Page 221 - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 205

Figure 2.254 Manifold absolute pressure Figure 2.255 Throttle controller and position sensor.
(MAP) sensor. (Source: Denso Media) (Source: Bosch Media)

Figure 2.256 Air temperature sensor

and operated by the throttle plate spindle. A throttle switch assembly has two
switches, one to indicate the closed throttle or idle position and the other for the
wide open throttle position. A throttle potentiometer is a variable resistor with a
rotary sliding contact. The sliding contact is moved along the rotary resistance
track to provide changes in voltage proportional to the position of the throttle.
The throttle potentiometer signals are used in the ECU for a number of functions.
At the closed throttle position, idle speed and deceleration fuel cut-off are
controlled. In the part open throttle position (about 5–70% open), there is
normal operation with close control of fuel delivery and exhaust emissions. In
the wide open throttle position (70–100%), full load enrichment and starting of a
fl ooded engine are provided. During rapid movement of the throttle plate there
is acceleration enrichment, depending on the rate of change of the throttle plate
and signal voltages from the sensor. Key fact
In order for the ECU to correctly calculate the required fuel for a correct mixture In order for the ECU to correctly
ratio, an accurate fi gure for air mass is necessary. However, air volume and calculate the required fuel for a
density are affected by changes in temperature. As the temperature rises, the air correct mixture ratio, an accurate
density falls. The air fl ow, or MAP measurement, therefore, must be corrected fi gure for air mass is necessary.
for temperature. The sensor is a temperature-dependent resistor with a negative
temperature coeffi cient (NTC) ( Fig. 2.256 ).