Content .. 1129 1130 1131 1132 ..
Dodge Durango (HB). Manual - part 1131
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:
•
Off-idle dashpot (throttle blade will close quickly but idle speed will not stop quickly)
•
Deceleration air flow control
•
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 last recorded number of steps for that
may have been 125. That value would be recorded in the memory cell so that the next time the PCM recognizes
the identical conditions, the PCM recalls that 125 steps were required to maintain the target. This program allows
for greater customer satisfaction due to greater control of engine idle.
Another function of the memory program, which occurs when the power steering switch (if equipped), or the A/C
request circuit, requires that the IAC stepper motor control engine rpm, is the recording of the last targeted steps
into the memory cell. The PCM can anticipate A/C compressor loads. This is accomplished by delaying compressor
operation for approximately 0.5 seconds until the PCM moves the IAC stepper motor to the recorded steps that
were loaded into the memory cell. Using this program helps eliminate idle-quality changes as loads change. Finally,
the PCM incorporates a
9
No-Load
9
engine speed limiter of approximately 1800 - 2000 rpm, when it recognizes that
the TPS is indicating an idle signal and IAC motor cannot maintain engine idle.
A (factory adjusted) set screw is used to mechanically limit the position of the throttle body throttle plate. Never
attempt to adjust the engine idle speed using this screw. All idle speed functions are controlled by the IAC
motor through the PCM.
HB
FUEL INJECTION
14 - 35