Diesel Engine Vehicle Speed Sensor


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The speed sensor detects the actual speed at which the vehicle is running. The sensor outputs the SPD signal, and the engine ECU uses this signal mainly to control the ISC system and the air-fuel ratio during acceleration or deceleration as well as other uses. The MRE (Magnetic Resistance Element) types are the main type of speed sensor used, but recently many models use the SPD signal from the ABS ECU.
1.MRE type
This sensor is installed on the transaxle, transmission, or transfer, and is driven by the drive gear of the output shaft. As shown in the illustration, the sensor is built-in and consists of a HIC (Hybrid Integrated Circuit) with a MRE and magnetic rings.
Other Type Speed Sensors
The MRE resistance changes depending on the direction of the magnetic force applied to the MRE. When the direction of the magnetic force changes according to the rotation of the magnet attached to the magnetic ring, the MRE output becomes an AC waveform as shown in the illustration. The comparator in the sensor converts this AC waveform into a digital signal and outputs it.
The waveform frequency is determined by the number of poles of the magnets attached to the magnetic ring. There are two types of magnetic rings, 20-pole type and 4-pole type, depending on the vehicle model. The 20-pole type generates a 20-cycle waveform (in other words, twenty pulses for each rotation of the magnetic ring), and the 4-pole type generates a 4-cycle waveform.
In some models, the signal from the speed sensor passes through the combination meter before arriving at the engine ECU, and in the other models, the signal from the speed sensor arrives directly at the engine ECU.
The output circuits of the speed sensor consist of the output voltage type and the variable resistance type.

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