This is intended to assist the fault finding of ones that don't work.
This is a simple guide to the basic workings of the speedo.
It is not an exhaustive fault finding guide to repair of electronics.
If you know a little electronics then this guide and the
Why does it stop working at high speed
page should give you all you need to know.
The 'speedo' can be divided into four pieces; the magnet (attached to the wheel), the sensor (on the fork leg), the lead (cord from the sensor) and the head (has the display, battery and buttons).
The magnet (attached to the wheel) passes the sensor, causing the sensor to go from an electrical open circuit to a short circuit. The lead makes the electrical connection from the sensor to the head. The head takes the signal generated by the sensor to do its calculations. No, I am not going into the internal workings of the head! Basically it is a small dedicated computer.
The magnet is simply that, to test use a screw driver and see if the magnet exerts some force on it. If not then the magnet needs replacing, magnets that are hit hard can loose their magnetism.
The sensor is a switch (sometimes called a reed switch), open circuit when the magnet is not near, closed circuit when the magnet is near. Test it using an ohm meter. You may also be able to hear it 'click' as the magnet is moved around the sensor. If the magnet does not get close enough then the sensor will not 'see' it! You can use any magnet to test for the 'click' sound, this avoids spinning the wheel and getting tangled in it. Move the wheel slowly and check that the sensor and magnet pass close to one another.
The lead simply connects the sensor to the head.
If the magnet 'clicks' the sensor but you have no result on
the ohm meter then possibly the lead is at fault.
Cut the lead close to the sensor and check for short circuit from
one end of the lead to the other end.
If it is open circuit on either connection then the lead is at fault.
The head contains the display, the 'brain'
and a battery. Measure the voltage where the lead would normally connect,
this is your battery voltage, about 1.2 volts.
If below 1 volt it is time to replace the battery!
You can also get water inside the unit.
Remove the battery to get good access,
wash it out using some methylated spirits to help dry out the water.
When the unit is dry, clean the battery and its' terminals
and reassemble the unit.
To check the head works is best done with two people.
One person holds the head and looks at the display.
The second person connects the two connectors together on
the back of the unit then disconnects the connectors, but very rapidly!
The connection could be done with a coin. The first person should see the
a speed other than zero, also the trip distance should increase.
The second person may find it easier to hold the head as well.
You should be able to get 6 km/h, anyone getting readings over 18km/h is very quick!
back to my home page & start again Dated 11 March 2001