Pontiac FIREBIRD 1972 Service Manual page 1304

12-34 1972 PONTIAC SERVICE MANUAL
Gage readings are made from five markings on gage face.
The left-hand line indicates empty, the centerline half-full
and the right line full.
The dash unit is an electromagnetic instrument consisting
of a permanent magnetic armature and spindle assembly
mounted in a pocket in center of a plastic core with a
quantity of silicone dampening fluid to restrict pointer
movement due to car motion. A pointer is attached to
spindle. Around the core, and surrounding magnet arma-
ture, are three coils that produce a magnetic field which
attracts the armature. The direction of the magnetic field
of coils is determined by resistance of the tank unit rheo-
stat.
Coil No. 1 is connnected through the gage terminal to
battery. The opposite end of coil No. 1 is a common point
to coil No. 2 and rheostat. The common point is another
gage terminal to which coils 1 and 2 are internally con-
nected and rheostat is externally connected. A third coil
is connected to opposite end of coil No. 2. The opposite
end of coil No. 3 is connected to ground.
Coils 1 and 2 are wound in opposite directions so that in
operation their magnetic fields oppose each other. A fixed
calibration resistor is connected across coil No. 1.
When the fuel tank is empty, the tank unit rheostat resist-
ance is approximately zero ohms. All current passes
through coil No. 1 and the armature and pointer align
themselves with the magnetic field of this coil. As the tank
fills up, the rheostat resistance increases and current now
flows in all three coils. The armature and pointer will align
themselves with resultant magnetic field of all three coils.
The exact position of armature and pointer depends on
magnitude of each field produced by coils. The total angu-
lar travel of armature and pointer from empty to full is 90".
The fuel gage tank unit consists of a float with linkage
connecting it to a movable contact arm and rheostat. As
the float rises with filling tank, the contact arm moves
over the rheostat which increases resistance into the dash
unit electrical circuit, and provides more current into coils
2 and 3, and causes greater dash unit pointer movement
toward FULL position.
Air conditioned cars are equipped with a vapor separator
system therefore, a special fuel tank gage unit is required
to accept the vapor return line to tank.
TEMPERATURE INDICATOR LAMP
The engine temperature indicator lamp is controlled by a
thermal switch which senses cylinder head temperatures.
When the ignition switch is turned to "start" position, a
test circuit is closed to indicate whether the red lamp is
functioning properly.
The engine thermostat is calibrated to control the coolant
temperature within certain limits of atmospheric pressure.
With installation of a 15 psi cap, the boiling point of
engine coolant is raised approximately 2.5"F. for each in-
crease in pressure. Thus, at atmospheric pressure, the boil-
ing point of coolant (if only water) will increase from
212°F. to approximately 250°F. at sea level.
The coolant pressure and temperature will vary during
car's operation. As engine warms up from cold start, heat
energy absorbed by coolant (circulated by water pump)
causes coolant to expand with resultant increase in cooling
system pressure. When car is stopped, as at an intersec-
tion, the temperature of coolant will increase because coo-
lant circulation is reduced and air flow through radiator
is at minimum. When car moves forward again, the engine
coolant is cooled due to recirculation through radiator
and added air flow across radiator. This rising and lower-
ing of temperature and pressure is a normal function of the
cooling system.
The engine thermostat control temperature also varies as
coolant mixture varies. All models specify the use of
195°F. thermostat with glycol type coolant.
Upon starting a cold engine there will be a period of time
before coolant reaches its normal operating temperature
and thermostat will remain closed until thermostat con-
trol temperature is exceeded; then the thermostat will
open. If coolant temperature reaches 248°F.
*
20", the
circuit to red lamp will be closed, causing warning lamp
to be illuminated.
If the cooling system should not hold pressure due to
pressure cap being left loose, or accident such as puncture
of radiator, rupture or disconnection of a hose, or use of
low boiling point anti-freeze, the calibration temperature
of red lamp heat indicator may not be reached, in spite of
boiling.
It is to be noted that a higher temperature thermostat
(e.g., 195°F.) will not provide faster warm-up. Since either
a 170°F. or 190°F. thermostat remains tightly closed until
their control temperatures are reached, rate of warm-up
is unaffected by thermostat. Heat is obtained from heater
at about 11 3°F. water temperature at 0°F. ambient.
NOTE: Low M n g coolants will not operate
lamp.
GENERATOR LAMP
The red generator indicator lamp, located in the instru-
ment cluster should light when the ignition switch is
turned on and engine is not running. If not, either the bulb
is burned out or generator has an open circuit.
When the generator voltage output becomes greater than
the battery voltage, the red lamp should go out. This does
not, however, indicate whether the battery is being
charged or regulator is functioning properly.