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How to check if the speed sensor is good?


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2006-07-30 15:08:13
2006-07-30 15:08:13

HOW IT WORKS The Vehicle Speed Sensor (VSS) is a variable reluctance sensor that generates a sine wave type waveform with a frequency that is proportional to the speed of the vehicle.It has a gear on it that meshes into either the transmission/transaxle or the differential (depending on the vehicle.Probably in the differential of a FORD RANGER) When the vehicle is moving slowly, the sensor produces a low frequency signal. As the vehicle speed increases, the sensor produces a higher frequency signal. This signal is sent to the PCM and is converted into a vehicle speed value, stored in RAM memory. The PCM utilizes this value to aid in controlling torque converter lock-up/transmission/transaxle shift points, control fuel injection, ignition timing and , coolant fan control, vehicle speed control, air/fuel ratio and ignition timing. MY OBERVATIONS AND COMMENTS It works like a little alternator of sorts, or probably more correctly like the rotor and pick-up in an electronic ignition system. When it is spinning with the transmission or differential gears it generates small electric pulses which are sent to the engine control computer. The faster it spins the higher the frequency. I have tested some of mine two different ways. One way is to remove it from the tailshaft of the transmission or the differential housing (depends on vehicle)and hook the two leads to a volt meter on the AC scale or if you have a digital meter with a frequency scale you can use that too. This will measure the frequency in HERTZ (Hz.),meaning cycles per second. Spin the little gear(probably nylon)as fast as you can by hand (drag your hand /arm across it real fast like you were winding up a toy car or top)and watch the voltmeter for a reading. If you get a reading ( a few volts or Hz.) it's probably still functioning. You could also turn the gear with a variable speed drill.Keep it slow to be on the safe side.I'm guessing under 1000 RPM (most variables top out at 1250rpm for 1/2" and 2250rpm for 3/8"). If it won't go in the drill chuck (some gears have a little nub on the end you can grab),then wrap the gear with some tape and press the gear onto the side of the chuck,sort of like a bicycle generator on a bike tire.The tape will protect the little gear and help increase the friction between the two so it spins easily. Another way is to leave the sensor in place, jack up the vehicle,drive wheels off the ground and turn the drive wheels by hand with your test apparatus hooked up to the leads on the sensor for similar results. The main thing is that you get varied voltages/frequencies at different speeds.If you get nothing it's busted. I DO NOT recommend that you run the vehicle while it is jacked up. Spinning the wheels by hand will tell you all you need to know. I don't know the exact voltage or frequency you should get because it's all based on how they designed the sensor, how they calibrated the computer,the gear ratios of the sensor to the transmission or differential, and how fast you spin it. If you get different voltage and/or frequency readings at different speeds,it's probably good. The resistance across the two terminals of the sensor for a 1993 FORD RANGER is supposed to be 190 to 250 OHMS. If the sensor checks out good ,you may have a problem in the connectors or wiring .Often a close visual inspection of the connectors and wiring will reveal the problem before you have to break out a meter and run continuity checks Also check out the condition of the gear on the sensor and make sure that it is not chewed up and is properly fixed to the shaft of the sensor and not spinning loose on the shaft. I tried the first method with the drill with a 1990 NISSAN speed sensor and at approx 1000 RPM I got about 7 volts AC and 270 Hz. Also, the measured resistance across the two terminals of the sensor was 258 OHMS.


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