Auto Service Professional

OCT 2017

Magazine for the auto service professional

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Page 28 of 77

27 | ASP October 2017 Technical asked to repair the ESC system that may have illuminated the warning lights on their dashboards, even without any driveability concerns. e basic components that all ESC systems rely on to make its decisions are similar. A controller that uses algorithms to make the system function, four individual wheel speed sensors (WSS), a steering angle sensor (SAS), yaw rate sensors and lateral accelerometers. Other components may be incorporated but not by all manufacturers; brake pressure sensor, longitudinal accelerometers, roll rate accelerometers and throttle position can further enhanced an ESC system. e latter two sensors are primarily involved in larger top-heavy SUVs and help prevent rollovers. ESC is a complex system that links together sensors typically already on the vehicle and makes safe driving decisions based on the data they provide. Understanding the basics of these sensors and their correlation to the decisions that the ESC will make is essential to diagnostics and repair. Gathering the information from... the wheel speed sensors e wheel speed sensors are an essential part that must accurately report each of the vehicle's wheel speeds to the ESC module. e familiar passive WSS uses a magnet, and generates an AC current and the resulting sine wave. is was the most common kind of WSS, but the active WSS is more accurate and can measure wheel speed all the way down to a complete stop and can even recognize that wheel's rotational direction (it is possible to know that the wheel is turning backwards). ese features have made the active WSS a popular choice for engineers on many of today's vehicles. Active WSS requires special testing and usually involves a scan tool (although there are some tests that can be done with a DVOM). When testing an active WSS it must be carefully back-probed (still connected) during testing and you will be looking for a supply voltage. is voltage can be 5.5 volts to up to 20 volts depending on the system. Many vehicles use 12 volts or battery voltage, and when testing it is imperative to make sure it's the same on all four sensors. Because an active WSS is a two-wire sensor, the control unit will measure the return signal, so it's not possible to test for a reference voltage or resistance across the connector terminals. Some Hondas, Acuras, and BMWs shut off the power supply if the sensor is disconnected, but it will come back if the sensor is hooked up and the key cycled. The accelerometers Yaw rate and lateral acceleration sensors are fancy names for G sensors or accelerometers. Essentially, they are just electronic gyro- scopes. e purpose is to sense a variation in direction, either left or right or side-to-side. This warning light will get the customer's atten- tion and is often the only reason the customer brings in the vehicle for an ESC issue. The ESC system can be called many things, but the purpose is the same: to prevent side skidding and the subsequent loss of control.

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