Ask The Guru

ATG (JanFeb16) – Speed Sensors

BM Writes: How do I check a speed sensor?

According to Roger: It is really not a difficult thing just a matter of understanding a few things. Most of the time if the car does a nose drive when you accelerate that is a strong indication of a defective sensor or magnet.

If the car cannot get to speed and just creeps along that is a good indicator. Again, those two things are just indicators and not a sure thing. Sometimes it is moisture or corrosion inside the connector, and you have to check that. Do not confuse erratic operation with these, as a potentiometer can simulate that.

With the car on the ground key on, in forward push the car. In one foot to two feet the car will roll easily then you will feel a slowdown and resistance. The backup buzzer should sound if it working. That basically indicates the sensor is working.

Use a voltmeter to see if correct voltages are present. Voltage can differ a little depending on the manufacturer, so you need to get the specifications first. Typically, you will see a fifteen volt reading on the positive input to the sensor (usually a red wire). Look at the car schematic to confirm wire color.

When operating the voltage can drop a little, maybe fourteen volts. You should see battery pack negative voltage on the black wire. The green wire and sometimes white is the output voltage and is a low voltage. You should see five volts as you push the car and drop to zero volts. If you do not see that pulse the sensor or magnet is defective. You can raise a wheel off the ground and rotate it by hand to get the same effect. You also want to make sure when you stop at five volts it stays at five or latches if you will. If not replace the sensor or magnet.

If you are missing the positive fourteen volts or the negative pack voltage that is not the sensor. That is a supply voltage. If either voltage is missing we have wiring issues, controller issues are there are wires in the wrong place. Always have a wiring diagram of the system you are working on in front of you! Confirm the wiring do not assume. Yes, you will need self-penetrating clips to connect to the wires.

If you do high volume testing you can build a test station. If you would like a Power Point for this just send me an email at and request one.

Basically all you need is a known good controller and a DC power supply. You can build your own connections with Molex connectors.