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Section 3 MERITOR_WABCO ABS Questions and Answers The Electronic Control Unit (ECU) How do you activate the ECU? In a constant-powered system, the ECU activates and then begins a self-diagnostic check of the system when you turn the ignition ON. In a stoplight-powered system, the ECU activates when you apply the brakes. All trailers manufactured on or after March 1, 1998 will be equipped with ABS that has constant power capability with stoplight power as back-up. What if the ECU finds a fault in an ABS component during normal operation? If the ECU senses a fault in the system (with an ABS valve, for example), the ECU turns the trailer ABS warning lamp on and returns the wheel controlled by that valve to standard braking. Or, if the ECU finds a fault with one wheel speed sensor in a system that has four sensors on a tandem axle, the ECU uses information from the other sensor on the same side of the tandem to ensure continuous ABS function. The ECU continues to provide full ABS function to the wheels unaffected by system faults. However, the ECU will turn the trailer ABS warning lamp on to tell the driver a fault has been detected in the system. How does the ECU respond to a wheel approaching lock-up? The ECU directs the ABS relay valve to function as a modulator valve and adjust air pressure to the chambers up to five times a second. This pressure adjustment allows a wheel (or wheels) to rotate without locking. Table B - Stoplight Power
The ABS Warning Lamp What is the function of the warning lamp? The warning lamp enables a driver to monitor the ABS at all times. Refer to the OEM operating manual for the mounting location of the warning lamp. How does the warning lamp operate? How the warning lamp operates depends on whether the ABS is powered by stoplight or constant power: · If the trailer was manufactured prior to February 28, 1998, or was manufactured outside of the United States, the ABS may be either stoplight or constant powered. · If the trailer was manufactured March 1, 1998 or later - and was manufactured in the United States - it will have constant power capability. This is mandated by Federal Motor Vehicle Safety Standard (FMVSS) 121. Check your vehicle specification sheet to determine the type of ABS power . Table B -Stoplight Power and Table C - Constant Power, in this section, illustrate warning lamp operation on stoplight and constant poweredABS systems. ABS Questions and Answers Table C - Constant Power
An ECU with part number 472 500 001 0 manufactured prior to September 1997 requires all sensed wheels to detect a 4 mph signal to shut off the ABS warning lamp. Do not confuse this with a faulty ABS system. If the warning lamp stays on when the brakes are applied to a moving vehicle, service the ABS system. Most trailers manufactured prior to February 1998 require that the brakes be applied to operate the ABS warning lamp. If the warning lamp stays on when the brakes are applied to a moving vehicle, service the ABS system. What does the trailer ABS warning lamp mean to service personnel? The trailer ABS warning lamp indicates the status of the trailer ABS. If it comes ON and stays ON when you apply the brakes to a moving vehicle, there is an ABS malfunction. It is normal for the lamp to come ON and go OFF to perform a bulb check, but it should not stay ON when the vehicle is moving above 4 mph. As with any safety system, it is important not to ignore this warning. If the warning lamp indicates a malfunction, the vehicle can be operated to complete the trip, but it is important to have it serviced as soon as possible using the appropriate maintenance manual to ensure proper braking performance and that the benefits of ABS remain available to your drivers. Typical ABS warning lamp mounting locations are illustrated in Figure 3.1. Figure 3.1
For more information, call the Meritor Customer Support Center, 800-535-5560. Can you continue to operate a vehicle when the warning lamp indicates a fault? Yes. When a fault exists in the ABS, standard braking returns to the affected wheel, and the ABS still controls other monitored wheels. This lets you complete the trip. You should not ignore the warning lamp and should have the vehicle serviced as soon as possible after the lamp comes ON and stays ON. Types of Faults What is a "fault" in the system? A fault in the system is a problem that can exist in the ABS or in the system's components. Faults can be either existing faults or intermittent stored faults. What is an existing fault? An existing fault is a problem that exists currently in the system. For example, a damaged sensor cable is an existing fault that the ECU will detect and store into memory until you identify the cause, repair the cable and clear the fault from the ECU. What is an intermittent fault? An intermittent fault is a problem that usually occurs only under certain driving conditions. For example, the ECU may detect a loose cable or wire or receive an erratic signal from a wheel sensor. Since intermittent faults can be unpredictable and may only happen periodically, you can use information stored in ECU memory to find and correct the loose cable or wire. Is an intermittent fault difficult to locate and repair? It can be, because you may not be able to easily see the cause of the problem. Meritor WABCO recommends that you write down intermittent faults to help you isolate a fault that recurs over a period of time. Can the ECU store more than one fault in memory? Yes. And the ECU retains existing and intermittent faults in memory even when you turn OFF the power to the ECU.
NOTE For part number information, refer to PB-96733. Copies are available from the Meritor Customer Support Center, 800-535-5560. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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