If you make a trip to the bike shop, you most likely won't find any cable-driven odometers or speedometers. Instead, you will find bicycle computers. Bicycles with computers like these have a magnet attached to one of the wheels and a pickup attached to the frame. Once per revolution of the wheel, the magnet passes by the pickup, generating a voltage in the pickup. The computer counts these voltage spikes, or pulses, and uses them to calculate the distance traveled.
If you have ever installed one of these bike computers, you know that you have to program them with the circumference of the wheel. The circumference is the distance traveled when the wheel makes one full revolution. Each time the computer senses a pulse, it adds another wheel circumference to the total distance and updates the digital display.
Many modern cars use a system like this, too. Instead of a magnetic pickup on a wheel, they use a toothed wheel mounted to the output of the transmission and a magnetic sensor that counts the pulses as each tooth of the wheel goes by. Some cars use a slotted wheel and an optical pickup, like a computer mouse does. Just like on the bicycle, the computer in the car knows how much distance the car travels with each pulse, and uses this to update the odometer reading.
One of the most interesting things about car odometers is how the information is transmitted to the dashboard. Instead of a spinning cable transmitting the distance signal, the distance (along with a lot of other data) is transmitted over a single wire communications bus from the engine control unit (ECU) to the dashboard. The car is like a local area network with many different devices connected to it. Here are some of the devices that may be connected to the computer network in a car:
- Engine control unit (ECU)
- Climate control system
- Power window controls
- Anti-lock braking system
- Air bag control module
- Body control module (operates the interior lights, etc.)
- Transmission control module
Many vehicles use a standardized communication protocol, called SAE J1850, to enable all of the different electronics modules to communicate with each other.
The engine control unit counts all of the pulses and keeps track of the overall distance traveled by the car. This means that if someone tries to "roll back" the odometer, the value stored in the ECU will disagree. This value can be read using a diagnostic computer, which all car-dealership service departments have.
Several times per second, the ECU sends out a packet of information consisting of a header and the data. The header is just a number that identifies the packet as a distance reading, and the data is a number corresponding to the distance traveled. The instrument panel contains another computer that knows to look for this particular packet, and whenever it sees one it updates the odometer with the new value. In cars with digital odometers, the dashboard simply displays the new value. Cars with analog odometers have a small stepper motor that turns the dials on the odometer.
For more information on odometers and other often-overlooked car parts, check out the links on the next page.