Lesson 4/Learning Event 3
The magnetic gage is very sensitive to road shock and sudden changes in voltage such as those
caused by the fuel sloshing in the tank. Because of this, the armature of the instrument panel gage
will be fitted with a damping device or a flywheel.
Pressure gages are used widely in automotive applications to keep track of things such as engine oil
pressure, fuel line pressure, air-brake system pressure, and the pressures of the hydraulic systems in
special purpose vehicles.
The instrument panel gage may be thermostatic or magnetic. The sending unit that is used with
each type gage varies as follows:
- Thermostatic. The sending unit used with this gage uses a flexible diaphragm that
moves a grounded contact. The contact that mates with the grounded contact is
attached to a heated bimetallic strip. The flexing of the diaphragm, which is done
with pressure changes, varies the point tension. The different positions of the
diaphragm produce gage readings in the same manner as the different positions of the
float arm of the tank sender in the paragraph above titled "Thermostatic Fuel Gage -
- Magnetic. The sending unit used with this gage also translates pressure into the flexing
of a diaphragm. In the case of the magnetic gage sending unit however, the
diaphragm operates a rheostat. The flexing of the diaphragm in the pressure sender
produces the same results as does the movement of the float arm in the tank sender
described in the paragraph above titled "Magnetic Fuel Gage." This type of sender
also can be used with a thermostatic gage if a voltage-limiting device is used.
The oil pressure warning light is used in place of a gage on many vehicles. The warning light,
although not an accurate indicator, is valuable because of its high visibility in the event of a low oil
pressure condition. Because an engine can fail or be damaged permanently in less than a minute of
operation without oil pressure, the warning light often is used as a backup for a gage to attract
instant attention to a malfunction. The warning light receives battery power through the ignition
switch. The circuit to ground is completed through the engine sender switch. The sender switch
consists of a pressure-sensitive diaphragm that operates a set of contact points. The contact points
are calibrated to turn on the warning light whenever the engine oil pressure drops below
approximately 15 PSI (103.4 kPa), depending on the equipment.