Troubleshooting a Vehicle Speed Sensor

A vehicle speedometer

When you are troubleshooting your vehicle speed sensor, there are several things that you need to look at. This could end up affecting your transmission, speedometer, ABS, power steering, and cruise control. A vehicle speed sensor works a lot like a wheel speed sensor.

As soon as you start to notice any problems with the aforementioned areas of your vehicle, it would help if you started troubleshooting immediately to fix the problem before it gets worse. Allowing it to go undiagnosed and unfixed will end up costing you a lot more in the long run. Testing the speed sensor is simple enough and can be done without a professional or any expensive equipment.

Testing Reluctance

If you are dealing with a variable reluctance type sensor, then there will be a permanent magnet that is wire wrapped. It has a magnetic field that partly collapses, thanks to a toothed ring. When your magnetic field collapses, there will be a voltage induced into the wire wrapped around said magnet.

You can test this with an ohmmeter. If there is an infinite reading, then this is a sign that there is no electricity going through. If using an analog ohmmeter will show this by staying on the same side as when no probes are touching. If you use a digital ohmmeter, then there will be no value shown.

Test Bad Sensor

There are two tests that you can do that will help identify a bad sensor. Use an OHM meter to test the integrity of the circuit of the sensor. This is to check for an open winding. Your sensor should be able to read some resistance. The other test is with a scan tool. Install the scan tool, and set it for a data stream.

The vehicle will need to be driven as you monitor the signal of the sensor. Take note of the code that shows up on the scan tool and look up what this code means.

Test MAG Resistive

If you happen to have a later model vehicle, your car will probably use the controller area network protocol. This particular sensor gives you a digital signal on the actual on-board computer in your vehicle. Test this with the help of a resistive magneto sensor.

With this sensor, an electromagnetic field is powered by the computer. The built-in resistive bridge circuit processes the signal given to it in the form of a digital square wave. The digital signal is then sent to your computer. This is how the speed is calculated. Your scan tool will be the easiest way to test this.

Manually Test

If you want to test it manually, you need to unplug the sensor. Leave the key turned on, with the engine still off—test for any power, as well as a ground from the computer. Plugin the sensor and start your vehicle. Test your signal return wire for a lab scope signal. Make sure that the drive wheels are suspended at the same time the transmission is put into gear.

Vehicle speed sensor accuracy is vital. These troubleshooting tips will help you regain that accuracy.

If you decide the element is truly busted, you'll probably want to replace it.