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Dado que la mayorÃa de los automóviles en la carretera hoy en dÃa tienen algún tipo de Sistema de frenos antibloqueo (ABS), son lo suficientemente importantes como para ver cómo funcionan y aclarar alguna información errónea sobre ellos.
Como siempre, lo que se describe aquà es cómo funciona la mayorÃa de los sistemas en general. Dado que diferentes fabricantes tienen sus propias versiones de ABS, sus especificaciones y nombres de partes pueden diferir. Si tiene un problema con el ABS en su vehÃculo, siempre debe consultar los manuales de servicio y reparación especÃficos para su vehÃculo.
El ABS es un sistema que evita el bloqueo de las ruedas al modular automáticamente la presión del freno durante una frenada repentina. Al evitar que las ruedas se bloqueen, permite al conductor mantener el control de la dirección y detenerse en la distancia más corta posible en la mayorÃa de las condiciones. Durante el frenado normal, la sensación del pedal de freno ABS y no ABS será la misma. Durante la operación del ABS, se puede sentir una pulsación en el pedal del freno, acompañada de una bajada y luego subida de la altura del pedal del freno y unos chasquidos.
Los vehÃculos con ABS están equipados con un sistema de doble freno accionado por pedal. El sistema básico de frenado hidráulico consta de lo siguiente:
- Válvulas de control hidráulico ABS y unidad de control electrónico
- Cilindro maestro del freno
- Tubos de freno y mangueras necesarios
El sistema de frenos antibloqueo consta de los siguientes componentes:
- Unidad de control hidráulico (HCU)
- Módulo de control de frenos antibloqueo
- Sensores de freno antibloqueo delantero / Sensores de freno antibloqueo traseros
Los sistemas de frenos antibloqueo (ABS) funcionan de la siguiente manera
- Cuando se aplican los frenos, el fluido es forzado desde los puertos de salida del cilindro maestro del freno a los puertos de entrada de la HCU. Esta presión se transmite a través de cuatro válvulas solenoides normalmente abiertas contenidas dentro de la HCU, luego a través de los puertos de salida de la HCU a cada rueda.
- El circuito primario (trasero) del cilindro maestro del freno alimenta los frenos delanteros.
- El circuito secundario (delantero) del cilindro maestro del freno alimenta los frenos traseros.
- Si el módulo de control del freno antibloqueo detecta que una rueda está a punto de bloquearse, según los datos del sensor del freno antibloqueo, cierra la válvula solenoide normalmente abierta para ese circuito. Esto evita que más fluido ingrese a ese circuito.
- El módulo de control del freno antibloqueo vuelve a mirar la señal del sensor del freno antibloqueo de la rueda afectada.
- Si esa rueda todavÃa está desacelerando, abre la válvula solenoide para ese circuito.
- Una vez que la rueda afectada vuelve a la velocidad, el módulo de control del freno antibloqueo devuelve las válvulas solenoides a su estado normal permitiendo que el fluido fluya al freno afectado.
- El módulo de control de frenos antibloqueo controla los componentes electromecánicos del sistema.
- El mal funcionamiento del sistema de frenos antibloqueo hará que el módulo de control del freno antibloqueo se apague o inhiba el sistema. Sin embargo, el frenado asistido normal permanece.
- La pérdida de lÃquido hidráulico en el cilindro maestro del freno desactivará el sistema antibloqueo. El sistema de frenos antibloqueo en las 4 ruedas se controla a sà mismo. Cuando el interruptor de encendido se gira a la posición de MARCHA, el módulo de control del freno antibloqueo realizará una autocomprobación preliminar en el sistema eléctrico antibloqueo indicado por una iluminación de tres segundos del indicador de falta de ABS amarillo.
- Durante el funcionamiento del vehÃculo, incluido el frenado normal y antibloqueo, el módulo de control del freno antibloqueo controla todas las funciones eléctricas antibloqueo y algunas operaciones hidráulicas.
- Cada vez que se conduce el vehÃculo, tan pronto como la velocidad del vehÃculo alcanza aproximadamente 20 km/h (12 mph), el módulo de control del freno antibloqueo enciende el motor de la bomba durante aproximadamente medio segundo. En este momento, se puede escuchar un ruido mecánico. Esta es una función normal de la autocomprobación del módulo de control de frenos antibloqueo.
- Cuando la velocidad del vehÃculo desciende por debajo de 20 km/h (12 mph), el ABS se apaga.
- La mayorÃa de los fallos del sistema de frenos antibloqueo y del sistema de control de tracción, si están instalados, harán que se ilumine el indicador amarillo de advertencia del ABS.
La mayorÃa de los camiones ligeros y SUV utilizan una forma de ABS conocida como ABS de la rueda trasera. El sistema antibloqueo de la rueda trasera (RWAL) reduce el bloqueo de la rueda trasera durante el frenado severo al regular la presión de la lÃnea hidráulica trasera. El sistema monitorea la velocidad de las ruedas traseras durante el frenado. El módulo electrónico de control de frenos (EBCM) procesa estos valores para producir controles de comando para evitar que las ruedas traseras se bloqueen.
Este sistema utiliza tres componentes básicos para controlar la presión hidráulica en los frenos traseros. Estos componentes son:
- Módulo de control electrónico de frenos
- Válvula de presión antibloqueo
- Sensor de velocidad del vehÃculo
El EBCM montado en un soporte al lado del cilindro maestro, contiene un microprocesador y un software para la operación del sistema.
Válvula de presión antibloqueo
La válvula de presión antibloqueo (APV) está montada en la válvula combinada debajo del cilindro maestro, tiene una válvula de aislamiento para mantener o aumentar la presión hidráulica y una válvula de descarga para reducir la presión hidráulica.
El Sensor de velocidad del vehÃculo (VSS) ubicado en la parte trasera izquierda de la transmisión en camiones con tracción en las dos ruedas y en la caja de transferencia de los vehÃculos con tracción en las cuatro ruedas, produce una señal de voltaje de CA que varÃa en frecuencia de acuerdo con la velocidad del eje de salida. En algunos vehÃculos, el VSS se encuentra en el diferencial trasero.
Durante el frenado normal, el EBCM recibe una señal del interruptor de la luz de freno y comienza a monitorear la lÃnea de velocidad del vehÃculo. La válvula de aislamiento está abierta y la válvula de descarga está asentada. Esto permite que el fluido bajo presión pase a través del APV y viaje al canal del freno trasero. El interruptor de reinicio no se mueve porque la presión hidráulica es igual en ambos lados.
Durante una aplicación de freno, el EBCM compara la velocidad del vehÃculo con el programa incorporado en él. Cuando detecta una condición de bloqueo de la rueda trasera, opera la válvula de presión antibloqueo para evitar que las ruedas traseras se bloqueen. Para hacer esto, el EBCM utiliza un ciclo de tres pasos:
- Mantener presión
- Disminuir presión
- Aumentar presión
Mantener presión
Durante la presión, mantener el EBCM energiza el solenoide de aislamiento para detener el flujo de fluido desde el cilindro maestro a los frenos traseros. El interruptor de reinicio se mueve cuando la diferencia entre la presión de la lÃnea del cilindro maestro y la presión del canal del freno trasero es lo suficientemente grande. Si esto sucede, pone a tierra el circuito lógico EBCM.
Durante la disminución de la presión, el EBCM mantiene el solenoide de aislamiento energizado y energiza el solenoide de descarga. La válvula de descarga se mueve de su asiento y el fluido bajo presión se mueve hacia el acumulador. Esta acción reduce la presión de la tuberÃa trasera evitando el bloqueo posterior. El interruptor de reinicio se basa en decirle al EBCM que se ha producido una disminución de la presión.
Durante el aumento de presión, el EBCM desenergiza los solenoides de descarga y aislamiento. La válvula de descarga vuelve a colocar y retiene el fluido almacenado en el acumulador. La válvula de aislamiento 9 se abre y permite que el fluido del cilindro maestro pase y aumente la presión hacia los frenos traseros. El interruptor de reinicio vuelve a su posición original por la fuerza del resorte. Esta acción le indica al EBCM que la disminución de presión ha finalizado y que la presión aplicada por el conductor se reanuda.
Cuando el interruptor de encendido se "ENCIENDE", el EBCM realiza una autocomprobación del sistema. Comprueba su circuito interno y externo y realiza una prueba de funcionamiento mediante el ciclo de las válvulas de aislamiento y descarga. El EBCM comienza su funcionamiento normal si no se detectan fallas de funcionamiento.
La pulsación del pedal del freno y el "chirrido" ocasional de los neumáticos traseros son normales durante la operación RWAL. La superficie del camino y la severidad de la maniobra de frenado determinan cuánto ocurrirán. Dado que estos sistemas solo controlan las ruedas traseras, aún es posible bloquear las ruedas delanteras durante ciertas condiciones severas de frenado.
El uso de la llanta de refacción suministrada con el vehÃculo no afectará el rendimiento del RWAL o del sistema.
El tamaño del neumático puede afectar el rendimiento del sistema RWAL. Los neumáticos de repuesto deben ser del mismo tamaño, rango de carga y construcción en las cuatro ruedas.
Contrariamente a la creencia popular, los frenos ABS no detendrán su automóvil más rápido. La idea detrás de los frenos ABS es mantener el control de su vehÃculo evitando el bloqueo de las ruedas. Cuando sus ruedas se bloquean, no tiene control de dirección y girar el volante para evitar una colisión no le servirá de nada. Cuando las ruedas dejan de girar, ya está hecho.
Al conducir en carreteras resbaladizas, debe permitir una mayor distancia de frenado, ya que las ruedas se bloquearán mucho más fácilmente y el ABS circulará mucho más rápido. La velocidad también es un factor, si vas demasiado rápido, incluso el control que te da el ABS no será suficiente para superar la inercia simple. Puede girar la rueda hacia la izquierda o hacia la derecha, pero la inercia lo mantendrá avanzando.
Si hay un fallo en el ABS, el sistema volverá al funcionamiento normal de frenado, por lo que no se quedará sin frenos. Normalmente, la luz de advertencia del ABS se encenderá y le informará que hay una fallo. Cuando esa luz está encendida, es seguro asumir que el ABS ha cambiado al funcionamiento de frenado normal (sin ABS) y se puede conducir teniendo en cuenta esa premisa.
Con suerte, esto le ha ayudado a comprender cómo funcionan los sistemas ABS. Es una tecnologÃa que ha estado en uso durante muchos años antes de que fuera adaptada para uso automotriz. Las aeronaves han estado utilizando algún tipo de ABS desde la Segunda Guerra Mundial y es un sistema probado y verdadero que puede ser de gran ayuda para evitar accidentes si se usa como se pretendÃa.
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Since most cars on the road today have some form of Anti-lock Brake System (ABS) they are important enough to take a look at how they work and clear up some misinformation about them.
As always, what is described here is how most systems work in general. Since different manufacturers have their own versions of ABS their specifications and part names may differ. If you're having a problem with the ABS on your vehicle you should always refer to the specific service and repair manuals for your vehicle.
The ABS is a four-wheel system that prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions. During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound.
Vehicles with ABS are equipped with a pedal-actuated, dual-brake system. The basic hydraulic braking system consists of the following:
- ABS Hydraulic Control Valves and Electronic Control Unit
- Brake Master Cylinder
- Necessary brake tubes and hoses
The anti-lock brake system consists of the following components:
- Hydraulic Control Unit (HCU)
- Anti-Lock Brake Control Module
- Front Anti-Lock Brake Sensors/Rear Anti-Lock Brake Sensors
Anti-Lock Brake Systems (ABS) Operate as Follows
- When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of the HCU to each wheel.
- The primary (rear) circuit of the brake master cylinder feeds the front brakes.
- The secondary (front) circuit of the brake master cylinder feeds the rear brakes.
- If the anti-lock brake control module senses a wheel is about to lock, based on anti-lock brake sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.
- The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel again.
- If that wheel is still decelerating, it opens the solenoid valve for that circuit.
- Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake.
- The anti-lock brake control module monitors the electromechanical components of the system.
- Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains.
- Loss of hydraulic fluid in the brake master cylinder will disable the anti-lock system. [li[The 4-wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a three-second illumination of the yellow ABS wanting indicator.
- During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.
- Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.
- When the vehicle speed goes below 20 km/h (12 mph), the ABS turns off.
- Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated.
Most light trucks and SUVs use a form of ABS known as Rear Wheel ABS. The Rear Wheel Anti Lock (RWAL) system reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. The system monitors the speed of the rear wheels during braking. The Electronic Brake Control Module (EBCM) processes these values to produce command controls to prevent the rear wheels from locking.
This system uses three basic components to control hydraulic pressure to the rear brakes. These components are:
- Electronic Brake Control Module
- Anti-Lock Pressure Valve
- Vehicle Speed Sensor
Electronic Brake Control Module
The EBCM mounted on a bracket next to the master cylinder, contains a microprocessor and software for system operation.
The Anti-Lock Pressure Valve (APV) is mounted to the combination valve under the master cylinder, has an isolation valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure.
The Vehicle Speed Sensor (VSS) located on the left rear of the transmission on two-wheel drive trucks and on the transfer case of four-wheel drive vehicles, produces an AC voltage signal that varies in frequency according to the output shaft speed. On some vehicles the VSS is located in the rear differential.
During normal braking, the EBCM receives a signal from the stop lamp switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the APV and travel to the rear brake channel. The reset switch does not move because hydraulic pressure is equal on both sides.
During a brake application the EBCM compares vehicle speed to the program built into it. When it senses a rear wheel lock-up condition, it operates the anti lock pressure valve to keep the rear wheels from locking up. To do this the
- Pressure Maintain
- Pressure Decrease
- Pressure Increase
Pressure Maintain
During pressure maintain the EBCM energizes the isolation solenoid to stop the flow of fluid from the master cylinder to the rear brakes. The reset switch moves when the difference between the master cylinder line pressure and the rear brake channel pressure becomes great enough. If this happens, it grounds the EBCM logic circuit.
During pressure decrease the EBCM keeps the isolation solenoid energized and energizes the dump solenoid. The dump valve moves off its seat and fluid under pressure moves into the accumulator. This action reduces rear pipe pressure preventing rear lock-up. The reset switch grounds to tell the EBCM that pressure decrease has taken place.
During pressure increase the EBCM de-energizes the dump and isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The isolation valve 9pens and allows the fluid from the master cylinder to flow past it and increase pressure to the rear brakes. The reset switch moves back to its original position by spring force. This action signals the EBCM that pressure decrease has ended and driver applied pressure resumes.
When the ignition switch is turned "ON," the EBCM performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the isolation and dump valves. The EBCM then begins its normal operation if no malfunctions are detected.
Brake pedal pulsation and occasional rear tire "chirping" are normal during RWAL operation. The road surface and severity of the braking maneuver determine how much these will occur. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain severe braking conditions.
Using the spare tire supplied with the vehicle will not affect the performance of the RWAL or system.
Tire size can affect the performance of the RWAL system. Replacement tires must be the same size, load range, and construction on all four wheels.
Contrary to popular belief ABS brakes will not stop your car faster. The idea behind ABS brakes is that you maintain control of your vehicle by avoiding wheel lock up. When your wheels lock up you have no steering control and turning the steering wheel to avoid a collision will do you no good. When the wheels stop turning, it's done and over.
When driving on slippery roads you need to allow for increased braking distance since the wheels will lock up much easier and the ABS will cycle much faster. Speed is a factor also, if you're going too fast even the control ABS gives you will not be enough to overcome plain inertia. You may turn the wheel to the left or right, but inertia will keep you going forward.
If there is an ABS failure, the system will revert to normal brake operation so you will not be without brakes. Normally the ABS warning light will turn on and let you know there is a fault. When that light is on it is safe to assume the ABS has switched to normal brake operation and you should drive accordingly.
Hopefully, this has helped you understand how ABS systems work. It is a technology that has been in use for many years before it was adapted for automotive use. Aircraft have been using some form of ABS since WW II and it is a tried and true system that can be a great help in avoiding accidents if it is used as it was meant to be used.
Part 1 of 6: Locate the ABS module
Materials Needed
Step 1: Refer to your specific repair manual to locate the ABS module. Usually, the repair manual will have a drawing with an arrow pointing at the area where the module is mounted.
At times, there will also be a written description that can be very useful.
- Tip: The ABS module has many metal brake lines connected to it. The module itself is bolted to the solenoid block and will need to be separated from it. This isn’t always the case as some manufacturers will require the module and the solenoid block to be replaced simultaneously.
Step 2: Find and identify the module on the car. You may need to lift the vehicle and remove some plastic covers, panels or other components to locate the ABS module.
- Note: Remember the ABS module will be bolted to a solenoid block that has many brake lines connected to it.
Part 2 of 6: Determine how to remove the ABS unit from the car
Step 1: Refer to the manufacturer’s repair instructions. You may be able to remove the ABS module from the vehicle as a complete unit or remove only the electrical module while the solenoid block remains attached to the vehicle.
- Tip: On some vehicles, you can get away with removing the module from the solenoid block while the solenoid block is still bolted to the car. Other vehicles may require the two components to be replaced as a unit. This is dependent on how well you can access it and how the new module is sold.
Step 2: Move onto part 3 or part 4. Skip to part 4 if you only need to remove the module and not the solenoid block and motor. If the ABS module, solenoid block, and the motor will be removed as a complete unit, move onto part 3.
Part 3 of 6: Remove the module and solenoid block as a unit
Step 1: Release brake line pressure. Some vehicle will have high pressure contained within the ABS unit. If this is the case with your vehicle, refer to the specific repair manual for your car to determine the correct methods for releasing the line pressure.
Step 2: Disconnect the electrical connector from the module. The connector will be large and have a retaining mechanism.
Each manufacturer uses different mechanisms to retain connectors.
- Tip: Be sure to mark the lines before removing them to ensure that you can reconnect them in their original positions.
Step 3: Remove the brake lines from the module. You will need the appropriate size line wrench to remove the lines without rounding them.
Once you have completely unthreaded all the lines from the block, pull up on them to remove them.
Step 4: Remove the ABS module with the solenoid block. Unbolt whatever bracket or bolts are used to mount the ABS module and solenoid block to the car.
This configuration will greatly depend on the make and model of car you are working on.
Step 5: Remove the ABS module from the solenoid block. Remove the bolts that attach the module to the solenoid block. Gently pry the module from the block.
This may require the use of a flat head screwdriver. Be sure to be gentle and patient.
- Note: The removal of the module from the solenoid block is not always necessary as it depends on how the new unit is supplied to you. Sometimes, it is sold as a complete unit with solenoid block, module, and motor. Other times, it will be just the module.
Step 6: Skip to part 6. Skip part 4 as it is for module replacement that doesn’t require the removal of the solenoid block and brake lines.
Part 4 of 6: Remove only the module
Step 1: Disconnect the electrical connector from the module. The connector will be large and have a retaining mechanism.
Each manufacturer uses different mechanisms to retain this connector.
Step 2: Remove the module. Remove the bolts that attach the module to the solenoid block. Gently pry the module from the block.
This may require the use of a flat head screwdriver. Be sure to be gentle and patient.
Part 5 of 6: Install the new ABS module
Step 1: Install the module onto the solenoid block. Gently guide the module onto the solenoid block.
Do not force it, if it isn’t sliding on smoothly, remove it and take a close look at what is going on.
Step 2: Start threading the bolts by hand. Before tightening any of the bolts, start threading them by hand. Ensure that they are snug before applying the final torque.
Step 3: Connect the electrical connector. Push the electrical connector on. Use the locking mechanism to firmly attach and secure it to the module.
Step 4: Program the new module to the car. This procedure is dependent on the manufacturer of your vehicle and often isn’t necessary.
Refer to your manufacturer’s repair manual for the instructions to program this module.
Part 6 of 6: Install the ABS unit onto the car
Step 1: Install the module to the solenoid block. This step is only necessary if the new module is supplied separately from the solenoid block.
Step 2: Install the ABS unit onto the car. Bolt the unit to the car as necessary.
Be sure to consider the alignment of the brake lines.
Step 3: Start the threads of the brake lines. Cross threading of the brake lines is a very real possibility that can lead to major problems.
Be sure to gently start each brake line by hand before using a wrench or applying the final torque.
Step 4: Torque all the brake lines. Make sure all the brake lines are tight and the flare end is firmly seated as you tighten the brake lines. Sometimes, this can be an issue. If it is, you will need to remove the leaking brake line and take a closer look at the flared end.
Step 5: Connect the electrical connector. Push the electrical connector on. Use the locking mechanism to firmly attach and secure it to the module.
Step 6: Program the new module to the car. This procedure will be dependent on the manufacturer of your vehicle and often isn’t necessary.
You will need to refer to your manufacturer’s repair manual to find the instructions for this process.
Step 7: Bleed the brake lines. Most of the time, you can bleed the brake lines at the wheels.
Some vehicles will have elaborate bleeding procedures that will need to be followed. Refer to your manufacturer’s repair manual for specific instructions.
Replacing an ABS module is a varied repair — on some vehicles, it can be very straightforward and simple while it is arduous and complicated on others. The complications can occur in the programming to the vehicle, the bleeding procedures, or the installation in cases where all the brake lines need to be removed.
Sometimes, the module is mounted in spots that require the removal of other components in order to access the ABS unit. Since brake systems span from the front to the rear of the car and on both sides, an ABS unit can be installed almost anywhere on a car. If you're lucky, it will be easily accessible and you will need to replace only the electrical portion of the ABS unit instead of having to do extensive disassembly, programming, and bleeding.
If your ABS light is on, you should always begin with a thorough diagnosis of the ABS system before replacing the ABS unit as ABS modules are expensive and complicated. Get a certified technician from YourMechanic to perform an inspection and diagnose the problem.
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