Auto Service Professional

OCT 2017

Magazine for the auto service professional

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Page 29 of 77

28 | ASP October 2017 Technical e yaw rate sensor will measure the rotation rate of the vehicle and determines how far off-axis the vehicle is "tilting" or "rolling" in a turn. e yaw rate sensor is usually mounted low and in the center of the vehicle (often under the center console to be as close to the vehicle's center of gravity as possible) so that its measurement is more precise. e lateral acceleration sensor measures the rate of change in side-to-side movement to calculate the vehicle's actual position in the turn. In many cases the lateral and yaw rate sensors are incorporated into one single unit or combined as a sensor cluster. ese sensors can be directly wired to the ESC controller or on the CAN-bus system, sharing information with other controllers. Transverse and roll-rate sensors can also be incorporated into the system to help detect and prevent vehicle rollovers. ese sensors can also be an all-in-one design or they can be mounted individually on the chassis. Most of these sensors can only be tested with a scan tool, but like an SAS sensor you can test for communication activity on the output line with a meter or scope. The steering angle sensor e steering angle sensor (SAS) reports the position of the steering wheel (where the driver wants to go) and how fast the steering wheel is being turned. e ESC module needs to know this information to compare it with yaw and lateral accelerometer info to decide what the driver really intended to do. ere are analog, digital and controller area network (CAN) bus designed SAS sensors with each containing a cluster of sensors for redundancy, precision and safety. An analog SAS will have a power (5V refer- ence), a ground, and signal outputs. Normal straight-ahead steering wheel position scanner values or back probing the connector will show 2.8V on one sensor and 0.4V on the other. e values should not match, just like TPS sensors on drive-by-wire systems. And the values may go positive or negative as the wheel is turned. e digital SAS or contactless sensors will use either a Hall Effect sensor or an optical sensor to produce a digital square wave output that will change frequency as the steering wheel is turned. e CAN-bus SAS can be directly wired to the ESC controller or the information created by the SAS can be shared along the CAN network of the vehicle, and used by other modules (directional headlights, EPS). is style is diagnosed using a scan tool. It is only possible to see if the SAS is communi- cating by checking the voltage output on the CAN wire, but you have no idea what is being said. How the system works e ESC controller, either a separate self- contained unit or integrated into another Anytime the steering system is disturbed, the steering angle needs to be reset. Servicing this evaporator core is a perfect example.

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