ABS Series: Bosch 5.3, Beyond ABS, Larry Carley, Brake & Front End, February 2001

As anti-lock brake technology continues to evolve, ABS systems are becoming more sophisticated, complicated and compact. Many systems today are capable of providing stability control, as well as traction control, and some have also taken over the job of handling front-to-rear brake proportioning. The Bosch 5.3 ABS system fits this description.

The 5.3 system is manufactured by the Robert Bosch Corporation and is the successor to the earlier Bosch 5 ABS system. Functionally, the two systems are nearly the same, but the 5.3 system is smaller, lighter (by nearly 3 lbs.), less expensive to manufacture and can, if the vehicle manufacturer chooses to do so, provide stability control and/or brake proportioning.

The first Bosch 5.3 system was introduced in 1997 on the Subaru Legacy and Toyota Camry. In 1998, it appeared on a number of General Motors’ models including the Cadillac Catera, Chevrolet Corvette and Camaro, Pontiac Firebird and Grand Prix, and Oldsmobile Intrigue. In 1999, Ford added it on the Contour, Mustang, Mercury Cougar, Mystique and Villager. Mazda also added it on the 626 and MPV minivan, and Nissan adopted it for its Altima, Frontier, Quest and Sentra models. Porsche also switched to the 5.3 system in 1999 on the 911. In 2000, the 5.3 system became standard on the Cadillac DeVille and Pontiac Bonneville, and for the 2001 model year you’ll find it on the Pontiac Aztek and a number of other new models.

Though the 5.3 system is fairly new and many recent applications are still under warranty, most of the earlier applications have now accumulated enough time and miles to be fair game for the aftermarket. Don’t expect a rush of repairs, though, because the 5.3 system has proved to be very reliable and trouble-free. Even so, as the miles add up, rust and corrosion will eventually take their toll on the more sensitive components within the system. The wheel speed sensors, electrical connectors and ABS solenoid valves are historically the parts that cause most ABS troubles.

HYDRAULIC COMPONENTS
Like most other late-model ABS systems, the Bosch 5.3 system is a four-wheel, nonintegral (add-on) ABS system. The Electronic Brake/Traction Control Module (EBTCM) and Brake Pressure Modulator Valve (BPMV) assembly are combined into one unit. The unit also includes a pump and relays for the pump motor and solenoids. The EBTCM or BPMV can both be replaced separately, but neither component is considered serviceable. So if an ABS solenoid valve, the pump or a relay fails, the whole BPMV assembly must be replaced.

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 (Corvette’s Active Handling system or Cadillac’s StabiliTrak). 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.

The ABS valves in the BPMV can isolate, decrease or maintain brake fluid pressure in each individual wheel circuit as needed. There are four inlet and four outlet solenoid valves. The normal position of the ABS inlet valves is open, while the normal state of the outlet valves is closed. On some applications, all four wheels have their own hydraulic circuit, but on others the rear wheels are paired.

The BPMV also contains one or two master cylinder isolation valves. One master cylinder isolation valve is used within the BPMV if the vehicle is equipped with ABS/TCS only. The valve isolates the master cylinder so the pump motor can build brake fluid pressure for the rear brakes when traction control is needed. On applications that also provide steering control, such as the Corvette active handling system, the BPMV has two master cylinder isolation valves. These valves isolate the master cylinder so the pump motor can apply pressure to individual front and/or rear brakes as needed when help is needed to maintain vehicle stability.

The BPMV also contains one or two prime valves. One is used if the vehicle is equipped with ABS/TCS only. This valve allows the pump to draw fluid from the master cylinder reservoir through the compensating ports in the master cylinder bore to build pressure in each rear brake as needed. Two prime valves are used in the BPMV if the vehicle is equipped with active handling so the pump can build pressure in any individual brake as needed.

ELECTRONIC CONTROLS
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.

ACTIVE HANDLING
On Corvette applications with active handling, as well as other GM cars equipped with Magna-steer variable-assist power steering, 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 sensor’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.

DIAGNOSIS
Problems with this ABS system are like those of any other. If the ABS warning lamp is on, it usually means the system has detected a fault and disabled the ABS system. Normal braking should still be available — unless, of course, the brake warning light is also illuminated. In this case the vehicle likely has a hydraulic problem (leaky brake hose, caliper, etc.).

If the ABS lamp is on, you’ll have to use a scan tool to pull the trouble codes. The catch here is that you must have the appropriate scan tool software to access the ABS codes, otherwise you won’t be able to read them. A properly configured scan tool will also allow you to exercise the BPMV solenoids, check pressure hold and release functions, check the ABS lamp circuit, activate traction control, do other system tests and bleed the BPMV.

Once you’ve identified a code, you then have to isolate the fault. If you’ve found a fault code for a wheel speed sensor, it may be due to an open or short in the sensor’s wiring harness, physical damage to the sensor or sensor ring, a buildup of metallic debris on the sensor tip or corrosion.

To measure a wheel speed sensor’s output voltage and circuit continuity at the same time, plug a breakout box into the ABS module’s wiring harness and attach the test leads from a DVOM to the appropriate sensor circuit pins on the breakout box. (The DVOM test leads can also be connected directly to the wheel speed sensor, but testing the sensor this way won’t show if the signal is getting through to the ABS control module or not.) Spin the wheel by hand and note the sensor’s voltage reading.

A good wheel speed sensor will generally produce an alternating current (AC) voltage reading of 50 to 700 MV when the wheel is spun at a speed of about one revolution per second.

If the voltage reading is low or nonexistent, check the sensor’s resistance (with the key off). This can be done through the breakout box with the DVOM. Checking resistance through the breakout box will tell you if the sensor’s wiring harness is OK. If you don’t get the specified value (990 to 1,210 ohms for the front sensors and 900 to 1,100 ohms for the rear sensors on a Corvette, for example), disconnect the sensor from its wiring harness and check the sensor’s resistance by attaching the DVOM test probes to the sensor leads.

A resistance reading that’s now within range tells you the problem is in the wiring, not the sensor. If the sensor has too much internal resistance (opens) or too little resistance (shorts), the sensor is defective and needs to be replaced.

Grounds or shorts in the wheel speed sensor cables can be found by checking continuity between the wiring connectors. If a defect is found in the wires that run between the sensor and the chassis, replacing the wires with new ones is a better repair choice than trying to fix or splice them. These wires undergo a great deal of flexing every time the suspension encounters a bump, so new wires will hold up better than ones that have been soldered, spliced or taped.

Another way to check wheel speed sensors is with an oscilloscope. By displaying the sensor’s output pattern graphically on the scope screen, you can often see problems that are difficult to find otherwise. A missing or damaged tooth on a sensor ring, for example, may not produce a noticeable change in the sensor’s output voltage, but the fluctuation that occurs in the signal every time the missing tooth passes under the tip of the sensor may be enough to affect the operation of the ABS module.

The scope connection can be made through the breakout box or can be hooked up directly to the wheel speed sensor. A scope pattern for a wheel speed sensor should show a classic sine wave alternating current pattern that changes both in frequency and amplitude with wheel speed. As the wheel is turned faster, signal frequency and amplitude should both increase. Damaged or missing teeth on the sensor ring will show up as flat spots or gaps in the sine wave pattern. A bent axle or hub will produce an undulating pattern that changes as the strength of the sensor signal changes with every revolution.

If the scope pattern produced by the sensor is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.