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.
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.
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.
- Set the parking brake and block the drive wheels.
- Route all tester cables away from the cooling fan, drive belt(s), pulleys and exhaust system.
- Do NOT operate the engine indoors, but provide proper ventilation.
- Always return the engine idle speed to normal before disconnecting the exerciser tool.
- Remove the engine cover.
- With the engine OFF, unplug the IAC motor wire connector.
- Plug the exerciser tool No. 7558 harness connector onto the IAC motor.
- 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.
- 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.
- Move the switch to the HIGH position. The engine speed should increase. Move the switch to the LOW position. The engine speed should decrease.
- 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.
- If the engine speed does not change, turn the ignition OFF and proceed to Step 6.
- Remove the IAC motor from the throttle body.
- 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.
- If the pintle does not move, replace the IAC motor. Start the engine and test the replacement motor operation as described in Step 5.
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.
- Remove the engine cover.
- Remove the air cleaner assembly.
- 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.
- 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.
- 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.
- Shut the engine off.
- Disconnect the negative battery cable.
- Disconnect the ECM wiring harness.
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.