The speed sensor on an electronic treadmill is most likely located at the front roller near the motor assembly, where it counts each revolution to calculate speed and distance. If that sensor begins to malfunction then it will give inaccurate readings and cause inconsistent, fluctuating speed. Either the sensor can be adjusted or it needs to be replaced. A home treadmill can be fixed and diagnosed with out a lot of extra tools or specific expertise.
Disconnect the treadmill from any power source to which it is connected. Make sure the treadmill has no power running through it at all. Remove the front covering over the motor of the treadmill to expose the main circuit board and motor assembly.
Disconnect the circuit board wires leading to the motor. In most cases, the wires are black and red. The motor should also be removed from its mount at this time.
Move the motor to slacken the pulley, then lift the motor to get underneath. That is where the treadmills speed sensor will be located. It is usually a square or rectangular shaped green module with connectors at both ends.
Use a finger to check the speed sensor's rigidity in its bracket. The malfunction could be because it needs to be tightened or adjusted in some cases. At this point, do a test. Remove the treadmill speed sensor and use the voltmeter to test the circuit. Check manufacturer specifications on the accurate use of the voltmeter.
Remove the speed sensor completely and replace it if necessary with your new speed sensor. Then reattach the wires that lead to the circuit board. Return the roller pulley and motor to their original positions and replace the protective cover.
Plug in the treadmill or attach power source. Once on test the treadmill to make sure the speed sensor has been fixed. This can be done by running the treadmill for 15 minutes at low speeds monitoring it carefully to see that it is performing as expected. Check manufacturer specifications to set up your treadmill to desired settings, (refer to owners manual).
Always take steps to keep the speed sensor area clean and free from dust and dirt particles to insure its proper and long lasting functionality.
Make sure there is no power running through the machine when attempting to replace any internal parts to avoid injury due to electrocution, shock or burns.