MOTOR TRUCK SERVICE MANUAL
(lower end first) through the lubricant back into the
When the bulb temperature increases as a result of
casing, leaving only a thin coating on the core. When
engine operation, the contained ether vaporizes, exerting
lubricating the core, leave four inches free of lubricant to
a vapor pressure on the inlet of the Bourdon tube, Fig.
prevent lubricant entering the instrument head.
20. The pressure causes the tube to straighten out. The
indicator point, which is attached to the Bourdon tube by
Speedometer and Tachometer Heads
the gear linkage, then rotates across the dial in an
provision for lubrication. Examine the tachometer head,
The gauge achieves a temperature scale, reading
and where lubricant openings are provided in the rear of
corresponding to the engine operating temperature.
the head at the cable take-off, lubricate as follows.
When the engine is shut off, the ether vapor condenses
and settles in the bulb as the engine cools.
Inject 1 to 2 drops of "3 in 1" oil every 15, 000 to 20,
000 miles of operation. Under severe service or
General gauge operation can be checked by
extremely hot climate lubricate more often.
removing the sending bulb from the engine block.
Submerge the bulb in cold water; the gauge should
AC MANUFACTURED INSTRUMENT GAUGES
descend downscale. Submerge the bulb in boiling water;
the gauge should ascend upscale. Because of the
The water temperature, oil pressure and air pressure
construction of the gauge, if either of these tests proves
gauges are mechanical with Bourdon tube type
unsatisfactory, replace the temperature gauge assembly.
construction. The electrical gauges are the fuel level and
battery-generating system indicator (voltmeter) utilizing
Oil Pressure Gauge:
the air core type construction. All gauges have direct
During engine operation oil is pumped throughout
the lubricating channels of the engine block. The level of
Gauge Operation and Servicing
pressure in the system depends upon the temperature,
oil supply and the condition of the system itself.
Water Temperature Gauge:
The oil pressure gauge measures the system's oil
The mechanical temperature gauge operates on a
pressure. The gauge operates with a Bourdon tube
Bourdon tube motion which is transmitted to a pointer
motion transmitted through a pointer by way of a sector
through a sector and pinion gear linkage. The
and pinion gear linkage, Fig. 20.
temperature gauge, unlike the mechanical air and oil
pressure gauges, is a hermetically sealed unit consisting
of a gauge, a capillary tube and a temperature bulb
which is filled with ether, Fig. 19.
Fig. 20. Oil Pressure Gauge
Fig. 19. Water Temperature Gauge