The other system mentioned operates on the same
principles but does not have a cold warning light. It has
the hot warning light and an engine temperature
sending unit with a single set of normally open
contacts. If the engine overheats, the contacts close and
turn on the hot warning light.
OIL PRESSURE LIGHT.--The oil pressure
warning light installed on some vehicles is also located
on the instrument panel to indicate when the oil
pressure is very low. The warning light is wired in
series with the ignition switch. The oil pressure switch
is located on the automotive engine.
The oil pressure switch contains a diaphragm and a
set of contacts. When the ignition is turned on, the
warning light should illuminate, because the light
circuit is energized through the closed contacts in the
oil pressure switch. When the engine is started, built-up
oil pressure compresses the diaphragm, opening the
contacts, and thereby opening the circuit and causing
the warning light to go out. The warning light is usually
red and indicates a bright "Oil" on the instrument panel.
alternator warning light mounted on the instrument
panel in place of the ammeter. The light indicates to the
operator when the alternator is not charging the battery.
The warning element is a panel-mounted window
(usually red) behind which is mounted a small light
bulb. The light comes on when the ignition is turned on
and the alternator is not charging. The circuit is
completed from the ignition switch, through the light,
to a terminal on the indicator light relay.
When the engine is running, the alternator speed is
sufficient to provide a voltage higher than that of the
battery. The operating coil of the indicator light relay
will be energized, opening the circuit between the
indicator light and the battery.
Some support equipment requires servicing or
maintenance after operating a certain amount of time.
Figure 6-80.--Hour meters.
Hour meters (fig. 6-80) provide maintenance personnel
a means of determining the specific amount of time the
equipment has operated. With the information from the
electrically driven type records elapsed time (clock
hour meters and maintenance instructions, mainte-
hours). The mechanically driven type records time after
nance personnel can determine what maintenance is
a certain number of functions have taken place. For
required and when to perform it.
example, the hour meter may be designed to advance an
hour after the crankshaft of an engine has completed as
There are two types of hour meters commonly used
many revolutions as it normally would during an hour's
on support equipment. These are the electrically driven
(most common) and the mechanically driven types. The