is maintained by the oil pressure in the system.
Operation of the gauge itself is based on the use of the
Bourdon tube (fig. 6-77), which is a flexible,
semicircular metal tube, elliptically shaped in cross
section, with one sealed end and one open end. The
open end is connected to the pressure system. As the
pressure of gas or liquid within the tube increases, the
tube tends to straighten. As the pressure decreases, the
tube resumes its normal semicircular shape. By fixing
the open end and allowing the sealed end to move
freely, the "straightening" movement of the tube can be
used to move a needle across a dial. A simple spring,
gear, and lever arrangement serves to return the needle
as the pressure is reduced and the tube resumes its
ELECTRICAL OIL PRESSURE GAUGE.--
Some automotive vehicles are equipped with electrical
oil pressure gauges. These gauges may be either the
balancing coil type (fig. 6-78) or the thermostatic type.
The thermostatic oil pressure gauge operates very
similarly to the thermostatic fuel gauge.
In the balancing coil type system, a variable resistor
is incorporated in the engine sending unit to become the
diaphragm of the engine sending unit (fig. 6-78, view
A). Increasing oil pressure causes the diaphragm of the
sending unit to become displaced (fig. 6-78, view B).
This increases the resistance, causing the right coil of
the pressure gauge to become stronger than the left coil.
As a result, the armature and pointer swing to the right,
indicating an increase in oil pressure. The opposite
takes place with a decrease in oil pressure.
Figure 6-78.--(A) Oil pressure sending unit; (B) Schematic
An absence of oil pressure during engine operation
diagram of balancing coil oil pressure indicating system.
indicates a faulty oil system or inoperative oil pressure
indicating system, and the engine should be stopped
immediately. The trouble must be located and repairs
made before the engine is restarted.
Temperature gauges, usually mounted on the
instrument panel, are used to indicate the temperature
of engine coolant, engine oil, transmission fluid, etc.
Temperature gauges are operated electrically or by the
Bourdon tube. The gauges discussed in the following
paragraphs were designed to indicate the temperature
of engine coolant.
B O U R D O N T U B E T E M P E R AT U R E
GAUGE.-- When the temperature gauge is a Bourdon
tube (previously described), it is actuated by pressure
conducted to it from a bulb that is screwed into the
water jacket of the engine. The heat of the water affects
the liquid in the bulb. The liquid vaporizes at a very low
Figure 6-77.--Bourdon tube.