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TM 9-8000
b. Electric Gage (Fig. 17-13). The instrument panel
operated by the bending of a bimetallic strip that is
calibrated to turn on the warning light at approximately
gage may be the thermostatic-type described in
2300F (1100C). Some models also utilize a cold
paragraph 17-3a, or the magnetic type described in
indication light that will indicate to the driver that the
paragraph 17-3d. The sending unit that is used with each
engine has not yet reached operating temperature. The
gage type varies as follows:
light usually is green in color, whereas the hot indication
is red. The sending unit for this application has an extra
(1) The sending unit that is used with the
terminal. Internally, the sender has an extra contact. The
thermostatic gage usually contains two bimetallic strips,
bimetallic strip simply completes the circuit to the cold
each of which has a contact point. One bimetallic strip is
light until engine temperature reaches approximately
heated electrically. The other bimetallic strip bends to
1500F (65.60C). The strip then will open the cold light
increase the tension of the contact points. The different
circuit. As long as the temperature of the engine Is
positions of the bimetallic strip will create gage readings
normal, the bimetal- lic strip will remain between the
in the same manner that the different float arm positions
contacts for the cold and hot indicator lights.
of the tank sender will in paragraph 17-3a.
(2) The sending unit that is used with the
17-6. Speedometers and Tachometers.
magnetic gage contains a device called a thermistor. A
thermistor is an electronic device whose resistance
a. Usage. Speedometers and tachometers are used
decreases proportionally as its temperature increases.
in  virtually  all  types  of  automotive  equipment.
This type of sending unit also can be used with a
Speedometers are used to indicate vehicle speed in
thermostatic gage if a voltage-limiting device like the one
miles per hour or kilometers per hour. The speedometer
described in paragraph 17-3b also is used.
in most cases also contains an odometer. An odometer
is a device that keeps a permanent record of the amount
c. Mechanical Gage (Fig. 17-14). The gage unit
of mileage that a vehicle has been used. Some
contains a bourdon tube and operates by the same
speedometers also contain a trip odometer that can be
principles as the mechanical oil pressure gage described
reset to zero at anytime so that individual trips can be
in paragraph 17-4c. The motivating force of the
measured. The odometer is calibrated to measure
mechanical temperature gage is a permanently attached
distance In miles or kilometers, depending on the
capillary tube that connects the gage to a bulb fitted in
application. A tachometer is a device that is used to
the engine water jacket. The bulb contains a liquid such
measure engine speed in revolutions per minute (rpm).
as ether, whose vapor pressure is proportional to its
The tachometer also may contain a device known as an
temperature. As the temperature increases, the ether
engine-hours gage. The engine- hours gage usually is
boils and expands. The vapor pressure acts on the gage
Installed on equipment that uses no odometer to keep a
through the capillary tube to produce temperature
record of engine use.
b. Mechanical Speedometers and Odometers. The
d. Indicator Lights (Fig. 17-15). The tem- perature
speedometer and odometer (fig. 17-16) usually are
warning light is used on many vehicles in place of a
combined into a single unit that is driven by a flexible
gage. The indicator light, although not as detailed as a
gage, Is valuable because of its high visibility in the event
of an overheat condition. Because an engine can
(1) The flexible shaft consists of a flexible outer
become  damaged  permanently  or  destroyed  by
casing that is made of steel or plastic and an inner drive
operating for short periods while overheated, warning
core that is made of wire-wound spring steel. Both ends
lights often are used as a backup for temperature gages
of the core are molded square so that they can fit into the
to attract Instant attention to a malfunction. The warning
driving member at one end and the driven member at the
light receives battery power through the ignition switch.
other end and be able to transmit torque.
The circuit to ground is completed through the engine
sender switch. The sender switch contains a set of
The flexible shaft, better known as a
speedometer cable, usually is driven by gears from the


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