Cessna 172R Pilot Operating Handbook page 180

Markings for the oil temperature gauge indicated a normal oper-
ating range of 100 to 245°F (green arc), and a maximum tempera-
ture of 245°F (red line).
Oil pressure signals are
generated from an
oil
pressure
line/transducer combination. An oil pressure line is routed from the
upper front of the engine case to the rear engine baffle. At the
baffle, the oil pressure line is connected to a transducer. This
transducer produces an electrical signal which translates into a
pressure reading at the instrument panel gauge.
Oil temperature signals are generated from a resistance-type
probe located in the accessory case. As oil temperature changes,
the probe resistance changes. This resistance is translated into oil
temperature readings on the cockpit indicator.
In addition, a separate low oil pressure indication is provided
through the panel annunciator. This annunciator is wired to a
pressure switch located on the rear of the engine accessory case.
When oil
pressure
is below 20 PSI, the switch grounds and
completes the annunciator circuit, illuminating the red OIL PRESS
light. When pressure exceeds 20 PSI, the ground is removed and
the OIL PRESS light extinguishes.
The engine driven mechanical tachometer is located on the
instrument panel to the right of the pilot's control wheel. The
instrument is calibrated in increments of 100 RPM and indicates
both engine and propeller speed. An hour meter in the lower section
of the dial records elapsed engine time in
hours and tenths.
Instrument markings include the normal operating range (green arc)
from 1900 to 2400 RPM.
An exhaust gas temperature (EGT) indicator is located on the
lower left instrument panel as part of the EGT/Fuel Flow gauge. A
thermocouple
probe
in
the tailpipe
measures exhaust
gas
temperature and transmits it to the indicator. The indicator serves as
a visual aid to help the pilot adjust mixture through monitoring of
exhaust gas temperature. Exhaust gas temperature varies with fuel
to air ratio, power, and RPM. However, the difference between the
peak EGT and the EGT at the cruise mixture setting is essentially
constant, and this provides a useful leaning aid.
7-20
Dec 2/96
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SECTION 7
CESSNA
AIRPLANE & SYSTEMS DESCRIPTION
MODEL 172R