3-12
and the instrument lights have +12 volts applied. The opposite side of each light is grounded. The
instrument panel lights are grounded through the dimming rheostat. This completes the path for current
flow from the negative side of the battery, through all the light bulbs (lamps), back to the positive side of
the battery. If no faults exist, the lamps will light.
When the light switch is pulled to the next position (on), the bar on the switch contacts the "off,"
"park," and "on" contacts of the switch. The lights that were illuminated before are still on, and the + 12
volt potential is now applied to the bright (B) side of the headlights through the dimmer switch. Since the
headlights are also grounded on one side, there is now a complete path for current flow, and the
headlights also light. If the dimmer switch is actuated, the positive potential is switched from the bright
filament to the dim filament of the headlights, and the lights dim.
The brake-light switch has +12 volts applied from point (1), directly to the stop lights (not fused). If
the brake pedal is pressed, the switch is actuated, and the +12 volts are applied to both stop lights (S).
Because one side of each light is tied to ground, there is a path for current flow, and the lights will light. If
the dimming rheostat for the instrument lights is turned in the direction that increases the resistance, more
voltage is dropped across the rheostat, less across the lights, and the lights will get dimmer.
The +12 volts at point (1) are also supplied to the OFF position of the ignition switch. When the
ignition switch is turned on, the +12 volts are felt at point (3). This is a common point to all the engine
instruments.
The gas gauge is a galvanometer with the dial graduated according to the amount of fuel in the tank.
The gas gauge tank unit is a rheostat mechanically linked to a float in the gas tank. When the tank is full,
the float rises to its highest level and positions the movable arm of the rheostat to a position of minimum
resistance. This allows maximum current flow through the galvanometer, and the dial rests at the "full"
mark on the gas gauge. As fuel is used by the engine, the float lowers, increasing the resistance of the
rheostat to ground. This reduces the current through the galvanometer, and the dial shows a lesser amount
of fuel.
The oil-pressure light gets its ground through a normally closed pressure switch. (When no pressure
is applied, the switch is closed.) When the engine is started, the oil pressure increases and opens the
switch. This turns the light off by removing the ground.
The water-temperature gauge is a galvanometer like the gas gauge, except its dial is graduated in
degrees of temperature. The water-temperature element is a thermistor with a negative temperature
coefficient. (A thermistor is a semiconductor device whose resistance varies with temperature.) When the
engine is cold, the resistance of the thermistor is at a maximum. This reduces the current through the
galvanometer, and a low temperature is indicated on the dial. As the water temperature of the engine
increases, the resistance of the thermistor decreases. This allows more current to flow from ground
through the galvanometer, and the temperature on the dial shows an increase.
On the voltage regulator shown, the "T" terminal is grounded anytime the alternator does not have an
output. This gives the alternator light a ground and causes it to illuminate.
Q10.
What type of diagram is the most useful in learning the overall operation of a system?
Q11.
Refer to the schematic diagram in figure 3-10. If the ignition switch is placed in the ON position
and all the engine instruments operate properly except the gas gauge, where would the fault
probably be?
Q12.
If the fuse shown on the schematic (figure 3-10) opens, what lights will operate?
