rotated by a flexible shaft. Surrounding the rotating
magnet is a metal cup that is attached to the indicating
needle. The revolving magnetic field exerts a pull on
the cup that forces it to rotate. The rotation of the cup is
countered by a calibrated hairspring. The influence of
the hairspring and the rotating magnetic field on the
cup produces accurate readings by the attached needle.
The flexible shaft consists of a flexible outer casing
that is made of either steel or plastic and an inner drive
core that is made of wire-wound spring steel. Both
ends of the core are molded square, so they can fit into
the driving member at one end and the driven member
at the other end and can transmit torque.
Gears on the transmission output shaft turn the
flexible shaft that drives the speedometer. This shaft is
referred to as the speedometer cable. A gear on the
ignition distributor shaft turns the flexible shaft that
drives the tachometer. This shaft is referred to as the
The odometer of the mechanical speedometer is
driven by a series of gears that originate at a spiral gear
on the input shaft. The odometer consists of a series of
drums with digits printed on the outer circumference
that range from zero to nine. The drums are geared to
each other so that each time the one furthest to the right
makes one revolution, it will cause the one to its
immediate left to advance one digit. The second to the
right then will advance the drum to its immediate left
one digit for every revolution it makes. This sequence
continues to the left through the entire series of drums.
The odometer usually contains six digits to record
99,999.9 miles or kilometers. However, models with
trip odometers do not record tenths, thereby contain
only five digits. When the odometer reaches its highest
value, it will automatically reset to zero. Newer
vehicles incorporate a small dye pad in the odometer to
color the drum of its highest digit to indicate the total
mileage is in excess of the capability of the odometer.
Electric Speedometers and Tachometers
The electric speedometer and tachometer use a
mechanically driven permanent magnet generator to
supply power to a small electric motor (fig. 2-84). The
electric motor then is used to rotate the input shaft of
the speedometer or tachometer. The voltage from the
generator will increase proportionally with speed, and
speed will likewise increase proportionally with
voltage enabling the gauges to indicate speed.
The signal generator for the speedometer is usually
driven by the transmission output shaft through gears.
The signal generator for the tachometer usually is
driven by the distributor through a power takeoff on
gasoline engines. When the tachometer is used with a
diesel engine, a special power takeoff provision is
made, usually on the camshaft drive.
Electronic Speedometers and Tachometers
Electronic speedometers and tachometers are self-
contained units that use an electric signal from the
engine or transmission. They differ from the electric
unit in that they use a generated signal as the driving
force. The gauge is transistorized and will supply
information through either a magnetic analog (dial) or
light-emitting diode (LED) digital gauge display. The
Figure 2-84.Electric speedometer and tachometer operation.