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1990 chevy 1500 speedometer not working
During normal driving, the speedometer and odometer do not run. The circuit schematic diagram of the instrument speed signal processing module is different from other models in that the vehicle speed signal is not directly sent to the odometer, but first sent to the processing module, and then sent to the odometer, ABS and engine control unit after being processed by the module. Through actual measurement, it is found that the signals output to the two control units are 5V square waves with the same number of pulses as the actual output, while the signals output to the odometer are frequency divided square waves. In terms of data, it is called the vehicle speed sensor calibration processing module. Observe the output waveform with an oscilloscope. Through oscilloscope measurement, it is found that the output signal amplitude of the vehicle speed sensor does not increase with the increase of vehicle speed, and the output signal amplitude is lower than the general signal. It is suspected that the sensor is damaged. When the vehicle speed is lower than 80km / h, the three output signals are regular square waves. When the vehicle speed is higher than 80km / h, the square wave becomes irregular and the frequency also decreases. After that, it directly forms a high level without waveform.
After measurement, it is found that when the signal received by the speedometer reaches 130Hz, the vehicle speed is expressed to the maximum speed of 180km / h. When the pointer of the speedometer is at the non-zero position, if the speedometer is powered off and powered on again, the pointer of the speedometer will return to zero. That is, the pointer of the speedometer rotates in the opposite direction for about 30s, and then the input vehicle speed signal can be normally indicated. This model is relatively rare in our local area, so it is difficult to find the speed sensor in the market, and even if it is found, the price is relatively high. Therefore, an old crankshaft position sensor is used to replace it successfully. The method is to measure the diameter difference between the two sensors, and then process a suitable bushing on the lathe, so that the modified sensor is just installed in the mounting hole, and through the thickness of the processed nylon sleeve, When the gap between it and the signal wheel is about 1mm, the output waveform can be correctly recognized by the processing module.
After the oscilloscope detection, the signal amplitude output by the sensor is significantly increased in the cold state, and the instrument can work normally at this time. Reassemble the instrument and conduct test run and troubleshooting.
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