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Engine Compartment

ABS + ESC: Diagnosis of Accelerometers and Yaw Sensors

Accelerometers measure gravitational pull in gravitational force or G-force. Some cars will have one or two sensors depending on sophistication of the system.

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Accelerometers measure gravitational pull in gravitational force or G-force. Some cars will have one or two sensors depending on sophistication of the system. Some systems group the accelerometers in a sensor cluster. The typical sensor may cost $400 to $800.
Lateral/Transverse 
These sensors are typically found under the center console near the vehicle’s center of gravity. 
 
Your scan tool will read the sensor output in G-force or G. A typical road vehicle will be able to corner around one G. Single-seat race cars can corner at 4- to 5-Gs. Most production car accelerometers have a limit of 4-Gs.  
Longitudinal 
These sensors measure braking and acceleration. This can be used to determine how hard the driver is accelerating or braking. Longitudinal sensors can also be used to calculate the angle of the vehicle for hill holding. 
 
It is very difficult and even dangerous to drive the vehicle and look at the readings from an accelerometer. You will see only minor movement in the sensor.
 
The best way to measure the performance of an accelerometer is to rotate it 90 degrees. When a lateral accelerometer is rotated 90º, it should read 1-G. When a longitudinal sensor is rotated on its end, it will read 1-G. This is because gravity is equal to 1-G.
 
Most accelerometers are wired with three wires. These include a  5-volt power, ground and signal wires. The signal wire will vary the voltage from 0-5-volts depending on G-force. 
Sensor Replacement
Whenever an accelerometer is replaced, it must be recalibrated. Also, some sensors must be tightened in a specific procedure. A loose accelerometer can generate codes because it will be measuring vibration and not G-force.
YAW Sensors
A yaw sensor measures the vehicle’s rotation around its vertical axis, while at the same time measuring the acceleration at right angles to the driving direction. 
 
By electronically evaluating the measured values, the sensor is able to differentiate between normal cornering and vehicle skidding movements. Yaw rate is measured in degrees per second. If you have a vehicles making a 90º turn in two seconds, it would have a yaw rate of 45º.
 
The Yaw rate sensor and accelerometers will often be mounted in a sensor cluster. To test a yaw rate sensor, rotate the unit so it is on a corner. The yaw sensor should react. 

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