Dakota 2WD V8-4.7L VIN J (2005)
Idle Speed/Throttle Actuator - Electronic: Description and Operation
The Idle Air Control (IAC) stepper motor (3) is mounted to the throttle body.
A separate IAC motor is not used with the 5.7L V-8 engine.
At idle, engine speed can be increased by retracting the IAC motor pintle and allowing more air to pass through the port, or it can be decreased by
restricting the passage with the pintle and diminishing the amount of air bypassing the throttle plate.
The IAC is called a stepper motor because it is moved (rotated) in steps, or increments. Opening the IAC opens an air passage around the throttle blade
which increases RPM.
The PCM uses the IAC motor to control idle speed (along with timing) and to reach a desired MAP during decel (keep engine from stalling).
The IAC motor has 4 wires with 4 circuits. Two of the wires are for
12 volts and ground to supply electrical current to the motor windings to operate the stepper motor in one direction. The other 2 wires are also for 12
volts and ground to supply electrical current to operate the stepper motor in the opposite direction.
To make the IAC go in the opposite direction, the PCM just reverses polarity on both windings. If only 1 wire is open, the IAC can only be moved 1 step
(increment) in either direction. To keep the IAC motor in position when no movement is needed, the PCM will energize both windings at the same time.
This locks the IAC motor in place.
In the IAC motor system, the PCM will count every step that the motor is moved. This allows the PCM to determine the motor pintle position. If the
memory is cleared, the PCM no longer knows the position of the pintle. So at the first key ON, the PCM drives the IAC motor closed, regardless of
where it was before. This zeros the counter. From this point the PCM will back out the IAC motor and keep track of its position again.
When engine rpm is above idle speed, the IAC is used for the following:
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Off-idle dashpot (throttle blade will close quickly but idle speed will not stop quickly)
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Deceleration air flow control
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A/C compressor load control (also opens the passage slightly before the compressor is engaged so that the engine rpm does not dip down when the
compressor engages)
-
Power steering load control
The PCM can control polarity of the circuit to control direction of the stepper motor.
IAC Stepper Motor Program: The PCM is also equipped with a memory program that records the number of steps the IAC stepper motor most recently
advanced to during a certain set of parameters. For example: The PCM was attempting to maintain a 1000 rpm target during a cold start-up cycle. The
