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Description & Operation

The Idle Air Control (IAC) motor is mounted to the throttle body and is operated by the engine controller. The throttle body has an air control passage that provides air for the engine at idle (when the throttle plate is closed). The IAC motor pintle protrudes into the air control passage and regulates airflow through it. Based on various sensor inputs, the engine controller adjusts engine speed by moving the pintle in and out of the air control passage. The IAC motor is positioned when the ignition is turned to the ON position.

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Operation

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:

• 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 "No-Load" 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.

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Operation

The Idle Speed Control (ISC) actuator is mounted to the throttle body and is controlled by the ECM. The ISC contains the idle contact switch and provides an input signal to the ECM. Various other sensors on the car give input to the ECM that helps to control the ISC. The ECM supplies current and a ground path to the ISC actuator. This enables the ECM to increase or decrease the throttle stop angle by extending or retracting the ISC actuator. The throttle lever rests against an adjustment screw at the end of the actuator. The actuator extends or retracts to control engine speed and to set throttle stop angle during deceleration.

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The Throttle Position (TP) sensor is a potentiometer that provides a signal to the ECM that is directly proportional to the throttle plate position. The TP sensor is mounted on the side of the throttle body and is connected to the throttle plate shaft. The TP sensor monitors throttle plate movement and position, and transmits an appropriate electrical signal to the ECM. These signals are used by the ECM to adjust the air/fuel mixture, spark timing and EGR operation according to engine load at idle, part throttle, or full throttle. The TP sensor is not adjustable.

The 3-wire TPS provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents the throttle blade position of the throttle body. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the output voltage of the TPS changes.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. This will vary in an approximate range of from .26 volts at minimum throttle opening (idle), to 4.49 volts at wide open throttle. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM needs to identify the actions and position of the throttle blade at all times. This information is needed to assist in performing the following calculations:

• Ignition timing advance

• Fuel injection pulse-width

• Idle (learned value or minimum TPS)

• Off-idle (0.06 volt)

• Wide Open Throttle (WOT) open loop (2.608 volts above learned idle voltage)

• Deceleration fuel lean out

• Fuel cutoff during cranking at WOT (2.608 volts above learned idle voltage)

• A/C WOT cutoff (certain automatic transmissions only)

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Idle air control motor testing

|WARNING |

|When the IAC motor is removed from the throttle body, do not extend the pintle more than 0.250 inch (6.35mm). If the pintle is |

|extended more than this amount, it may separate from the IAC motor and the motor will have to be replaced. |

To perform a complete test of the IAC motor, you will need the DRB-II scan tool, or equivalent. This test is a test of the IAC motor only. You will need access to the special factory IAC motor exerciser tool No. 7558, or equivalent.

1. Set the parking brake and block the drive wheels.

2. Route all tester cables away from the cooling fan, drive belt(s), pulleys and exhaust system.

3. Do NOT operate the engine indoors, but provide proper ventilation.

4. Always return the engine idle speed to normal before disconnecting the exerciser tool.

5. Remove the engine cover.

6. With the engine OFF, unplug the IAC motor wire connector.

7. Plug the exerciser tool No. 7558 harness connector onto the IAC motor.

8. Connect the red clip of the exerciser tool to the positive battery terminal, and the black to the negative battery terminal. The red lamp will illuminate when the tool is properly connected.

9. Start the engine. When the switch is in the high or low position, the lamp on the exerciser tool will flash. This indicates that the voltage pulses are being sent to the IAC stepper motor.

10. Move the switch to the HIGH position. The engine speed should increase. Move the switch to the LOW position. The engine speed should decrease.

11. If the engine speed changed predictably while using the exerciser tool, the IAC motor is working correctly. Disconnect the exerciser tool and install the IAC motor wiring connector on IAC motor.

12. If the engine speed does not change, turn the ignition OFF and proceed to Step 6.

13. Remove the IAC motor from the throttle body.

14. With the ignition OFF, cycle the exerciser tool switch between the HIGH and LOW positions. Keep your attention on the pintle. It should move in-and-out of the motor.

15. If the pintle does not move, replace the IAC motor. Start the engine and test the replacement motor operation as described in Step 5.

16. If the pintle operates properly, check the IAC motor bore in the throttle body for blockage and clean as needed. Install the IAC motor and retest. If blockage is not found, more complete testing will be required using the DRB-II scan tool or equivalent.

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|PATH: Engine Performance & Emission Controls > Components & Systems > Idle Speed Control Actuator > Testing |[pic]Print[pic] |

Testing

Animations and Tech Tips

[pic]Click to view: Typical actuator control signal

[pic]Click to view: Typical pulse width control signal

|WARNING |

|Testing the wiring harness with the ECM still connected can cause serious damage to the processor. ALWAYS disconnect the ECM before|

|Testing the wiring harness., unless instructed otherwise. |

1. Remove the engine cover.

2. Remove the air cleaner assembly.

3. While watching the ISC actuator, have an assistant start the engine, shut it off and turn the ignition OFF. The ISC actuator should move outward to preset a fast idle after the next start, after the ignition is turned OFF. If it does not move outward, continue diagnostics. If the actuator moves outward, start engine and let idle to verify proper idle speed, if idle speed is out of specifications, set the idle speed using the procedure on reinstallation of the ISC.

4. Check the ISC actuator to see if it is frozen or sticking, if it is, replace it and retest. If it is OK, continue diagnostics.

5. Start the engine and backprobe the ISC connector to voltage is getting to ISC actuator. Voltage should be fluctuating between 2.0 and 6.0 volts. If voltage is OK, continue diagnostics. If voltage is not OK, repair circuit and retest.

6. Shut the engine off.

7. Disconnect the negative battery cable.

8. Disconnect the ECM wiring harness.

9. Using an ohmmeter, measure the resistance on the ground circuit, using the supplied wiring diagrams, between the ISC connector and ECM harness connector. Resistance should be less than 5.0 ohms, if not repair ground circuit and retest. If resistance is OK, replace ISC motor.

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|PATH: Engine Performance & Emission Controls > Components & Systems > Throttle Position Sensor > Testing |[pic]Print[pic] |

Testing

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[pic][pic]What the throttle position sensor (TPS) detects; when diagnostic trouble codes (DTCs) are set; when failsafe mode is triggered; what can cause excessively high or low voltage; and TPS testing

1. Remove the engine cover.

2. Remove the air cleaner assembly.

3. With the engine OFF and the ignition ON, check the voltage at the center terminal of the TP sensor by carefully backprobing the connector.

4. Voltage should be between 0.2 and 1.4 volts at idle, and less than 4.8v at Wide Open Throttle (WOT). If the TP sensor does not meet these specifications, replace it.

5. If no voltage is present, check the wiring harness for supply voltage (5.0v) and ground (0.3v or less), by referring to your corresponding wiring guide. If supply voltage and ground are present, but no output voltage from TP, replace the TP sensor. If supply voltage and ground do not meet specifications, make necessary repairs to the harness or ECM.

6. Replace the air cleaner assembly.

7. Replace the engine cover.

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