Lesson 4/Learning Event 3
- the resistance of the indicator lamp so that their parallel arrangement will produce a
zero-voltage drop. When the field relay is open, alternator field current is supplied
through the resistor-indicator light combination. The magnetic coil of the field relay is
energized directly from the alternator stator through the stator terminal. When the
ignition switch is closed, before the engine is started, current flows through the
resistor and the indicator lamp to the alternator field, causing the indicator lamp to
light. After the engine is started, the alternator begins to produce current, energizing
the field relay coil from the stator. The relay coil pulls the relay points closed,
shunting the alternator field directly to the battery. This results in a zero potential
across the indicator lamp, causing it to go out.
Most fuel gages are operated electrically and are composed of two units--the gage (mounted on the
instrument panel) and the sending unit (mounted on the fuel tank). The ignition switch is included
in the fuel gage circuit so that the electrical fuel gage operates only when the ignition switch is on.
Operation of the electrical gage depends on either coil action or thermostatic action.
Thermostatic Fuel Gage - Self-Regulating
This gage configuration consists of an instrument panel gage and an electromechanical sending unit
that is located inside the fuel tank. The instrument panel gage contains an electrically heated
bimetallic strip that is linked to a pointer. The bimetallic strip consists of two dissimilar metals
that, when heated, expand at different rates, causing it to deflect or bend. In the case of the
instrument panel fuel gage, the deflection of the bimetallic stip will result in the movement of the
pointer, causing the gage to give a reading. The sending unit consists of a hinged arm with a float
on the end of it. The movement of the arm controls a grounded point that makes contact with
another point that is attached to an electrically heated bimetallic strip. The heating coils in the tank
unit and the gage are connected to each other in series.