How they work.


Mechanical odometers have been counting the miles for ages. Although they are a dying breed, they are simple! A mechanical odometer is nothing more than a gear train with a gear ratio. This odometer  has a 1690:1 gear reduction! That means the input shaft of this odometer has to spin 1,690 times before the odometer will register 1 mile. Odometers like this are being replaced by digital odometers that provide more features and cost less. This odometer uses a series of three worm gears to achieve its 1690:1 gear reduction. The input shaft drives the first worm, which drives a gear. Each full revolution of the worm only turns the gear one tooth. That gear turns another worm, which turns another gear, which turns the last worm and finally the last gear, which is hooked up to the tenth-of-a-mile indicator. When your odometer "rolls over" a large number of digits (say from 19,999 to 20,000 miles), the "2" at the far left side of the display may not line up perfectly with the rest of the digits. A tiny amount of gear lash in the white helper gears prevents perfect alignment of all the digits. Usually, the display will have to get to 21,000 miles before the digits line up well again. Mechanical odometers like this one are rewindable. When you run the car in reverse, the odometer actually can go backwards -- it's just a gear train.

While that does work on older mechanical odometers, it does not work on the new electronic ones. 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. The computer in the car knows how much distance the car travels with each pulse, and uses this to update the odometer reading.  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
  • Dashboard
  • Power window controls
  • Radio
  • 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. 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.



Standard Disclaimer On The Use Of Our Products And Services

It is unlawful to use our products, software and or services to misrepresent mileage. Our products, software, and services are designed for the restoration of displayed mileage on vehicles that require the need for it. Thank you.

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