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.