| Temperature
Sensors |
| There
are a few temperature sensors located throughout
the engine compartment. The hollow threaded sensors
contain a semiconductor material whose resistance
decreases as the temperature increases.
One
sensor measures the temperature of the coolant
while the other sensor or sensors measure the
temperature of the air. A cold engine requires
more fuel than a engine at operating temperature
since it must overcome additional friction and
the effects of fuel condensation. The electronic
control unit constantly monitors the resistance
values of these sensors. The electronic control
unit uses these resistance values to calculate
the proper amount of fuel to be injected based
on coolant and air temperature.
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| As
we have stated above, a cold engine requires more
fuel than a warm one to overcome additional friction
and combustion deficiency. A cold engine also
needs to produce more torque. The extra torque
is achieved by the introduction of more air into
the engine. Not only does this increase the idle
speed slightly, the electronic control unit will
react to this additional air by injecting an additional
amount of fuel. Thus the requirement of extra
torque and extra air and fuel is met.
They're
a few different styles of auxiliary air valve
but they essentially perform the same function,
which is the introduction of additional air
behind the throttle plate. The size of the opening
in the auxiliary air valve is determined by
temperature. The colder it is, the larger the
opening that will be uncovered inside the auxiliary
valve. The larger the opening uncovered inside
the auxiliary air valve the more bypassed air
flows behind the throttle plate. The opening
in the auxiliary air valve will be slowly reduced
and the temperature rises and will close at
operating temperature, at which time bypassed
air will not flow behind the throttle plate
through the auxiliary valve.
The
two most common types you will see are the electrically
heated bimetallic type and the wax pellet type,
which is immersed in engine coolant.
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| Trigger
contacts, also referred to as trigger points, are housed
in the lower section of the ignition distributor underneath
the mechanical centrifugal advance plates. The opening
and closing of the trigger contacts indicate the speed
of the engine since the ECU monitors the interval of
time between trigger pulses.
Tech
Tip…the trigger contacts signal the "start"
of injection while the "duration" of injection,
which is how long the injectors remain open, is determined
by the inputs of the pressure sensor, engine speed,
and the various other engine sensors to the electronic
time switch of the ECU.
The
trigger points, on a V8 engine contain four sets of
points. The trigger points are not that dissimilar
from ignition points in looks except they do not carry
voltage. As each set of points is opened and closed
the ECU knows when to fire each set of corresponding
injectors based on crankshaft position. Since the
distributor is installed at top dead center, the trigger
points corresponding lobes inside the distributor
correlate to the position of the crankshaft. Consequently
the injectors are pulsed at the precise time.
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