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INTRODUCTION An antilock braking system (ABS) is an electronically managed brake system that monitors and controls wheel speed during braking. An ABS is designed to work with standard air brake systems. It is a requirement of ABS that it has full system redundancy to a Federal Motor Vehicle Safety Standard No. 121 (FMVSS 121) pneumatic control circuit in the event of an electronic failure.
A fully electronic braking system (EBS) is a computerized air brake system in which the control circuit is managed electronically and in which the brake pedal is simply an input to an EBS control module. Although full EBS is used in Europe, when it is used in North America it must be in conjunction with an FMVSS 121–compliant pneumatic control system. This makes it an expensive option, so it is not regularly used.
ABS OBJECTIVES The idea behind ABS is simple. The system monitors wheel speeds at all times and controls brake application pressures when wheel lock is imminent. The main benefits of ABS are better vehicle stability and control during severe braking. The ABS electronic control unit (ECU) (the system controller) receives and processes signals from the wheel speed sensors (which signal wheel speed to the ECU).
When the ECU detects a wheel lockup, the unit activates the appropriate modulator valve (system output, an electrically controlled slave valve), and service air pressure is pulsed. ABS permits a less experienced truck driver to maintain much better control during panic stops.
SPLIT-COEFFICIENT STOPS Because in many systems the brakes on each side of the vehicle are individually controlled, ABS permits maximum efficiency stops even when the vehicle is run on a split-coefficient surface. A split-coefficient surface might occur when one side of the truck is running on ice and the other on bare pavement.
Wheel Speed Sensors • Sensors use a pulse generator principle • Consists of a toothed tone wheel that cuts through the magnetic field located in the stationary sensor (acts like generator) • The analog alternating current (AC) voltage output from the sensors is transmitted to the ECU. • The ECU ignores the voltage value and uses the AC frequency to determine wheel speed.
Wheel and Axle Speed Sensors 100 tooth sensors
Sensor Resistance The sensor circuit resistance must be 700 to 3,000 ohms. Resistance can be measured at the sensor connector or at the pins on the ECU connector using a DMM.
Sensor Signal (Output) Voltage Test Voltage must be at least 0.200 volt AC at 30 rpm. 1. Turn the ignition off. 2. Disconnect the ECU (see the wiring diagram). 3. Raise the wheel from the ground and release the brakes. 4. Use a DMM on AC volts mode range set to read up to 1V. 5. Rotate the wheel by hand at 30 rpm (one revolution per 2 seconds).
Make sure that your DMM is in V-AC mode. Spin the tire at 30 rpm (1 rev per 2 sec) and check voltage. See manual. Resistance should be 700-3,000 ohms.
ELECTRONIC CONTROL UNIT The ECU is a simple computer. It receives input signals from the wheel speed sensors. Because these input signals are in analog form, they must be converted to a digital format before they can be processed. The ECU then processes the information and prevents wheel lock up by signaling the brake modulators.
ABS VOLTAGE CHECK • Voltage should measure between 11 and 14 volts on a 12-volt system (18 and 30 volts for a 24-volt system). • Remember, the ignition key must be turned on for this test.
An older style modulator controller assembly with a diagnostic LED
Brake Modulators • Solenoid controlled valve capable of dumping air being delivered to the brake chambers • Typically will contain 1-4 solenoid valves depending on number of brake circuits being regulated • May be integrated with the relay/quick release valve.
Brake Modulators or ABS Valves valve in balanced position service application. exhausting during a normal service application
Testing an ABS Modulator Valve Measure the resistance across each valve solenoid coil terminal and ground on the ABS valve to ensure 4 to 8 ohms for a 12-volt system (8 to 16 ohms for a 24-volt system).
ABS CONFIGURATIONS There are a number of types of ABS on trucks today. Just as in automobiles, there is some difference between high-performance and basic systems. The type of ABS that will function best on a truck depends on number of axles, axle configuration, axle load, brake circuit distribution, and brake force distribution.
DEFINING CHANNELS The term channel refers to either an input or output path to the ECM. However, in describing ABS, each manufacturer uses the word to mean slightly different configurations.
Channel Arrangements Regardless of the number of ABS channels, on most tractors or straight trucks, there is only one ABS module managing the system. Truck ABS, whether air or hydraulic, can operate as either split- or diagonally arranged systems. A split system means that the modulator valve would manage a pair of service chambers located on one side of the vehicle.
SIX/FOUR CHANNEL SYSTEM The 6S/4M ABS locates a wheel speed sensor on each of the six wheels of the vehicle, so the speed of each wheel is monitored. However, the system produces only four outputs, meaning that four of the six wheels are managed in pairs.
CAUTION: ABS effectiveness is fully dependent on the foundation brake adjustment. Use stroke indicators to check brake adjustment status.
Power Line Carrier A primitive form of multiplexing known as power line carrier (PLC) has been used to permit trailer-to-truck ABS communications since 1985. PLC enables communications transactions to take place on a nondedicated communication wire, specifically the auxiliary wire in a standard ATA 7-pin tractor/trailer connector (J-560). PLC communications were introduced when ABS became an option on highway trailers, but its use became mandatory in 2001, 3 years after the introduction of mandatory ABS.
TRAILER ABS WARNING LIGHT Trailer ABS should have a failure warning light mounted on the trailer. The warning light stays on until the self-check is completed. A self-check includes checking the output of the wheel speed sensors.
Shop Talk Bulb Test: Depress the ABS switch for 16 seconds. The ABS light should illuminate even when no codes are present.
Shop Talk Never use a battery charger to perform a dynamic check of a trailer ABS because ECU damage can result.
Shop Talk Blink codes in most systems do not display faults in the order they were recorded in the ECU memory. Also, if codes are blinked that do not appear in the OEM code chart, the ECU may be defective.
SELF-DIAGNOSTICS Self-diagnostics are written into the software of all the current systems. Most ABS are designed to perform a self-test on startup. When the ABS circuit is first energized, a clicking noise can be heard from the modulator valve solenoids.
ESTs • Display active and inactive codes • Clear fault codes • Display valve and sensor configuration data • Display auxiliary codes
PC-Based Software This type of full feature software has all of the capabilities of the handheld ESTs but additionally will: • Display odometer readings • Display an audit trail (archived faults) • Accept tire data programming • Provide service reminder data • Analyze brake performance and display percentage of braking by each wheel • Archive data trails
TIRE SIZE RANGE Proper ABS/ATC operation with a standard ECU requires that the front and rear tire sizes be within 14 percent of each other. If this tire size range is exceeded without electronically modifying the ECU (PROM), system performance may be affected and the warning lamp can be illuminated. Some ECUs can be reprogrammed and special ECUs can be ordered in cases where reprogramming is not possible.
Dynamometer Testing Vehicles with ATC Another typical method of disabling ATC is to press and hold the blink code switch for at least 3 seconds. When the system configuration code appears, ATC has been disabled. The ATC lamp illuminates and stays on while the system is disabled. Some systems require that the ATC fuse or breaker be removed.
EBS Advantage Electrical signals travel at closer to the speed of light. In a true EBS, the pneumatic control circuit would be eliminated. It would be replaced by a “brake by-wire” electrical control circuit, which would respond much faster to changes in the command signals from the driver’s boot. The actual braking force is still provided by the potential energy of compressed air metered to brake chambers by electrically controlled modulators.
