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1F – 414 ENGINE CONTROLS
through a resistor in the ECM and measures th
change in voltage to determine the intake air tempera-
ture. The voltage will be high when the manifold air is
cold and low when the air is hot. The ECM knows
the intake air temperature by measuring the voltage.
The IAT sensor is also used to control spark timing when
the intake air is cold.
A failure in the IAT sensor circuit sets a diagnostic
trouble code P0112 or P0113.
IDLE AIR CONTROL VALVE
Notice: Do not attempt to remove the protective cap
and readjust the stop screw. Misadjustment may result
in damage to the Idle Air Control (IAC) valve or to the
throttle body.
The idle air control (IAC) valve is mounted on the throttle
body where it controls the engine idle speed under the
command of the en-
gine control module (ECM). The ECM sends volt-
age pulses to the IAC valve motor windings, causing the
IAC valve pintle to move in or out a given distance (a
step or count) for each pulse. The pintle movement con-
trols the airflow around the throttle valves, which in turn,
controls the engine idle speed.
The desired idle speeds for all engine operating condi-
tions are programmed into the calibration of the
ECM. These programmed engine speeds are based on
the coolant temperature, the park/neutral switch status,
the vehicle speed, the battery voltage, and the A/C sys-
tem pressure (if equipped).
The ECM "learns" the proper IAC valve positions
to achieve warm, stabilized idle speeds (rpm) desired for
the various conditions (park/neutral or drive, A/C on or
off, if equipped). This information is stored in ECM
"keep alive" memories (information is retained after the
ignition is turned OFF). All other IAC valve positioning is
calculated based on these memory values. As a result,
engine variations due to wear and variations in the mini-
mum throttle valve position (within limits) do not affect
engine idle speeds. This system provides correct idle
control under all conditions. This also means that dis-
connecting power to the ECM can result in incor-
rect idle control or the necessity to partially depress the
accelerator when starting until the ECM relearns
idle control.
Volts
4.9
4.4
kPa
100
90
in Hg
29.6
26.6
e
MAP
3.8
3.3
2.7
80
70
60
23.7
20.7
17.7
Engine idle speed is a function of total airflow into the
engine based on the IAC valve pintle position, the
throttle valve opening and the calibrated vacuum loss
through accessories. The minimum throttle valve posi-
tion is set at the factory with a stop screw. This setting
allows enough air flow by the throttle valve to cause the
IAC valve pintle to be positioned a calibrated number of
steps (counts) from the seat during "controlled" idle op-
eration. The minimum throttle valve position setting on
this engine should not be considered the "minimum idle
speed," as on other fuel injected engines. The throttle
stop screw is covered with a plug at the factory following
adjustment.
If the IAC valve is suspected as being the cause of im-
proper idle speed, refer to "Idle Air Control System
Check" in this section.
MANIFOLD ABSOLUTE PRESSURE
SENSOR
The Manifold Absolute Pressure (MAP) sensor mea-
sures the changes in the intake manifold pressure which
result from engine load and speed changes and con-
verts these to a voltage output.
A closed throttle on engine coast down produces a rela-
tively low MAP output. MAP is the opposite of vacuum.
When manifold pressure is high, vacuum is low. The
MAP sensor is also used to measure barometric pres-
sure. This is performed as part of MAP sensor calcula-
tions. With the ignition ON and the engine not running,
the engine control
module (ECM) will read the manifold pressure as baro-
metric pressure and adjust the air/fuel ratio accordingly.
This compensation for altitude allows the system to
maintain driving performance while holding emissions
low. The barometric function will update periodically dur-
ing steady driving or under a wide open throttle condi-
tion. In the case of a fault in the barometric portion of the
MAP sensor, the ECM will set to the default value.
A failure in the MAP sensor circuit sets a diagnostic
trouble code P0106, P0107 or P0108.
The following tables show the difference between abso-
lute pressure and vacuum related to MAP sensor out-
put, which appears as the top row of both tables.
2.2
1.7
1.1
50
40
30
14.8
11.8
8.9
0.6
0.3
0.3
20
10
0
5.9
2.9
0
DAEWOO T-154 BL2,3
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