronic Brake and Traction Control Module (EBTCM). Depending on what features the vehicle is equipped with
, the EBTCM not only monitors inputs from the individual wheel speed sensors (WSS) and brake pedal switch, but also a yaw sensor, steering angle sensor and lateral acceleration sensor (Corvette only).
The EBTCM monitors and compares wheel speeds to decide when wheel slip is excessive and/or the vehicle is becoming unstable. If the vehicle is braking, wheel slip will trigger ABS braking. If the vehicle is accelerating, wheel slip will trigger traction control. And if the vehicle is cornering or making a sudden steering maneuver, undesirable changes in the yaw rate will trigger corrective braking to restore handling control.
Each wheel in a Bosch 5.3 ABS system has its own wheel speed sensor. They are the traditional magnetic type that generate an AC signal that increases both frequency and amplitude with wheel speed. The sensor and toothed sensor ring are usually part of an integral hub/bearing assembly, and do not have an adjustable air gap. Shielded wiring is used to help reduce electromagnetic interference that can cause false or noisy WSS inputs to the EBTCM.
On vehicles equipped with traction control and stability control, there is a traction control/active handling on/off switch so the driver can shut off either system for personal or diagnostic reasons. On the Corvette, pressing and holding the TCS/active handling on/off switch on the center console for five seconds places the system in a special "competitive" driving mode. The competitive driving mode turns the TCS off, but leaves active handling on.
On Corvette applications with active handling, a steering wheel position sensor is used to monitor driver inputs. The sensor outputs a pair of digital reference signals (called "Phase A and Phase B"), and an analog signal. This information is used to determine when the steering wheel is centered, when the wheels are being steered, and how quickly the wheels are being steered. The information is then used to vary steering assist as well as to activate the active handling system if the vehicle is becoming unstable.
The EBTCM runs a centering routine when the vehicle speed goes above 30 km/h (18 mph). When the vehicle reaches 30 km/h, the EBTCM monitors the steering sensor signals (Phase A, Phase B and analog voltage) to see if the steering wheel is moving. If the steering wheel is not moving for a set period of time, the EBTCM assumes the vehicle is going in a straight line. At this point, the EBTCM looks at the analog voltage signal and reads the voltage. This voltage, normally around 2.5 volts, is then considered the center position and the digital degrees also become zero at the same time.
This centering routine is necessary to compensate for wear in the steering and suspension which can change in the relationship between the steering wheel and the front tires when driving in a straight line.
The Corvette also has a Lateral Accelerometer to monitor cornering forces. The sensor uses a reference voltage of five volts and varies its output voltage from 0.25 to 4.75 volts depending on the G-forces encountered. The sensorâ€™s operating range is -1.5 to +1.5G. Zero lateral acceleration results in an output signal of 2.5 volts, while a maximum G-force of 1.5 would yield an output voltage of 4.75 volts.
The yaw rate sensor also uses a reference voltage of five volts, and outputs a signal that can vary from 0.25 to 4.75 volts over a range of -75 to +75 degrees/second. A zero yaw rate would read 2.5 volts.
The EBTCM uses the lateral accelerometer along with the steering wheel position sensor and the wheel speed sensors to calculate the desired yaw rate. The EBTCM compares the desired yaw rate to the actual yaw rate as read by the yaw rate sensor. The difference between the two is the "yaw rate error." The EBTCM then uses its program algorithms to figure out how much correction is needed to maintain vehicle stability. This is called the "Bank Angle Compensation Calculation." As soon as the EBTCM has this information, it brakes one or more individual wheels to help correct the steering and reduce the yaw error. The active handling continues until the vehicle straightens out and completes the steering maneuver.
The Corvette also has a brake pressure sensor to monitor how hard the brakes are being applied. The sensor can read pressures from zero to 2000 psi, generating an output signal of 0.20 to 4.80 volts. The EBTCM uses the input from this sensor when active handling is engaged to provide more accurate control over the braking system.
Something to keep in mind concerning the Corvette active handling system is that changing the tire/wheel size can upset the operation of the Bosch 5.3 system. If someone has installed a different size tire or wheel, the wheel/tire combo may not work well with the stock calibration of the active handling system.
Diagnostics are via a Tech 1, Tech 2 or equivalent scan tool. A scan tool is also required to clear trouble codes. Special diagnostic modes include solenoid inlet and outlet tests, traction control system test and lamp test.
Manual or pressure bleeding can be used to flush the brakes, but if air is trapped in the BPMV you'll have to use a scan tool to run the "auto bleed" procedure. This cycles the ABS solenoids and runs the pump to purge air from the secondary circuits which are normally closed off during normal braking.